Sedimented hydrothermal vents are likely to be widespread compared to hard
substrate hot vents. They host chemosynthetic microbial communities which
fix inorganic carbon (C) at the seafloor, as well as a wide range of macroinfauna,
including vent-obligate and background non-vent taxa. There are no previous
direct observations of carbon cycling at a sedimented hydrothermal vent. We
conducted 13C isotope tracing experiments at three sedimented sites in the
Bransfield Strait, Antarctica, which showed different degrees of
hydrothermalism. Two experimental treatments were applied, with 13C
added as either algal detritus (photosynthetic C), or as bicarbonate
(substrate for benthic C fixation). Algal 13C was taken up by both
bacteria and metazoan macrofaunal, but its dominant fate was respiration, as
observed at deeper and more food-limited sites elsewhere. Rates of 13C
uptake and respiration suggested that the diffuse hydrothermal site was not
the hot spot of benthic C cycling that we hypothesised it would be. Fixation
of inorganic C into bacterial biomass was observed at all sites, and was
measurable at two out of three sites. At all sites, newly fixed C was transferred
to metazoan macrofauna. Fixation rates were relatively low compared with
similar experiments elsewhere; thus, C fixed at the seafloor was a minor C
source for the benthic ecosystem. However, as the greatest amount of benthic
C fixation occurred at the “Off Vent” (non-hydrothermal) site (0.077±0.034mgCm-2 fixed during 60 h), we suggest that benthic fixation of
inorganic C is more widespread than previously thought, and warrants further
study.
Introduction
Sedimented hydrothermal vent (SHV) sites are areas where hydrothermal fluid
diffuses through soft sediment cover on its way to mixing with oceanic
bottom water. This creates hot (up to ∼100∘C)
sediments with porewaters rich in dissolved sulfide and methane. This
supports microbes that conduct chemosynthetic C fixation via a range of
pathways (Bernardino et al., 2012). These hydrothermally influenced
sediments are likely to be more spatially extensive than hard substrate
vents, although their diffusive nature makes their extent hard to quantify.
Sedimented hydrothermal vents have been shown to influence biological
community composition and nutrition at adjacent sites which were otherwise
characterised as “inactive” or “off-vent” (Levin et al., 2009; Bell et al.,
2016a, b, 2017a). However, the ecology of
sedimented hydrothermal sites has received relatively little study. There is
only one modelling study that has focused on the interaction between benthic
ecosystems and C cycling at SHVs (Bell et al., 2017b), and there are no
direct observations of SHV C cycling by components of the benthic ecosystem.
So far, a limited number of studies have used natural stable isotopic
analysis to determine carbon sources and their fixation pathways utilised by
infauna at SHVs (Levin et al., 2009; Soto, 2009; Sweetman et al., 2013; Bell
et al., 2016a; Portail et al., 2016). Evidence has shown that C fixed during the
anaerobic oxidation of methane, oxic methanotrophy, sulfide oxidation, as
well photosynthetic organic matter (OM) sinking from the surface, are all
utilised by macrofauna to varying extents at SHVs (Levin et al., 2009;
Bernardino et al., 2012). It is challenging to quantify the relative
contributions of different C sources to macrofaunal diets, both because the
natural isotopic ranges of some C sources overlap, and because often the
isotopic compositions of those end-members could not be measured (Levin et
al., 2009; Bell et al., 2016a). Unknown variability in trophic
discrimination factors also currently precludes quantitative estimates of the
relative contribution of different C sources.
Stable isotope tracing experiments offer a way to overcome some of these
issues. The experimental addition of labelled C sources, either
photosynthetic OM or dissolved inorganic C (bicarbonate) to SHV sediment
allows for the production of chemosynthetic OM, and the transfer of different OM
types into the macrobenthos and other C pools in the short term to be
directly observed. Such experiments (using only photosynthetic OM) have been
conducted at a wide range of (ostensibly) non-chemosynthetic benthic sites,
and have shown a wide variation in the relative importance of different
biological C processing pathways (Woulds et al., 2009, 2016). At food-limited sites in the deep sea, respiration tends to be the dominant fate of
added OM (van Oevelen et al., 2011, 2012). Shallower, more food-rich settings
such as coastal fjords and estuaries, with greater sedimentary organic C
concentrations and higher macrofaunal biomass, show a pattern of biological
C processing in which uptake by fauna is a more important process, and at
unusual and particularly food-rich sites, such as the lower margin of the
Arabian Sea oxygen minimum zone (∼1000m depth), macrofaunal
C uptake can even be the dominant process (Woulds et al., 2009, 2016).
The occurrence of chemosynthesis in a benthic habitat represents an
additional source of fresh, labile OM in an environment that would otherwise
be more severely food limited. For this reason, it has been suggested that
hydrothermally influenced sites can be biomass hot spots, where
biogeochemical cycling is rapid (Bernardino et al., 2012). However, due to
the environmental toxicity created by hydrothermal fluid, and the fact that
the majority of taxa inhabiting SHVs are background rather than
vent endemic, the difference in faunal biomass between SHVs and adjacent
non-vent sites is highly variable (Levin et al., 2009; Bernardino et al.,
2012; Bell et al., 2016b). Therefore, it seems possible that biological C
processing at SHVs will show a distinct complement of biological C
processing patterns unlike those observed elsewhere in the deep sea. The
food-rich, high-biomass characteristics of some SHVs may lead to biological
C processing that is more similar to shallower, food-rich environments.
On the contrary, spatially variable biomass patterns, as well as the
metabolic costs associated with potentially high temperatures and porewater
toxicity could counteract the effect of enhanced food availability. As
direct measurements of biological C processing rates and pathways have not
previously been made at SHVs or in the Southern Ocean, a gap remains
in our understanding of sedimentary C and N cycling.
In this study we conducted stable isotope tracing experiments at three sites
of variable hydrothermal activity in the Bransfield Strait, Antarctica. To
the best of our knowledge this is the first isotope tracing experiment in
this type of system. The following hypotheses were tested:
Hydrothermally influenced sites exhibiting chemosynthesis will show elevated
rates of biological C processing.
At hydrothermally influenced sites bicarbonate will be fixed by
chemoautotrophs and transferred to the macrofauna.
Preference for feeding on photosynthetic versus chemosynthetic OM will be
taxon dependent.
MethodsStudy sites
In this study we focus on a SHV in the Bransfield Strait, close to the tip
of the Antarctic Peninsula. The discovery of hydrothermal venting in the
Bransfield Strait was reported by Klinkhammer et al. (2001), who detected
hydrothermal plumes in the water column, and recovered hot “soupy” sediment
from Hook Ridge. In addition, a species of Sclerolinum (Sahling et al., 2005; Georgieva
et al., 2015) has been described there, and porewater geochemistry and
hydrothermal flux rates have been published (Sahling et al., 2005; Aquilina
et al., 2013).
Map of study sites, adapted from Bell et al. (2016b). The “Off Vent”
site is marked “Off-Axis Control”, and the Middle Sister site is located
where “Three Sisters” is marked. Depths in metres.
Experiments were conducted at three sites in the Bransfield Strait,
Antarctica (Fig. 1). Two of the sites lay on raised edifices, known as Hook
Ridge and Middle Sister, along the axis of the basin, and were selected as
being likely to exhibit diffuse hydrothermal venting, and the former was the
location where diffuse venting had been identified. A third site, at a
similar depth but along the north side of the basin, was chosen as an
off-vent control (hereafter known as “Off Vent”).
Porewater geochemistry at Middle Sister and Off Vent were consistent with
microbial processes without the influence of hydrothermal activity. Porewater
NO3- and NH4+ profiles were indicative of nitrate
reduction, but downcore declines in SO42- and Cl- were
lacking over the ∼40cm depth sampled. In contrast, at Hook
Ridge SO42- was depleted by up to 11 % compared with seawater, and
Cl- was depleted by up to 7 %, allowing for the calculation of hydrothermal advection of
9–33 cmyr-1 (Aquilina et al., 2013).
Site characteristics, all of the information except that on bacterial biomass is from Bell et
al. (2016b).
Sediment organic carbon (Corg) concentrations were lower at Hook Ridge (0.97 wt % Corg) than at the Off Vent and Middle Sister sites, which showed
similar values (1.35 wt % and 1.4 wt % Corg respectively; Table 1). The sites
differed in biomass of different groups, with Hook Ridge and Middle Sister
showing higher bacterial biomass and lower macrofaunal biomass than the
Off Vent site (Table 1). Hook Ridge was the only site classified as
hydrothermally active by Aquilina et al. (2013). Porewaters were enriched in
sulfide, methane and dissolved metals and depleted in chloride. Macrofauna
tended to be representative of the background taxa of the region.
Polychaetes were numerically dominant (41 %–56 %), except at Hook Ridge,
which was dominated by peracarids. Oligochaetes were the next most dominant
at all sites. Vent-endemic fauna were represented by two species of
siboglinid polychaete – S. contortum at Hook Ridge, and Siboglinum sp. elsewhere – and they were always a minority constituent of the community.
Isotope tracing experiments
Sediment cores (10 cm internal diameter) were recovered using a multiple corer, and kept
in the dark at seafloor temperatures (Table 1) using cooled incubators.
Experiments were initiated by the addition of isotopically enriched substrates.
Cores were then sealed and incubated for ∼60h, during which
core-top water was continuously stirred.
Duplicate cores were subjected to each of two treatments. In the “algae”
treatment, lyophilised algal cells (Chlorella, Cambridge Isotope Laboratories,
CNLM-455-1) enriched in 13C and 15N (both ∼100 atom %) were allowed to settle on the sediment surface, giving a final dose of
436±30mgCm-2. This was equivalent to ∼1.6 %
of total OC in the surface 1 cm of sediment, or ∼9 % of the
annual OC input (Bell et al., 2017b). It is recognised that such organic
detritus is less degraded than the sinking photosynthetic material which
normally reaches the depths of our study sites. This is a limitation of the
method common to all such experiments in the literature, and means that
rates for processing of added C in algae experiments should be considered
maximal. Further, diatom detritus would have been more representative of
local photosynthetic material, but was unfortunately not available.
In the “bicarbonate” treatment a solution of 100 % 13C labelled
sodium bicarbonate and 100 % 15N labelled ammonium chloride was
injected into the surface 5 cm of sediment porewater, to give a dose of 306 mgCm-2 and 2.52 mgNm-2, and an estimated porewater bicarbonate
concentration of 1 mM.
At intervals (T0 and every ∼12h thereafter) during the
incubation, core-top water samples were withdrawn from algae treatment
cores, and stored in crimp-cap vials poisoned with HgCl2 for dissolved
inorganic carbon (DIC) analysis. At the end of the experiment cores were
extruded and sectioned at intervals of 0–1, 1–2, 2–3, 3–5 and 5–10 cm. Half
of each section was frozen at -20∘C, and the other half was
preserved in buffered 10 % formalin.
Sample processing and analysis
Overlying water samples were analysed for the concentration and isotopic
composition of DIC in triplicate on a Thermalox TOC analyser coupled to a
Thermo DELTA V Advantage IRMS via a Conflo IV interface, using a Thermo
TriPlus autosampler. The reaction column was filled with
H3PO4-coated beads.
Frozen sediment samples were freeze dried, and surface 0–1 cm horizons were
analysed for phospholipid fatty acids (PLFAs) following Main et al. (2015).
Briefly, samples were extracted in a modified Bligh and Dyer extraction
solution of chloroform:methanol:citrate buffer (1:2:0.8). The polar fraction
was obtained by loading samples onto ISOLUTE SPE columns, washing with
chloroform and acetone, and eluting with methanol. After the addition of
nonadecanoic acid (C19:0) as an internal standard, extracts were derivatised
in the presence of KOH in methanol. Derivatisation was quenched with water
and acetic acid, and the organic fraction was extracted by washing with 4:1isohexane:chloroform. Samples were dried and then taken up in isohexane for
analysis on a TRACE Ultra GC, connected via a GC Combustion III to a DELTA V
Advantage IRMS (Thermo Finnigan, Bremen). The isotopic signature of each
PLFA was measured against a CO2 reference gas which is traceable to
IAEA reference material NBS 19 (TS-Limestone), with a precision of ±0.31 ‰, and corrected for the C atom added during
derivatisation.
Sediment horizons between 0 and 10 cm preserved in formalin were sieved over
a 300 µm mesh. Macrofauna were extracted under a binocular microscope,
identified to broad taxonomic level, air dried in pre-weighed tin capsules,
and weighed. In some cases multiple individuals were pooled to create
samples large enough for analysis. Fauna were de-carbonated by dropwise
addition of 0.1 M HCl, followed by air drying at 50 ∘C. Calcareous
foraminifera and bivalves that were too small for manual removal of shells
were de-carbonated with 6 N HCl. Fauna were analysed for their C content and
isotopic signature using a FlashEA 1112 series elemental analyser connected
via a Conflo III to a DELTAPlus XP isotope ratio mass spectrometer (all
Thermo Finnigan, Bremen). Carbon contents was quantified using the area
under the mass spectrometer response curve, against National Institute of
Standards and Technology reference material 1547 peach leaves (repeat
analysis gave a precision of ±0.35 %). Isotopic data were traceable to
IAEA reference materials USGS40 and USGS41 (both L-glutamic acid), with a
precision of ±0.13 ‰.
Data treatment
Respiration of added algal C was calculated for cores subjected to the algae
treatment. The amount of excess DI13C in each sample was calculated by
first subtracting the natural abundance of 13C in DIC. This was scaled
up to give the total amount of DIC from the added algae at each sample
time point, and corrected for water removed and added during sampling.
The respiration rate was calculated for each core by placing a line of best fit
through the amount of added 13C over time, and normalised to surface
area.
Bacterial incorporation of 13C was calculated by first subtracting the
natural abundance of 13C from the isotopic signature of each PLFA (data
published in Bell et al., 2017a), where the difference exceeded the precision
of the analytical technique, to give the amount of added C in each compound.
Bacterial incorporation was then calculated using the four bacteria-specific
PLFAs isoC14:0, isoC15:0, antisoC15:0 and isoC16:0, following Boschker and
Middelburg (2002). Uptake of 13C into these bacteria-specific PLFAs was
summed, and scaled up on the basis that they together account for 14 % of
total bacterial PLFA, and that PLFAs account for 5.6 % of total bacterial
biomass. For samples in the bicarbonate treatment further upscaling was
applied, to account for the fact that the addition of 13C bicarbonate
was calculated to result in a porewater DIC pool that was 22 atom %
13C.
Faunal uptake of added 13C was calculated by subtracting 13C
attributable to its natural abundance in the appropriate taxon (data
published in Bell et al., 2017a) from faunal isotopic signatures, where the
difference exceeded the precision of the analytical technique, and
multiplying by the quantity of organic C in each specimen. Specimens were
summed for each core, and the value was multiplied by 2 to account for only
half of each horizon being used for faunal extraction.
Results
Data files can be accessed at 10/dgsn (Woulds et al., 2019).
Respiration
Respiration rates measured in algae addition experiments varied from 0.03 mgCm-2h-1 at the Off Vent site to 0.15 mgCm-2h-1 at
Middle Sister (Fig. 2).
Respiration rates measured in algae addition experiments. A and B
refer to the two replicate cores in each experiment.
Bacterial uptake and PLFA suite
In the algae addition experiments, total bacterial uptake of C throughout
the experiment was maximal at Middle Sister and Hook Ridge (1.30–1.91 and
1.25 mgCm-2 respectively), and minimal at the Off Vent site
(0.25–0.77 mgCm-2, Fig. 3). In bicarbonate addition experiments, in
which incorporation of 13C into bacterial PLFAs represents
chemosynthesis, bacterial incorporation of bicarbonate was maximal at the
Off Vent site (0.05–0.10 mgCm-2), and was also detectable in one of
the replicates at Middle Sister (0.003 mgCm-2, which was close to detection
limits, so this value is treated with caution); however, bacterial incorporation of bicarbonate was not
detectable at Hook Ridge.
Bacterial uptake measured in (a) algae addition experiments and (b) bicarbonate addition experiments. Uptake was not quantifiable at Hook Ridge B and Middle Sister A, and a sample was not available from Hook Ridge A. A and
B refer to the two replicate cores in each experiment.
The PLFA suites at all sites were qualitatively similar. They were dominated
by C16:0, C16:1ω7c and C18:1ω7, which together constituted
42±2 % of total PLFAs (Fig. 4). This is at the high end of
contributions from these compounds elsewhere, such as 34 %–45 % in the
Arabian Sea and 41 % on the Galicia Bank (Kunihiro et al., 2014). The
relatively high proportions of C16:1ω7 and C18:1ω7 are
indicative of the presence of chemosynthetic and specifically sulfide
oxidising bacteria (Colaco et al., 2007). In addition C18:1ω9, which
is linked to endosymbionts in vent mussels, and C18:1ω13, which is
associated with methylotrophic bacteria, were also present (Colaco et al.,
2007).
Example PLFA suites – each data series is from one sample, as
opposed to being an average across two replicates. (a) PLFA suite as percentage of the
total PLFAs in the algae addition experiments (figure for bicarbonate addition
experiments very similar and not shown). (b) The composition of 13C uptake
into PLFAs in algae addition experiments. (c) The composition of 13C
uptake into PLFAs in bicarbonate addition experiments.
In both algae and bicarbonate addition experiments, 13C incorporation
into PLFAs was dominated by C16:0, followed by C18:1ω9 and the
sulfide oxidiser indicators C16:1ω7 and C18:1ω7 (Fig. 4).
Faunal uptake
Faunal uptake of added C differed between the A and B replicate cores in all
experiments except the algae addition at the Off Vent site, and bicarbonate
addition at Middle Sister (Fig. 5).
Faunal uptake in (a) algae addition experiments and (b) bicarbonate
addition experiments. A and B refer to the two replicate cores in each
experiment.
In algae addition experiments faunal uptake was similar between the Off Vent
site and one of the Hook Ridge cores (∼0.03mgCm-2),
whereas the other Hook Ridge core showed very low faunal C uptake.
Considerably greater faunal uptake (0.12 mgCm-2) was observed in one
of the replicate cores from Middle Sister (Fig. 5).
In bicarbonate addition experiments, measurable uptake of 13C by fauna
was observed at all sites. It was maximal at Hook Ridge (0.02 mgCm-2
in one replicate), and the Off Vent and Middle Sister sites showed similar
values (Table 2, Fig. 5).
Amount of C in pools at the end of the experiment, and respiration rates
(algae addition experiments only). N/A indicates that it was not appropriate
to measure respiration in bicarbonate addition experiments, n.d. indicates
no data due to missing sample, and “low” indicates an unmeasurably low value.
The value marked * indicates detectable bacterial 13C uptake, but very
close to detection limits – thus, the value should be treated with caution.
The small size of individuals meant that organisms had to be pooled for isotopic
analysis, limiting the taxonomic resolution of the faunal uptake data.
Although limited in this way, the data show that faunal uptake of 13C
in both algae and bicarbonate addition experiments was mostly carried out by
either polychaetes, or “mixed macrofauna” (Fig. 6). This latter category
contained variously bivalves, crustaceans, echinoderms, nematodes and
foraminifera, in cases where the groups were not present in sufficient
numbers to enable separate reporting of their C uptake. When a group was present
in a sufficient quantity, it was analysed separately. As with total macrofaunal
13C uptake, there was considerable variability between replicate cores
in the most abundant taxonomic groups. In addition, meiofaunal organisms
took up 13C at Middle Sister, and the bicarbonate 13C that was
transferred to macrofauna at Hook Ridge was mostly observed in amphipod
crustaceans.
Distribution of C uptake amongst taxonomic groups in the (a) algae
addition experiments and (b) bicarbonate addition experiments.
DiscussionOccurrence of inorganic C fixation
The results of the bicarbonate addition experiments show evidence for the occurrence
of benthic C fixation at all sites, and transfer of that C to the
macrofauna, in the form of isotopic enrichment of bacterial PLFAs at the
Off Vent and Middle Sister sites (Fig. 3), and of macrofauna at the Hook
Ridge and Middle Sister sites (Fig. 5). The quantities of bicarbonate
13C detected in bacterial and faunal biomass were low, and tended to be
1 to 2 orders of magnitude smaller than equivalent values for the algae addition
experiments (Table 2). We have confidence that the values reported are above
detection limits, in that data were only used for areas where the enrichment of
organisms or PLFAs above their natural background signatures was greater
than the analytical precision of the method. The greatest quantities of
bacterial uptake were measured at the Off Vent site (Fig. 3), and the
greatest quantity transferred to the fauna was measured at Hook Ridge (Fig. 5); however, due to the low values measured and the evident patchiness of
faunal communities, we do not feel these differences are suitable for further
discussion.
The most striking result of the bicarbonate addition experiments was that
evidence for benthic C fixation was found at all sites, not only at the
hydrothermally influenced Hook Ridge. Further, the site showing the largest
amount of bicarbonate 13C incorporation into bacterial PLFAs was the
Off Vent “control” site (Table 2, Fig. 3). This is consistent with the
occurrence of siboglinids at all sites. These host chemosynthetic
endosymbionts, most of which conduct sulfide oxidation (Thornhill et al.,
2008; Georgieva et al., 2015). It should be noted that the evidence for
inorganic C fixation comes from PLFAs in the bulk sediment, while isotopic
signatures of siboglinids did not show enrichment above background values.
Therefore, the occurrence of benthic C fixation is not only associated with
siboglinids.
Experiments were designed to replicate natural conditions as far as
practically possible, while being limited to shipboard rather than in situ
methods. One result of this is that the sediment contained in cores was
detached from the upward flux of hydrothermal fluid, and the electron donors
it supplied. This could have limited inorganic C fixation, which would have
impacted the rates measured at Hook Ridge. We suggest, however, that this is
not a serious limitation, as Hook Ridge was rather mildly hydrothermal. Vent-endemic fauna were almost absent (Bell et al., 2016b), there was no increase
in faunal biomass close to venting, downcore profiles of alkalinity, nitrate
and ammonium were consistent with normal microbial processes, and
hydrothermal advection rates were 9–33 cmyr-1 (Aquilina et al., 2013).
At these low advection rates we suggest that there would not have been
sufficient time during our ∼60h experiments for a noticeable
depletion in the availability of electron donors supplied by hydrothermal fluid.
The evidence suggests that while the amount of benthic C fixation was always
low, it was not restricted to environments typically thought of as
chemosynthetic (sedimented or hard substrate hydrothermal vents, methane
seeps or organic falls; Bernardino et al., 2012). Thus, benthic C fixation
appears to play a role in benthic C cycling at a much wider range of sites
and over a much larger area of the seafloor than previously thought. This is
supported by linear inverse modelling of C cycling at the sites in this
study, which led Bell et al. (2017b) to suggest that chemosynthetic support
for ecosystems may have a far greater spatial extent than previously
thought, extending beyond those which are directly hydrothermally
influenced. Similar results have also been reported in non-hydrothermal, but
methane-rich sediments on the South Georgia margin, where the assimilation of
13C labelled bicarbonate into bacterial biomass and transfer into
macrofauna was also observed (Would et al., 2019). In addition, in situ
observations of benthic C fixation have also been made at mesotrophic,
abyssal sites in the eastern equatorial Pacific, which were not associated
with hydrothermal or methane seep activity (Sweetman et al., 2018). In that
study incorporation of 13C labelled bicarbonate into bacterial PLFAs
was observed at two sites separated by hundreds of kilometres, at rates similar
to bacterial assimilation of phytodetritus C at the same sites. Together
with global-scale modelling completed by Middelburg (2011), these studies
suggest that chemoautotrophic C fixation may be considerably more widespread
than previously thought. It is, therefore, deserving of further study so that
it can be quantitatively incorporated into our understanding of the marine
C cycle.
In their study using linear inverse modelling of the benthic food web and C
cycle, based on natural isotopic and biomass data, Bell et al. (2017b)
modelled a rate for chemosynthesis of 5.76–8.4 mgCm-2d-1 at
Hook Ridge and < 0.006 mgCm-2d-1 at the Off Vent site.
These modelled rates at Hook Ridge are considerably higher than Hook Ridge
benthic C fixation measured in this study, for which there was evidence
(labelled PLFAs), but a rate could not be calculated. The higher modelled
rates by Bell et al. (2017b) may be explained by the fact that a
temperature of 50 ∘C was used for the Hook Ridge site, based on
previously published conditions for the site (Klinkhammer et al., 2001).
Unfortunately, equipment was not available while at sea for measurement of
sediment temperature at the study sites; therefore, all experiments,
including that at Hook Ridge, were conducted at measured bottom water
temperatures of 0–1 ∘C. It is likely that the rates measured here
for chemosynthetic incorporation of labelled bicarbonate are lower than
those that would have been measured in situ. It is also probable that
measurable rates could have been detected at Hook Ridge had more samples
been available for replicate analyses.
The maximal rate of benthic C fixation measured in this study was 0.050 mgCm-2d-1, which occurred in one core at the Off Vent site. This value
remains considerably lower than the 0.24–1.02 mgCm-2d-1
measured by Molari et al. (2013; rates calculated in Sweetman et al., 2018)
at depths ranging between 1207 and 4381 m on the Iberian margin and in the
Mediterranean, and the 1.29 mgCm-2d-1 measured by Sweetman et
al. (2018) at a depth of approximately 4100 m in the Clarion Clipperton Zone.
The Bransfield Strait sites in this study were shallower, had higher
concentrations of sedimentary organic C and slightly lower bottom water
temperatures than either of the previous studies cited. The very low
temperatures at which experiments were conducted (1 ∘C at Hook
Ridge and 0 ∘C at the Off Vent site) are likely to have contributed
to the slow rates of benthic C fixation measured. Another factor which may
influence benthic C fixation is the annual flux of photosynthetic C from the
surface (Molari et al., 2013; Bell et al., 2017a). The annual flux of POC to
the sediments in the Bransfield Strait is greater than in the Clarion
Clipperton Zone, and probably than in the Mediterranean as well (Masque et
al., 2002; Sweetman et al., 2017), and this may be an additional driver
behind the low benthic C fixation rates observed. Archaeal abundance has
been shown to correlate with dark C fixation, and addition of labile organic
material has been shown to increase inorganic C fixation rates, perhaps
via a combination of heterotrophy and mixotrophy (Molari et al., 2013).
Overall, the factors governing benthic C fixation rates require
investigation. In addition, the pathways (i.e. autotrophic C fixation versus
anapleurotic C fixation by heterotrophs, Wegener et al., 2012), energy
sources (e.g. sulfide and methane) and organisms responsible for benthic
inorganic C fixation have not been identified, and warrant further study.
Carbon uptake by macrofauna
Uptake of added C by fauna in isotope tracer experiments usually shows a
degree of spatial patchiness (e.g. Woulds et al., 2007), but this seems to
have been particularly marked in the Bransfield Strait, mainly at those
sites with hydrothermal influence. This is consistent with the patchiness of
S. contortum in replicate cores at Hook Ridge (Bell et al., 2016b). At both Hook Ridge and
Middle Sister there was a very marked difference in faunal uptake of algal C
between the A and B replicate cores in the algae addition experiments (Fig. 5),
and this was considerably greater than that observed, for example, in
experiments on the Pakistan margin (Woulds et al., 2007). This is likely due to differences in the biomass of fauna present in each core, and such
marked small-scale patchiness in faunal communities has been noted
previously as a particular feature of SHVs (Levin et al., 2009; Bernardino
et al., 2012). Fine-scale distribution of fauna is related to variations in
concentrations of substrates such as sulfide and methane (Levin et al.,
2003); therefore, the patchiness observed, especially at Hook Ridge, is likely
related to spatial and temporal fluctuation in hydrothermal advection.
Faunal uptake of added C appeared to be greatest at Middle Sister in the algae
addition experiments, and at Hook Ridge in the bicarbonate addition experiments;
however, the variation between replicate cores limits the conclusions that can be
drawn. Previous isotope tracing experiments have noted correlations between the
biomass of taxa and the amount of C that they take up (e.g. Woulds et al., 2007).
Further, there was no systematic variation in biomass-specific C uptake
(0.026–0.13 µg C uptake / mg C biomass) between sites; therefore, the
patterns observed here in faunal C uptake are likely to have resulted from
variation in the biomass present in each experimental core.
Similarly, the identities of fauna responsible for 13C uptake were
variable between replicate cores (Fig. 6), and this is also likely to have
been driven by variation in the macrofaunal community present in each core.
The prevalence and variable importance of the “mixed macrofauna” category
indicates that a fairly diverse assemblage was engaged in C
uptake and processing in some cases.
Distribution of biologically processed C between processes for the (a) algae addition experiments and (b) bicarbonate addition experiments. The absence of data represents the combination of the lack of sample and an undetectable
faunal 13C uptake.
Previous studies have suggested that SHVs tend to exhibit relatively high-biomass macrofaunal communities, sustained by the additional food source
provided by chemosynthesis (Bernadino et al., 2012), and this leads to an
expectation that the macrofauna may be particularly active in the processing of
organic C in the sediment, in line with other food-rich environments such as
estuaries and fjords (Moodley et al., 2000, 2005; Witte et al., 2003a). This
was not the case in the algae addition experiments, with faunal uptake
accounting for only 0.05 %–2.2 % of total biological 13C processing
(Fig. 7). This is similar to the role of faunal C uptake in overall C
processing seen at deep, organic carbon-poor sites such as at a depth of 2170 m
off north-western Spain (2.2 %, Moodley et al., 2002), or at a depth of 1552 m in the
eastern Mediterranean (0.2 %, Moodley et al., 2005), and is lower than
that at a depth of 4800 m on the Porcupine Abyssal Plain (1.5 %–26 %, Witte et
al., 2003b). Such sites tend to have lower OC concentrations and lower
macrofaunal biomass (Woulds et al., 2016) than was observed in the
Bransfield Strait; therefore, the unusually small role of macrofaunal in C
uptake in the Bransfield Strait may be due to low temperatures. Both low
temperature and food scarcity have previously been observed to limit
metabolic rates in polar environments (Brockington and Peck, 2001; Sommer
and Portner, 2002). Another possible explanation for the rather small amount
of macrofaunal C uptake at the Hook Ridge site may be that the macrofaunal
community, which was composed almost entirely of non-vent-obligate, ambient
Southern Ocean taxa (Bell et al., 2016b), had reduced levels of function due
to the stress imposed by living at a site influenced by hydrothermal fluid.
The toxicity and relatively high temperature of their environment (compared
with non-hydrothermal Southern Ocean benthic settings) may have resulted in
reduced C uptake activity. Therefore, macrofaunal biomass and C processing
activities were limited by a hydrothermal flux that was sufficient to limit
functioning and preclude the occurrence of some locally common taxa, but
insufficient to sustain a high-biomass, vent-endemic macrofaunal community
as seen in other SHVs (Bell et al., 2016b).
Siboglinid polychaetes, known to host chemosynthetic endosymbionts, were
present at all study sites (Bell et al., 2016b), but were not found to make
a substantial contribution to uptake of added 13C. This is to be
expected in the algae addition experiments, as siboglinids would have direct
access to algal C (except possibly via DOC). Most specimens recovered from the
bicarbonate addition experiments also showed δ13C values
indistinguishable from their natural signature, with one exception at the
Middle Sister site which was enriched by 3.2 ‰ compared with the natural signature. The fact that siboglinids did not have a major
role in C fixation and cycling in our experiments may have been partly due
to their low abundances in experiment cores compared with patches where they
were maximally abundant (Bell et al., 2016b), or because experiments were
not long enough to allow for uptake by endosymbionts. Nonetheless, our findings show
a much reduced role for siboglinids compared with suggestions made in previous
publications. Aquilina et al. (2014) suggested that Siboglinum sp. at Hook Ridge may be
sufficiently abundant to be conduits for a quantitatively meaningful flux of
dissolved iron out of the sediment, and Bell et al. (2017b) found that they
may be a key taxon facilitating input of chemosynthetic C into the food web.
In agreement with the point made by Bell et al. (2016b), the spatial
distribution of siboglinids is extremely patchy, and thus their role in
benthic biogeochemical processes is spatially heterogeneous (Bell et al.,
2017a, b).
Carbon processing and SHVs as biogeochemical hot spots
Respiration rates measured in the algae addition experiments were maximal at
Middle Sister, and minimal at the Off Vent site (Fig. 2). Temperature is
often recognised as a dominant control on benthic respiration rates (e.g.
Moodley et al., 2005; Woulds et al., 2009), however these experiments were
all conducted within 1 ∘C of each other, so temperature is
unlikely to have driven differences in respiration rates. Instead, the
differences between sites may have been driven by differences in bacterial
biomass (Table 1), which was maximal at Middle Sister and minimal at the
Off Vent site. The bacteria are often found to account for a large majority
of benthic community biomass, and are thus usually assumed to be responsible
for the majority of benthic community respiration (e.g. Heip et al., 2001).
The measured respiration rates were similar to those measured at 2170 m on
the north-western margin of Spain (Moodley et al., 2002), and on the Porcupine Abyssal
Plain (Witte et al., 2003b), both of which were considerably deeper, and had
lower sediment organic C concentrations, but higher bacteria biomass (Woulds
et al., 2016). They were also lower than respiration rates measured at
similar depths in the eastern Mediterranean (Moodley et al., 2005), and
Arabian Sea (Woulds et al., 2009). These sites showed similar bacteria
biomass to the Bransfield Strait, but were all considerably warmer
(7–14 ∘C, Woulds et al., 2016); therefore, the low ambient
temperatures of the Southern Ocean appeared to reduce respiration rates.
Total biological C processing during the (a) algae addition
experiments and (b) bicarbonate addition experiments. The absence of data represents the combination of the lack of sample and an undetectable
faunal 13C uptake.
It has been suggested that reducing benthic environments are often hot spots
of faunal biomass and biogeochemical cycling due to the increased
availability of labile food sources supplied by chemosynthesis (Bernardino
et al., 2012). In this study, the hydrothermally active site Hook Ridge
showed rates of respiration and bacterial uptake of algal C that were
intermediate between the two non-hydrothermally active sites (Figs. 2, 3).
Whilst comparison between sites is limited by very marked faunal patchiness,
the amount of faunal uptake of algal 13C at Hook Ridge was similar to
that at the Off Vent control site, whereas that at Middle Sister was, in one
replicate, considerably greater (Fig. 5). This suggests that SHVs are not
necessarily biogeochemical cycling hot spots, as in the algae addition
experiments the overall amount of added C processed by the benthic community
was not greater than that observed at non-hydrothermal sites (Fig. 8). In
line with this, biological processing of added C in the algae addition
experiments did not show a major role for faunal C uptake as we
hypothesised, but was instead dominated by respiration, as is typically
observed at relatively deep, cold sites (Woulds et al., 2009). The Middle
Sister site showed the greatest amount of biological processing of added
algal C, which was probably attributable to it having the greatest bacterial
biomass and organic carbon concentrations, and the fact that the macrofaunal
community, composed mostly of ambient Southern Ocean taxa, will have been
functioning without the stress imposed by hydrothermal fluid.
Conclusions
The main fate of photosynthetic C was respiration in common with other
deeper and more food-limited sites. The rates of respiration and C uptake by
both macrofaunal and bacteria that we measured were comparatively low, and
this is attributable to the low temperature of the experiments, and the
toxicity and thermal stress caused by hydrothermal fluid. The hydrothermal
site (Hook Ridge) in this study did not show more rapid C cycling than other
similar experiments, as we hypothesised it would.
Benthic fixation of inorganic C was observed at all sites, and quantified at two
out of three sites. While the rates were low compared with other similar
experiments, the fact that the greatest amount of benthic C fixation
occurred at the Off Vent site suggests that benthic C fixation may not be
restricted to hydrothermal and other reducing settings. We suggest that it
could be an important aspect of the marine C cycle, and warrants further
study.
Data availability
Data sets can be found at 10.5285/98ccf93a-5e01-15e1-e053-6c86abc050ce (Woulds et al., 2019).
Author contributions
Experiments were conducted by CW and AGG. All authors
contributed to analysis of samples, and commented on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank Paul Tyler, as well as
the officers and crew of RRS James Cook, and the on-board scientific party
on cruise JC 55. We would also like to thank Elisa Neame for assistance with
extracting macrofauna.
Financial support
This research has been supported by Antarctic Science Ltd. and the Natural Environment Research Council (grant no. NE/J013307/1).
Review statement
This paper was edited by Tina Treude and reviewed by three anonymous referees.
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