BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-15-6139-2018Latitudinal trends in stable isotope signatures and carbon-concentrating
mechanisms of northeast Atlantic rhodolithsLatitudinal trends in stable isotope signaturesHofmannLaurie C.lhofmann@mpi-bremen.delaurie.c.hofmann@awi.deHeeschSvenjaMicrosensor Group, Max Planck Institute for Marine Microbiology,
Bremen, 28359, GermanyCentre National de Recherche Scientifique, UMR 8227, Station
Biologique de Roscoff, Roscoff, 29680, Francecurrent address: Marine Aquaculture Group, Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Bremerhaven, 27515, GermanyLaurie C. Hofmann (lhofmann@mpi-bremen.de, laurie.c.hofmann@awi.de)18October201815206139614920September201717October20172August201826September2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://bg.copernicus.org/articles/15/6139/2018/bg-15-6139-2018.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/15/6139/2018/bg-15-6139-2018.pdf
Rhodoliths are free-living calcifying red algae that form
extensive beds in shallow marine benthic environments (<250 m),
which provide important habitats and nurseries for marine organisms and
contribute to carbonate sediment accumulation. There is growing concern that
these organisms are sensitive to global climate change, yet little is known
about their physiology. Considering their broad distribution along most
continental coastlines, their potential sensitivity to global change could
have important consequences for the productivity and diversity of benthic
coastal environments. The goal of this study was to determine the plasticity
of carbon-concentrating mechanisms (CCMs) of rhodoliths along a latitudinal
gradient in the northeast Atlantic using natural stable isotope
signatures. The δ13C signature of macroalgae can be used to
provide an indication of the preferred inorganic carbon source (CO2 vs.
HCO3-). Here we present the total (δ13CT) and
organic (δ13Corg) δ13C signatures of
northeast
Atlantic rhodoliths with respect to changing environmental conditions along
a latitudinal gradient from the Canary Islands to Spitsbergen. The δ13CT signatures (-11.9 to -0.89) of rhodoliths analyzed in this
study were generally higher than the δ13Corg signatures,
which ranged from -25.7 to -2.8. We observed a decreasing trend in δ13CT signatures with increasing latitude and temperature, while
δ13Corg signatures were only significantly correlated to
dissolved inorganic carbon. These data suggest that high-latitude rhodoliths rely more on CO2
as an inorganic carbon source, while low-latitude rhodoliths likely take up
HCO3- directly, but none of our specimens had ∂13Corg signatures less than -30, suggesting that none of them
relied solely on diffusive CO2 uptake. However, depth also has a
significant effect on both skeletal and organic δ13C
signatures, suggesting that both local and latitudinal trends influence the
plasticity of rhodolith inorganic carbon acquisition and assimilation. Our
results show that many species, particularly those at lower latitudes, have
CCMs that facilitate HCO3- use for
photosynthesis. This is an important adaptation for marine macroalgae,
because HCO3- is available at higher concentrations than CO2
in seawater, and this becomes even more extreme with increasing temperature.
The flexibility of CCMs in northeast Atlantic rhodoliths observed in our
study may provide a key physiological mechanism for potential adaptation of
rhodoliths to future global climate change.
Introduction
Rhodoliths are free-living calcifying red algae that form extensive beds in
shallow marine benthic environments (<250 m) and provide
important habitats and nurseries for marine organisms and contribute to
carbonate sediment accumulation. There is growing concern that these
organisms are sensitive to global climate change
(Hofmann and Bischof, 2014; McCoy and Kamenos,
2015), particularly ocean acidification. Ocean acidification may be
detrimental to rhodoliths by reducing calcification rates or increasing
dissolution rates (Hofmann and Bischof, 2014), which would have
severe impacts on the communities supported by rhodolith beds and coastal
carbonate accumulation. Considering their global distribution, it is
important to understand how rhodoliths may be affected by changing
environmental conditions, and if they have physiological mechanisms that
will allow them to adapt. The response of marine macroalgae to ocean
acidification may be closely linked to their inorganic carbon uptake
mechanisms
(Cornwall
et al., 2017; Hepburn et al., 2011). Marine macrophytes have diverse
physiological mechanisms for concentrating CO2 (carbon-concentrating
mechanisms, CCMs) that allow them to overcome the low concentration of
CO2 in seawater relative to HCO3- by direct uptake of
HCO3- or enzymatic conversion of CO2 to HCO3- via
carbonic anhydrase (Giordano et al., 2005). The
species of inorganic carbon (CO2 or HCO3-) taken up by marine
macroalgae influences the stable carbon isotope signature of the tissue
(Maberly et al., 1992;
Raven et al., 2002). Therefore, the ratio of stable carbon isotopes
(∂13C) in macroalgal tissue can be used as an indicator of
whether or not HCO3- is being used
(Raven et al., 2002) using the
formula
∂13C=13C/12Csample13C/12CPDB,
where 13C/12C is the ratio of the natural abundance of the
carbon stable isotopes in the macroalgal tissue sample and carbonate from
the Cretaceous Pee Dee Belemnite. Values greater than -10 ‰ indicate that a CCM is present and the macroalga is
able to take up HCO3-, while values less than -30 ‰ indicate there is no CCM present and the macroalga
relies solely on diffusive CO2 uptake. Values in between -30 ‰ and
-10 ‰ indicate uptake of both CO2 and HCO3-.
Many studies have investigated the presence and absence of CCMs using ∂13C signatures across taxonomic groups and environmental gradients
such as depth and light, CO2, and latitude
(Hepburn
et al., 2011; Moulin et al., 2011; Raven et al., 2011; Stepien, 2015;
Stepien et al., 2016), but so far few studies have investigated the
flexibility of CCMs in a single species or group of marine macroalgae
(Cornelisen
et al., 2007; Cornwall et al., 2017; Mackey et al., 2015). Therefore, we
investigated the plasticity of CCMs in Lithothamnion spp. and other rhodolith species
across a latitudinal gradient from the Canary Islands to Spitsbergen using
natural stable isotope signatures.
Materials and methodsSample collection and treatment
Rhodolith samples were collected at each site from 3 to 10 m in depth via
snorkeling or scuba diving, with the exception of the samples from
Mosselbukta, which were collected during the MSM55 cruise with the manned
submersible JAGO at 11, 25, and 40 m in depth. Samples were air-dried or
dried at 60 ∘C for 48 h. After drying, samples were ground to
a powder in stainless steel shaking flasks with two chromium steel grinding
balls in a micro-dismembrator ball mill for 30 s at 2000 rpm (B. Braun
Biotech International, Melsungen, Germany). For δ13CT
analysis, approximately 10 mg of powder was packed in a 5×9 mm tin capsule
(HEKAtech GmbH, Wegberg, Germany) for each sample and sent to the University of California, Davis
stable isotope facility. For δ13Corg analysis, the
rhodolith powder was treated with 300 µL 1 M HCl in 10.5×9 mm
silver capsules (HEKAtech GmbH, Wegberg, Germany) to dissolve all inorganic
carbon. The samples were left for several days in a fume hood until the HCl
evaporated and all inorganic carbon was dissolved. The organic fraction was
then washed with distilled water and redried before the capsules were
packed and sent to the University of California, Davis stable isotope facility, where the samples
were analyzed for 13C and 15N isotopes using a PDZ Europa ANCA-GSL
elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass
spectrometer (Sercon Ltd., Cheshire, UK). Samples were combusted at
1000 ∘C in a reactor packed with chromium oxide and silvered
copper oxide. Following combustion, oxides were removed in a reduction
reactor (reduced copper at 650 ∘C). N2 and CO2 were
separated on a Carbosieve gas chromatography column (65 ∘C, 65 mL min-1) before
entering the isotope ratio mass spectrometer (IRMS). During analysis, samples were interspersed with several
replicates of different laboratory standards (glutamic acid and peach
leaves), which were previously calibrated against NIST Standard Reference
Materials. In addition to ∂13C, ∂15N
signatures, the percentage of organic tissue as nitrogen (%Norg),
and the percentage of tissue as carbon (total: %CT and organic:
%Corg) were obtained for each sample.
Species identification
Rhodoliths were checked under a dissecting microscope for the presence of
epi- and endophytes or other adhering foreign matter. Clean fragments were
ground to a fine powder with a sterile mortar and pestle, and DNA was
extracted using the Qiagen DNeasy Blood and Tissue Kit (Qiagen, Crawley,
UK), following the modifications of the manufacturers' protocol given in
Broom et al. (2008). PCR amplifications and sequencing of the
rbcL gene were performed according to the methods given in Heesch et al.
(2016), using the primers F57 and R1150
(Freshwater and Rueness, 1994).
Sequences were deposited in ENA/GenBank (see Supplement Table S1 for accession
numbers).
pH drift experiments
The pH drift experiments were conducted for two Irish species, Phymatolithon calcareum (Pallas)
W. H. Adey & D. L. McKibbin ex. Woelkering & L. M. Irvine and P. lusitanicum (P. Crouan &
H. Crouan) Woelkerling & L. M. Irvine, the Canary Islands samples, and
the samples from Akia peninsula, Greenland. These drift experiments can be
used to determine if an efficient CCM is present because if the seawater pH
in the sealed incubation becomes greater than 9.0, no more CO2 is
present, and it can be assumed that HCO3- is taken up during
photosynthesis (Maberly, 1990).
The incubations were conducted during a 24–70 h light exposure with 35 µmol photons m-2 s-1 at 15, 20, or
4 ∘C (for the Irish, Canary Islands, and Greenland samples,
respectively) in natural seawater (34 psu) in 200–300 mL glass or plastic
jars, depending on the size of the specimens. The duration of the light
exposure depended on the specimens. The Greenland specimens were incubated
longer due to their slower metabolic rates. The seawater was vacuum filtered
using 0.22 µm Durapore membrane filters (Merck Millipore, Darmstadt,
Germany). The jars contained stir bars and were placed on a multi-position
stir plate. The rhodoliths were held on custom-made stands inside the jars
to allow space for the stir bars below. The start and finish pH values (NBS)
were recorded using a SenTix 51 pH electrode with an integrated temperature
probe connected to a WTW 3110 pH meter (Weilheim, Germany). The seawater
used during the incubation was left open to the ambient air for
re-equilibration to make sure the change in pH was due to the metabolism of
the algae. Oxygen production rates were measured either simultaneously or in
separate 1 h incubations with oxygen sensor spots (OSXP5) glued to the
inside of the glass jars (Pyroscience, Aachen, Germany). Four to five
individuals of each species were measured.
Data analysis and statistics
In order to standardize the data used for the environmental parameters at
each site, we obtained surface temperature, salinity, pH, total dissolved
inorganic carbon (DIC), and total alkalinity (TA) data from locations closest
to our sampling sites compiled on the Ocean Data View website
(http://odv.awi.de/en/data/ocean/global-alkanity-tco2/, last access: October 2017) for global
alkalinity and total dissolved carbon estimates from Goyet et al. (2000). Only surface data were used from the data set. For quality
control, these data were compared to data from long-term monitoring stations
at our sampling sites when available. From these data, the remaining
parameters of the seawater carbonate system were calculated using the R
package seacarb in RStudio (version 1.0.143).
In order to investigate specific relationships between the ∂13C signatures and environmental variables, a multiple regression
analysis was conducted for δ13Corg using DIC, temperature,
latitude, %Norg, ∂15Norg, %CT,
%Corg, ∂13CT, salinity, and pH as regression
factors. A relative importance test was conducted to determine the relative
importance of each regression factor using the relaimpo package in R. The
function calc.relimp was used to calculate relative importance metrics for
the linear model, and the function boot.relimp was used to calculate
bootstrap confidence intervals for the relative importance of each
regression factor using the Pratt method.
A principal component analysis (PCA) was applied to the data to identify
patterns among physiological characteristics, species, collection site,
and environmental variables. Environmental variables that had correlation
coefficients greater than 0.95 were not both included in the analysis to
avoid multicollinearity. A skewness transformation was applied, and the data
were centered and scaled prior to PCA. The PCA was conducted using
the prcomp function in the R stats package and the results were visualized
using the ggbiplot function in the R package ggplot2.
The effect of species on ∂13C signatures was tested using a
multivariate analysis of variance with species as the independent variables
and ∂13Corg or ∂13CT as response
variables. Tukey's HSD tests were conducted to determine which species
differed from each other.
The effect of depth on ∂13CT, ∂13Corg, ∂15N, %Norg, %Corg, and
%CT was analyzed for the rhodolith samples from Mosselbukta using a
multivariate analysis of variance. The data were checked for normality and
homogeneity of variance using the Shapiro–Wilk and Bartlett tests,
respectively. Significant differences among depths for each dependent
variable were tested using Tukey's HSD tests.
Results
The collection site, species name, and accession numbers of the identified
specimens are summarized in Table S1. The rhodoliths from Greenland were
collected from two sites, Købbe Fjord (64.14, -51.59) and Akia peninsula
(64.19, -51.91). The samples were identified as a mixture of two closely
related species, L. glaciale Kjellman (from Akia and Købbe fjords) and a second entity
from Købbe Fjord, which also occurred in Oslo Fjord. This specimen was most
closely related to L. erinaceum Melbourne & J. Brodie, a species recently described and
reported from the UK (Melbourne et al.,
2017). The specimens from western Ireland were a mixture of three species:
Phymatolithon lusitanicum (V. Peña), P. calcareum (Pallas) W. H. Adey & D. L. McKibbin, and Lithophyllum incrustans Philippi. P. lusticanicum was
collected from Carraroe, while the other two species were collected from
Mannin Bay. The samples from Brest were identified as a mixture of L. corallioides
(P. L. Crouan & H. M. Crouan) P. L. Crouan & H. M. Crouan and a closely
related, undefined species. The Gran Canaria rhodoliths were tentatively
identified as Lithothamnion sp., although both Lithothamnion and Phymatolithon sp.
2
(Pardo et al., 2014) are
present off the eastern coast of Gran Canaria and the descriptions of these
species are still in flux (Viviana Peña, personal communication, 2017). The rhodoliths
collected from Mosselbukta were identified as L. glaciale by Teichert et al. (2014). Our molecular analysis
confirmed that these samples were dominated by L. glaciale, but we also identified a
specimen of L. lemoineae Adey from our subsampling of this collection.
The (a) organic (∂13Corg) and (b) total
(∂13CT) stable carbon isotope signatures of rhodoliths
collected for this study grouped by genus (green: Lithothamnion; red: Lithophyllum;
blue:
Phymatolithon) and species (Lc: L. corallioides; Le: L. erinaceum; Lg: L. glaciale; Lg2: L. glaciale2; Lsp: Lithothamnion sp.; Li: L. incrustans; Pc: P. calcareum; Pl: P. lusitanicum).
The organic (∂13Corg) and total (∂13CT)
stable carbon isotope signatures of all Lithothamnion spp. collected for
this study as a function of latitude, excluding the Mosselbukta samples
collected deeper than 11 m.
The δ13CT signatures of rhodoliths analyzed in this study
ranged from -11.9 to -0.89 and were generally higher than the δ13Corg signatures, which ranged from -25.7 to -2.8 (Fig. 1).
Both Phymatolithon species collected from Ireland had significantly higher ∂13CT
signatures than Lithothamnionglaciale, and P. calcareum had significantly higher
∂13CT than species in all other genera. Differences were less
pronounced for ∂13Corg, for which P. lusitanicum had higher signatures
than L. glaciale and L. incrustans. Within the Lithothamnion genus,
both ∂13CT and ∂13Corg showed a decreasing trend with increasing latitude (Fig. 2a, b).
The PCA explained 79 % of the variance within the first three
components (Fig. 3). The first axis (PC1) separated the
cold-temperate/Arctic samples from the temperate and subtropical samples
based on higher [DIC] and lower temperature, pH, [CO32-], and
salinity. The second axis (PC2) separated the samples based on ∂15N and ∂13C. The Oslo, Brest, and Irish Lithophyllum samples had
relatively high ∂15Norg ratios compared to the others,
while samples from Gran Canaria and the Irish Phymatolithon spp. had the highest
∂13C ratios. The Greenlandic specimens were strongly
separated from all other groups due to the site having the lowest pH and
[CO32-]. The three species collected from Ireland were
distinguished based on ∂13C and ∂15N
signatures. The ∂13C signatures (both organic and inorganic)
increased in the order L. incrustans to P. crispatum to P. lusitanicum, and the ∂15N signatures
showed the exact opposite trend.
Principal component analysis (PCA) of the response variables
measured (∂13Corg, ∂13CT, ∂15Norg, % CaCO3, C:N ratio) and environmental factors
(DIC: total dissolved inorganic carbon; pH; salinity; CO3:
[CO32-]; Temp: temperature; ∘C; Long: longitude).
Latitude, the remaining carbonate chemistry parameters (HCO3-,
pCO2, Ωaragonite,), and %Corg were not included to
avoid multiple colinearity. The points are grouped by collection site (GRN: Greenland; OSLO: Oslo Fjord; BREST: Bay of Brest; GC: Gran
Canaria; IR: Ireland; SPIT: Spitsbergen; MOSS: Mosselbukta) and
labeled by species (Lc: L. corallioides; Le: L. erinaceum; Lg: L. glaciale; Lg2: L. glaciale2;
Lsp:
Lithothamnion sp.; Li: L. incrustans; Pc: P. calcareum; Pl: P. lusitanicum.
Multiple regression analysis of only the Lithothamnion spp. showed a significant
correlation between ∂13Corg and DIC, ∂13CT, ∂15Norg, %Corg, and salinity
(Fig. 4). The proportion of variance explained by the regression model was
94.4 %. The total and organic fraction ∂13C signatures were
most strongly correlated, and this correlation accounted for 86 % of the
R2 in the model. Figure 5a shows the relationship between ∂13Corg and ∂13CT by species (the dominant
species at each site). All species showed a strong linear relationship.
Figure 5b shows the relationship between ∂13Corg and
percentage of organic carbon. All species showed an increasing ∂13Corg with increasing organic carbon content, with the exception
of L. corallioides and L. glaciale from Mosselbukta. Temperature, latitude, and salinity were the environmental variables
with the strongest contributions to the model (32 %, 23 %, and 22 %,
respectively; Fig. 6a). The proportion of variance explained by the
regression model using ∂13CT as a response variable
was 93.8 %, and DIC was the most important environmental factor (16.5 %
of the R2; Fig. 6b). Closer examination of the relationship between
carbonate content and ∂13Corg signatures showed that
within species, there was a negative linear relationship between carbonate
content and ∂13Corg signatures (Fig. 7).
The linear relationships between ∂13Corg
signatures of Lithothamnion spp. collected for this study and ∂13CT,
organic carbon content, total dissolved inorganic carbon (DIC), ∂15Norg, and salinity. Linear regression analysis showed
significant correlation coefficients for these factors.
The relationship between ∂13Corg and (a)∂13CT and (b) percent organic carbon for each species at
each site (Brest: Bay of Brest; GC: Gran Canaria; IR: Ireland;
MOSS: Mosselbukta; OSLO: Oslo Fjord; SPIT: Spitsbergen).
Depth had a significant effect on ∂13CT (F=4.923,
p=0.02271), %Norg (F=5.4079, p=0.01704), %Corg
(F=3.809, p=0.0459), and %CT (F=9.99, p=0.0017) of the
Mosselbukta rhodoliths (Fig. 8). The ∂13C signatures (total
and organic) of rhodoliths collected at 11 m were significantly lower than
those collected at 25 and 40 m (Fig. 8a, b). There was no significant effect of depth on ∂15Norg signatures
(Fig. 8c). The %N and %CT were significantly higher in the rhodoliths collected at 11 m compared to at 25 and 40 m,
while the rhodoliths collected from 40 m had significantly lower %Corg than the rhodoliths collected at 11 and 25 m (Fig. 8d–f).
The pH drift experiments showed that the seawater pH actually decreased after a
24 h light incubation for both Phymatolithon calcareum and P. lusitanicum (Fig. 9). However, these samples
were small, and it is possible that the incubation period was not long
enough to detect a significant change in pH. In comparison, the Canary
Islands samples elevated the seawater pH up to 9.07. The Greenland samples
also increased the seawater pH up to 9.7, but the seawater pH did not return
to ambient levels after being exposed to the atmosphere. After 5 days, the
seawater pH was lower than when the rhodoliths were present, but still
higher than the starting pH value. The Greenland samples produced high
amounts of dissolved organic carbon (DOC) during the incubation period (data not
shown), which could have strongly affected the pH compensation point, and
suggests that pH drift experiments for these specimens are not reliable
methods for determining the pH compensation point.
(a) The mean skeletal (∂13C: symbol color) and
organic (∂13Corg: symbol size) stable carbon isotope
signatures of Lithothamnion spp. mapped in relationship to surface ocean temperature,
excluding the Mosselbukta samples collected deeper than 11 m. (b) The
skeletal stable carbon isotope signatures as a function of total dissolved
inorganic carbon (DIC).
Relationship between carbonate content (% calcium carbonate)
and ∂13Corg signatures for each species (separated by
panels) and collection site (indicated by color). Lc: Lithothamnion corallioides; Le: L. erinaceum;
Lg:
L. glaciale; Lg2: L. glaciale2; Lsp: Lithothamnion sp.; Li: Lithophyllum incrustans;
Pc: Phymatolithon calcareum; Pl: P. lusitanicum. Collection
sites (GRN: Greenland; OSLO: Oslo Fjord; BREST: Bay of Brest; GC: Gran Canaria; IR: Ireland; SPIT: Spitsbergen;
MOSS:
Mosselbukta).
Discussion
Our results demonstrate that ∂13C signatures in rhodoliths
are highly variable, and that this variability is closely related to
environmental factors, particularly temperature and seawater chemistry.
However, ∂13C signatures are also species dependent since
two different species collected from the same site (L. incrustans and P. calcareum) showed
significant differences in ∂13C signatures. In general, all
the rhodoliths collected in this study had ∂13Corg
signatures greater than -30, suggesting that none of them rely solely
on diffusive CO2 uptake. The mean ∂13Corg
signatures for most species were close to -10, suggesting that most of the
rhodoliths examined in this study have relatively efficient CCMs. Both
Phymatolithon spp. consistently had ∂13Corg signatures greater than
-10, suggesting that these species primarily take up HCO3-
directly. The pH drift experiments also support the hypothesis that most
rhodoliths investigated have an active CCM involving HCO3- uptake
since several individuals from the highest and lowest latitude investigated
had pH compensation points above 9.0. There was high variability in pH
compensation points due to the variability in size and shape of the
rhodoliths used for the incubations. For the Canary Islands samples, most
individuals were similar in size, but some were more or less solid or hollow
than others.
The ∂13Corg signatures measured in our study are
relatively high for red algae (Rhodophyta) in general and are higher than
other coralline algae collected from Brittany (Schaal et al., 2009, 2012).
Red algae have been reported to have generally lower ∂13C
signatures than green (Chlorophyta) and brown algae (Phaeophyceae)
(Hepburn et al., 2011; Moulin et al., 2011; Raven et al., 2002; Stepien,
2015). However most of the reported values supporting this trend are for
fleshy macroalgae (Maberly et al., 1992; Marconi et al., 2011; Raven et al.,
2002). Our data complement the findings by Stepien (2015), who analyzed
published ∂13C signatures of marine macrophytes and reported
that calcifying algae had the highest ∂13Corg signatures
compared to all other functional groups. Furthermore, the presence or
absence of CCMs in red algae is apparently not a conserved trait, as
families within this group are strongly variable with respect to the
presence or absence of CCMs (Stepien, 2015). Our study demonstrates that
northeast Atlantic rhodoliths contribute to this variability, as they
deviate from the typical trend in red macroalgae lacking or having
inefficient CCMs. The high ∂13Corg signatures measured
in our study support previous work suggesting that crustose coralline algae
(CCA) can directly take up HCO3- for calcification and
photosynthesis (Comeau et al., 2013; Hofmann et al., 2016). Because CCA take
up HCO3- for calcification (Comeau et al., 2013), the same
transporters are likely used to supply inorganic carbon for photosynthesis,
which would explain the high ∂13Corg signatures. The
positive linear relationship between ∂13CT and ∂13Corg across all specimens suggests that there is a strong use
of respiratory CO2 during calcification in northeast Atlantic
rhodoliths (Lee and
Carpenter, 2001).
The linear relationship between skeletal ∂13C signatures
(∂13CT) and DIC observed in our study supports the
notion that these signatures can be used as a proxy for seawater DIC in
long-lived coralline algae (Williams et al., 2011). Although the ∂13Corg signatures of rhodoliths from multiple genera collected in
our study did not show a strong relationship to DIC, when only rhodoliths
from a single genus (Lithothamnion) were considered there was a strong linear
relationship to DIC. The increasing trend in Lithothamnion spp. ∂13C
signatures with decreasing DIC indicates that this genus may exhibit
physiological plasticity in CCMs that could
facilitate adaptation to changing seawater chemistry induced by ocean
acidification. Cornwall et al. (2017) has recently shown that macroalgae
with flexible CCMs whose CO2 use increased with CO2 concentration
were more abundant at natural CO2 seeps. Although the authors also
found that obligate calcifiers were less abundant at natural CO2 seeps,
as other studies have also shown (Fabricius et al., 2015; Hall-Spencer et
al., 2008), it my be possible that the physiological plasticity of
rhodoliths observed in this study may nevertheless facilitate adaptation at
a rate comparable to current global climate change.
The high ∂15N signatures in the samples from the Bay of Brest and
Oslo Fjord may be due to uptake of anthropogenic sources of nitrogen because
of the close proximity of the collection sites to major cities and high
agricultural activity. Sewage discharge has been shown to elevate ∂15N signatures of macroalgae (McClelland et al., 1997; McClelland and
Valiela, 1998). These values are higher than those reported for the
coralline alga Corallina elongata collected from northern Brittany (Schaal et al., 2009,
2012), where anthropogenic nitrogen inputs are lower than in the Bay of
Brest, which is influenced by strong agricultural activity. From September
2015 to 2017, the maximum nitrate concentration in the Bay of Brest was 37 µM NO3-, compared to 8.4 µM in Roscoff (Service
d'Observation en Milieu Litoral, INSU-CNRS, Roscoff). A comparison of fleshy
macroalgae from the Brest harbor and Île de Batz, a pristine environment
off the coast of Roscoff, showed that ∂15N signatures were
enriched in macroalgae from Brest harbor (Schaal et al., 2010). Therefore,
the ∂15N signatures of rhodoliths also appear to be impacted
by anthropogenic nitrogen inputs, like in other macroalgae.
Box plots of the (a) organic and (b) total ∂13C
signatures, (c)∂15N signatures, (d) organic nitrogen content,
(e) organic carbon content, and (f) total carbon content of L. glaciale collected from
three depths at Mosselbukta. Asterisks indicate significant differences
among depths.
The increase in ∂13C signatures we observed with depth was
surprising, considering most studies have observed decreases in ∂13C signatures with increasing depth (Hepburn et al., 2011; Stepien,
2015). Light has been shown to be an important factor influencing ∂13C signatures in macroalgae (Cornwall et al., 2015; Murru and
Sandgren, 2004; Raven et al., 2002; Stepien et al., 2016). Considering that
rhodoliths are low-light-adapted subtidal algae, it is possible that other
factors such as temperature, DIC, or water velocity have a stronger influence
on ∂13C signatures than light in the case of rhodoliths. In
fact, there may be a relationship between ∂13C signatures and
DOC availability in rhodolith beds since
rhodolith food webs depend strongly on external inputs of organic matter
(Grall et al., 2006; Gabara, 2014), and the biogeochemical cycling within the
rhodolith bed food web influences isotopic signatures. A comparison of
∂13Corg values measured in this study with
concentrations of DOC collected along a similar latitudinal gradient
(compiled by the GLODAPv2 Group; Key et al., 2015) shows a similar decreasing
trend with latitude (Fig. S1), but any direct relationship is purely
speculative at this time. Investigating the influence of DOC on rhodolith
physiology and ∂13C signatures is a topic that should be
investigated further. Alternatively, different rates of DIC recycling at
different depths in Arctic rhodoliths could influence their stable isotope
signatures. Our data suggest that calcification in northeast Atlantic
rhodoliths is strongly influenced by respiratory CO2, but there could
additionally be recycling of DIC from precipitated carbonate material that
is redissolved in Arctic rhodoliths. If more DIC from redissolved
carbonate is recycled in deep Arctic rhodoliths than in shallow specimens,
that could explain the higher ∂13C signatures we observed at
25 and 40 m compared to 11 m.
In conclusion, our results show that many northeast Atlantic rhodolith
species, particularly those at lower latitudes, have CCMs that facilitate HCO3- use for photosynthesis. This is
an important adaptation for marine macroalgae because HCO3- is
available at higher concentrations than CO2 in seawater, and this
becomes even more extreme with increasing temperature. The flexibility of
CCMs in northeast Atlantic rhodoliths observed in our study may provide a
key physiological mechanism for potential adaptation of rhodoliths to future
global climate change.
The pH compensation points (maximum pH reached during pH drift
experiment) for the rhodoliths from Greenland (L. glaciale), Gran Canaria
(Lithothamnion sp.), and Ireland (P. calcareum and P. lusitanicum).
The data have been archived in PANGAEA (10.1594/PANGAEA.881865, last access: October 2017) (Hofmann and Heesch, 2017). The DNA sequences of samples used in
this study will be deposited in GenBank and the assigned accession numbers
will be published in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/bg-15-6139-2018-supplement.
LCH designed and carried out the experiments and wrote the paper. SH
provided molecular taxonomy analysis for species identification and edited
the paper.
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank the following individuals for collection of
rhodolith samples used in this study: Max Schwanitz, Kate Schoenrock, Stein
Fredriksen, Sophie Martin, Fernando Tuya, and Joy Smith. This research was
funded by the National Science Foundation Ocean Sciences International
Postdoctoral Research Fellow program awarded to Laurie C. Hofmann (grant
number 1521610): “Plasticity of inorganic carbon use in marine calcifying
macroalgae across a latitudinal gradient and consequences of global
change.”The article processing charges for this open-access publication were covered by the Max Planck Society.
Edited by: Caroline P. Slomp
Reviewed by: two anonymous referees
References
Broom, J., Hart, D., Farr, T., Nelson, W., Neill, K., Harvey, A., and
Woelkerling, W.: Utility of psbA and nSSU for phylogenetic reconstruction in
the Corallinales based on New Zealand taxa, Mol. Phylogenet. Evol., 46,
958–973, 2008.Comeau, S., Carpenter, R. C., and Edmunds, P. J.: Coral reef calcifiers buffer their
response to ocean acidification using both bicarbonate and carbonate, P. R.
Soc. B, 280, 20122374, 10.1098/rspb.2012.2374, 2013.Cornelisen, C. D., Wing, S. R., Clark, K. L., Hamish Bowman, M., Frew, R. D.,
and Hurd, C. L.: Patterns in the δ13C and δ15N signature of
Ulva pertusa: Interaction between physical gradients and nutrient source
pools, Limnol. Oceanogr., 52, 820–832, 10.4319/lo.2007.52.2.0820,
2007.Cornwall, C. E., Revill, A. T., and Hurd, C. L.: High prevalence of diffusive uptake of
CO2 by macroalgae in a temperate subtidal ecosystem., Photosynth.
Res., 124, 181–190, 10.1007/s11120-015-0114-0, 2015.Cornwall, C. E., Revill, A. T., Hall-Spencer, J. M., Milazzo, M., Raven, J.
A., and Hurd, C. L.: Inorganic carbon physiology underpins macroalgal
responses to elevated CO2, Sci. Rep., 7, 46297, 10.1038/srep46297,
2017.Fabricius, K. E., Kluibenschedl, A., Harrington, L., Noonan, S., and
De'ath, G.: In situ changes of tropical crustose coralline algae along
carbon dioxide gradients, Sci. Rep., 5, 9537, 10.1038/srep09537, 2015.Freshwater, D. W. and Rueness, J.: Phylogenetic relationships of some
European Gelidium (Gelidiales, Rhodophyta) species, based on rbcL nucleotide
sequence analysis, Phycologia, 33, 187–194,
10.2216/I0031-8884-33-3-187.1, 1994.
Gabara, S. S.: Community structure and energy flow within rhodolith
habitats at Santa Catalina Island, CA, Master's Thesis, San José State
University, San José, CA, USA, 2014.Giordano, M., Beardall, J., and Raven, J. A.: CO2 concentrating mechanisms in
algae: mechanisms, environmental modulation, and evolution, Annu. Rev.
Plant Biol., 56, 99–131, 10.1146/annurev.arplant.56.032604.144052,
2005.Goyet, C., Healy, R., Ryan, J., and Kozyr, A.: Global distribution of total
inorganic carbon and total alkalinity below the deepest winter mixed layer
depths, available at:
https://www.osti.gov/scitech/servlets/purl/760546 (last access: 24 August 2017),
2000.Grall, J., Le Loc'h, F., Guyonnet, B., and Riera, P.: Community structure and food
web based on stable isotopes (δ15N and δ13C) analysis of a North Eastern
Atlantic maerl bed, J. Exp. Mar. Biol. Ecol., 338, 1–15, 10.1016/J.JEMBE.2006.06.013, 2006.Hall-Spencer, J. M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S. M.,
Rowley, S. J., Tedesco, D., and Buia, M.-C.: Volcanic carbon dioxide vents show
ecosystem effects of ocean acidification, Nature, 454, 96–99, 10.1038/nature07051,
2008.Heesch, S., Pažoutová, M., Moniz, M. B. J., and Rindi, F.:
Prasiolales (Trebouxiophyceae, Chlorophyta) of the Svalbard Archipelago:
diversity, biogeography and description of the new genera Prasionella and
Prasionema, Eur. J. Phycol., 51, 171–187,
10.1080/09670262.2015.1115557, 2016.Hepburn, C. D., Pritchard, D. W., Cornwall, C. E., Mcleod, R. J., Beardall,
J., Raven, J. A., and Hurd, C. L.: Diversity of carbon use strategies in a
kelp forest community: Implications for a high CO2 ocean, Glob. Chang.
Biol., 17, 2488–2497, 10.1111/j.1365-2486.2011.02411.x, 2011.Hofmann, L. C. and Bischof, K.: Ocean acidification effects on calcifying
macroalgae, Aquat. Biol., 22, 261–279, 10.3354/ab00581, 2014.Hofmann, L. C. and Heesch, S.: Latitudinal trends in stable
isotope signatures and carbon concentrating mechanisms of northeast Atlantic
rhodoliths, PANGAEA, available at: 10.1594/PANGAEA.881865, 2017.Hofmann, L. C., Koch, and de Beer, D.: Biotic control of surface pH and
evidence of light-induced H+ pumping and Ca2+-H+ exchange in a
tropical crustose coralline alga, PLoS One, 11, e0159057,
10.1371/journal.pone.0159057, 2016.Key, R. M., Olsen, A., van Heuven, S., Lauvset, S. K., Velo, A., Xiaohua,
L., Schirnick, C., Kozyr, A., Tanhua, T., Hoppema, M., Jutterström, S.,
Steinfeldt, R., Jeansson, E., Ishii, M., Perez, F. F., and Suzuki, T.: Global
Ocean Data Analysis Project, version 2 (GLODAPv2), available at:
http://cdiac.ornl.gov/oceans/GLODAPv2/NDP_093.pdf (last access: October 2017),
2015.Lee, D. and Carpenter, S. J.: Isotopic disequilibrium in marine calcareous
algae, Chem. Geol., 172, 307–329, 10.1016/S0009-2541(00)00258-8, 2001.Maberly, S. C.: Exogenous sources of inorganic carbon for photosynthesis by
marine macroalgae, J. Phycol., 26, 439–449,
10.1111/j.0022-3646.1990.00439.x, 1990.Maberly, S. C., Raven, J. A., and Johnston, A. M.: Discrimination between12C
and 13C by marine plants, Oecologia, 91, 481–492, 10.1007/BF00650320,
1992.Mackey, A. P., Hyndes, G. A., Carvalho, M. C., and Eyre, B. D.: Physical and
biogeochemical correlates of spatio-temporal variation in the δ13C
of marine macroalgae, Estuar. Coast. Shelf Sci., 157, 7–18,
10.1016/j.ecss.2014.12.040, 2015.Marconi, M., Giordano, M., and Raven, J. A.: Impact of taxonomy, geography,
and depth on δ13C and δ15N variation in a large
collection of macroalgae, J. Phycol., 47, 1023–1035,
10.1111/j.1529-8817.2011.01045.x, 2011.McCoy, S. J. and Kamenos, N. A.: Coralline algae (Rhodophyta) in a changing
world: integrating ecological, physiological, and geochemical responses to
global change, J. Phycol., 51, 6–24, 10.1111/jpy.12262, 2015.McClelland, J. W. and Valiela, I.: Linking nitrogen in estuarine producers
to land-derived sources, Limnol. Oceanogr., 43, 577–585,
10.4319/lo.1998.43.4.0577, 1998.McClelland, J. W., Valiela, I., and Michener, R. H.: Nitrogen-stable isotope
signatures in estuarine food webs: A record of increasing urbanization in
coastal watersheds, Limnol. Oceanogr., 42, 930–937,
10.4319/lo.1997.42.5.0930, 1997.Melbourne, L. A., Hernández-Kantún, J. J., Russell, S., and Brodie,
J.: There is more to maerl than meets the eye: DNA barcoding reveals a new
species in Britain, Lithothamnion erinaceum sp. nov. (Hapalidiales, Rhodophyta), Eur. J. Phycol.,
52, 166–178, 10.1080/09670262.2016.1269953, 2017.Moulin, P., Andría, J. R., Axelsson, L., and Mercado, J. M.: Different
mechanisms of inorganic carbon acquisition in red macroalgae (Rhodophyta)
revealed by the use of TRIS buffer, Aquat. Bot., 95, 31–38,
10.1016/j.aquabot.2011.03.007, 2011.Murru, M. and Sandgren, C. D.: Habitat matters for inorganic carbon
acquisition in 38 species of red macroalgae (Rhodophyta) from Puget Sound,
Washington, USA, J. Phycol., 40, 837–845,
10.1111/j.1529-8817.2004.03182.x, 2004.Pardo, C., Lopez, L., Peña, V., Hernández-Kantún, J., Le Gall,
L., Bárbara, I., and Barreiro, R.: A Multilocus Species Delimitation
Reveals a Striking Number of Species of Coralline Algae Forming Maerl in the
OSPAR Maritime Area, PLoS One, 9, e104073,
10.1371/journal.pone.0104073, 2014.Raven, J. A., Johnston, A. M., Kübler, J. E., Korb, R., McInroy, S. G.,
Handley, L. L., Scrimgeour, C. M., and Walker, D. I.: Mechanistic
interpretation of carbon isotope discrimination by marine macroalgae and
seagrasses, Funct. Plant Biol., 29, 335–378, 10.1071/PP01201, 2002.Raven, J. A., Giordano, M., Beardall, J., and Maberly, S. C.: Algal and
aquatic plant carbon concentrating mechanisms in relation to environmental
change, Photosynth. Res., 109, 281–296, 10.1007/s11120-011-9632-6,
2011.Schaal, G., Riera, P., and Leroux, C. D.: Trophic significance of the kelp
Laminaria digitata (Lamour.) for the associated food web?: a between – sites
comparison, Estuar. Coast. Shelf Sci., 85, 565–572,
10.1016/j.ecss.2009.09.027, 2009.Schaal, G., Riera, P., Leroux, C., and Grall, J.: A seasonal stable isotope
survey of the food web associated to a peri-urban rocky shore, Mar. Biol.,
157, 283–294, 10.1007/s00227-009-1316-9, 2010.Schaal, G., Riera, P., and Dric Leroux, C.: Food web structure within kelp
holdfasts (Laminaria)?: a stable isotope study, Mar. Ecol., 33, 370–176,
10.1111/j.1439-0485.2011.00487.x, 2012.Stepien, C. C.: Impacts of geography, taxonomy and functional group on
inorganic carbon use patterns in marine macrophytes, edited by A. Austin, J.
Ecol., 103, 1372–1383, 10.1111/1365-2745.12451, 2015.Stepien, C. C., Pfister, C. A., and Wootton, J. T.: Functional Traits for
Carbon Access in Macrophytes, edited by: Dam, H. G., PLoS One, 11,
e0159062, 10.1371/journal.pone.0159062, 2016.Teichert, S., Woelkerling, W., Rüggeberg, A., Wisshak, M., Piepenburg,
D., Meyerhöfer, M., Form, A., and Freiwald, A.: Arctic rhodolith beds and
their environmental controls (Spitsbergen, Norway), Facies, 60, 15–37,
10.1007/s10347-013-0372-2, 2014.Williams, B., Halfar, J., Steneck, R. S., Wortmann, U. G., Hetzinger, S.,
Adey, W., Lebednik, P., and Joachimski, M.: Twentieth century δ13C
variability in surface water dissolved inorganic carbon recorded by coralline
algae in the northern North Pacific Ocean and the Bering Sea, Biogeosciences,
8, 165–174, 10.5194/bg-8-165-2011, 2011.