Fine scale variability in methanol uptake and oxidation in the micro-layer and near-surface waters of the Atlantic

The aim of this research was to make the first depth profiles of the microbial assimilation of methanol carbon and its oxidation to carbon dioxide and use as an energy source from the microlayer to 1000 m. Some of the highest reported methanol oxidation rate constants of 0.5–0.6 d−1 were occasionally found in the microlayer and immediately underlying waters (10 cm depth), albeit these samples also showed the greatest heterogeneity compared to other depths down to 1000 m. Methanol uptake into the particulate phase was exceptionally low in microlayer samples, suggesting that any methanol utilised by microbes in this environment is for energy generation. The sea surface microlayer and 10 cm depth also showed a higher proportion of bacteria with a low DNA content, and bacterial leucine uptake rates in surface microlayer samples were either less than or the same as those in the underlying 10 cm layer. The average methanol oxidation and particulate rates were however statistically the same throughout the depths sampled, although the latter were highly variable in the near-surface 0.25–2 m compared to deeper depths. The statistically significant relationship demonstrated between uptake of methanol into particles and bacterial leucine incorporation suggests that many heterotrophic bacteria could be using methanol carbon for cellular growth. On average, methanol bacterial growth efficiency (BGEm) in the top 25 m of the water column is 6% and decreases with depth. Although, for microlayer and 10 cm-depth samples, BGEm is less than the near-surface 25–217 cm, possibly reflecting increased environmental UV stress resulting in increased maintenance costs, i.e. energy required for survival. We conclude that microbial methanol uptake rates, i.e. loss from seawater, are highly variable, particularly close to the seawater surface, which could significantly impact upon seawater concentrations and hence the air–sea flux.


Introduction
Oxygen containing volatile organic species (OVOCs) such as low molecular weight alcohols, aldehydes and ketones substantially influence the oxidising capacity and ozone forming potential of the atmosphere (Singh et al., 2001, and references therein).Methanol is the predominant OVOC in the troposphere (Singh et al., 2000(Singh et al., , 2001)), and the second most abundant organic gas after methane (Jacob et al., 2005) with atmospheric ubiquity (Singh et al., 2000).However, there is still considerable uncertainty over sources of all OVOCs, and critically, we continue to be unsure of the importance of the oceans in controlling the abundance of these compounds in marine air (Singh et al., 2001(Singh et al., , 2003;;Williams et al., 2004;Lewis et al., 2005;Beale et al., 2010).The surface ocean is thought to play an important role in the budget of methanol, and atmospheric observations in combination with air-sea exchange models generally predict an air to sea flux, of varying magnitude (e.g., Singh et al., 2001Singh et al., , 2003;;Sinha et al., 2007;Carpenter et al., 2004;Williams et al., 2004;Dixon et al., 2011a).These studies have generally used a surface methanol concentration of the order 100 nM, but recent seawater analysis, using a membrane inlet-proton transfer reaction mass spectrometry, has shown concentrations up to 429 nM (Beale et al., 2011).Recent flux calculations in the oligotrophic North Atlantic suggest that the atmosphere is unlikely to be a dominant source of methanol, and invoke in situ production mechanisms (Dixon et al., 2011b).The measured seawater concentration of methanol is a product of its sources, e.g., production mechanisms and rates and sinks.Methylotrophic organisms by definition must use one carbon compounds more reduced than CO 2 , like methanol, as sole energy sources, and for assimilation of carbon into cell biomass (Murrell and McDonald, 2000;Chistoserdova, 2011).Biological methanol turnover times as low as 1 day have been found in surface waters of the subtropical North East Atlantic (Dixon et al., 2011a).Molecular approaches have identified methylotrophs like Hyphomicrobium and Methylophaga spp in remote marine and coastal locations (Dixon et al., 2011b;Neufield et al., 2007Neufield et al., , 2008)).Other bacteria, such as undescribed gammaproteobacteria Figures

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Full ( Neufield et al., 2007Neufield et al., , 2008) ) have also been found to uptake methanol carbon into their cells.Recently, Sun et al. (2011) also revealed that the most abundant heterotroph in the oceans, SAR11 alphaproteobacteria, who play a major role in mineralising marine dissolved organic matter, use methanol to produce energy.
Previous results have focussed only on bulk surface waters (5-10 m), and have reported microbial methanol oxidation rate between 2-8 nM d −1 in more northern temperate waters (Dixon et al., 2011b), and between 2-146 nM d −1 in oligotrophic tropical waters of the North Atlantic (Dixon et al., 2011a), giving turnover times of between 7-33 and 1-25 days, respectively.The aim of this research was to make the first depth profiles for microbial methanol use as an energy source and a growth substrate.We further profiled the fine scale variability in these rates in the top 2 m of the water column and the sea surface microlayer, to assess whether bulk surface, i.e., 5-10 m measurements, typically used in air-sea flux calculations, were truly representative of the near-surface (< 2 m).Temporal and surface spatial changes in microbial methanol use are also compared.

Methods
Sampling was conducted at a series of stations west of the Iberian Peninsula in the NE Atlantic aboard RRS Discovery on the SOLAS UK (Surface Ocean Lower Atmosphere Study) DOGEE II (Deep Ocean Gas Exchange Experiment II) D320 research cruise (Fig. 1).

Sampling devices
Ten with the top bottle at 25 cm below the sea surface, and the bottom one at 220 cm (as described in Tilstone et al., 2010).An additional microlayer sampling device that was mounted on a Catamaran (C) was also fitted with a subsurface pole that enabled sampling from 10 cm, 1 and 2 m below the sea surface (as described in Brooks et al., 2009).The sea surface microlayer was also sampled by hand from a rigid inflatable boat using a Garrat Screen (GS, Garret 1965).Briefly, this screen consisted of a 50 cm 2 frame enclosing a stainless steel mesh and is typically considered to sample the top 300-400 µm (Cunliffe et al., 2009;Zuev et al., 2001).Additional 10 cm samples were also taken by hand from the inflatable boat using a 10 ml long handled pipette.Microlayer samples were also obtained using the Catamaran, as it was fitted with a rotating drum device mounted between the hulls.The rotating glass drum picks up the sea surface by tension, and is thought to typically sample 20-100 µm (Zuev et al., 2001).

SF 6 tracer deployment
A diel experiment was undertaken using surface waters that had been labelled with the inert tracer SF 6 (as used in an air-sea gas transfer experiment detailed in Salter et al., 2011).We used this tracer to check that we were consistently sampling the same body of water during the diel experiment.

Methanol oxidation and uptake into particulate phase
We incubated surface seawater samples with nanomolar concentrations of 14 C labelled methanol ( 14 CH 3 OH) and determined both uptake into particles and oxidation to CO 2 (Dixon et al., 2011a,b), which we interpret as microbial cellular incorporation (cell growth, G) and energy use (E ), respectively.For full experimental details please refer to Dixon et al. (2011b).In brief, seawater samples were treated with approximately 10 nM final added concentration of 14 CH 3 OH, and were always < 1 % of the sample volume.We generally incubated oxidation samples for 1-3 h, and uptake onto particles The data is reported as apparent rate constants, k (h −1 ) calculated from a ratio of the sample 14 C counts (from the particles on the filter or from the precipitate which contains the captured 14 CO 2 as Sr 14 CO 3 , nCi ml −1 h −1 ) divided by total 14 CH 3 OH added to the sample (nCi ml −1 ).All rate constants were corrected by subtracting killed sample (TCA, 5 % final concentration) counts.Evaluation of control samples suggests that ≤ 2 % of the added 14 CH 3 OH is counted in the resultant precipitate (for determining oxidation) and ≤ 0.3 % is recovered on the filters.

Bacterial production
Heterotrophic bacterial production rates were determined by measuring the incorporation of 3 H-leucine (20 nM final concentration) into bacterial protein synthesis on 1.7 ml seawater samples following the method of Smith and Azam (1992).

Cellular abundance
The numbers of bacterioplankton cells were determined by flow cytometry on SYBR Green I DNA-stained cells from 1.8 ml seawater samples fixed in paraformaldehyde (5 %, final concentration).Numbers of Prochlorococcus spp and Synechococcus spp were analysed on unstained samples by flow cytometry.Surface concentrations of chlorophyll-a were estimated using a composite Aqua-MODIS chlorophyll-a remotely sensed image for stations 4 and 5, as unfortunately the continuous surface fluorometer onboard the ship was not working correctly.

The sea surface microlayer and 10 cm
Methanol oxidation rate constants in the sea surface microlayer were highly variable and ranged between 0.002-0.59d −1 (Fig. 2a).Methanol uptake into the particulate Figures

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Full phase in the microlayer showed the lowest ever rate constants reported ranging between 0.1−2 × 10 −3 d −1 (Fig. 2b), with highest values coincident with the highest methanol oxidation rates.For the majority of sampling occasions there was little difference between methanol rate constants (oxidation and particulate uptake) in the sea surface micro-layer and 10 cm depth.The greatest difference in oxidation and uptake rates between the micro-layer and 10 cm samples was observed at times of maximum rates, although there was no consistent trend.However, the two highest particulate methanol rate constants were found in the 10 cm depth samples (not the micro-layer).For micro-layer samples which were obtained with either the Garratt Screen and/or the Catamaran Skimmer, no consistent differences could be identified for any parameter/rate determined (Fig. 2).Numbers of Prochlorococcus cells ranged between 1.9−10.6× 10 4 and 2.2−7.0 × 10 4 cells ml −1 in micro-layer and 10 cm depth samples, respectively (Fig. 2c).The highest biomass of Prochlorococcus cells was shown in a micro-layer sample.Numbers of Synechococcus cells ranged between 0.6−5.5 × 10 4 and 0.7−5.5 × 10 4 cells ml −1 in micro-layer and 10 cm depth samples, respectively (Fig. 2d), whilst pico-eukaryotic cells ranged between 2.2−6.6 × 10 3 cells ml −1 (Fig. 2e).In general, cell numbers showed no convincing differences or trends between micro-layer and 10 cm samples.Bacterial numbers ranged between 3.6−13.8× 10 5 cells ml −1 and 4.7−12.1 × 10 5 cells ml −1 in micro-layer and 10 cm depth samples, respectively (Fig. 3a,b), and were highest in both during sampling times in June, compared to July.However, there was no overall trend in bacterial numbers between the micro-layer and 10 cm depth samples.There was however a greater number of bacteria (52-64 %) containing a relatively low DNA content compared to a high DNA content (as seen on the Flow Cytometer, Fig. 3a) for all surface micro-layer samples.Bacterial numbers from 10 cm depth showed more variability in the proportion of low DNA containing bacteria (44-71 %) (Fig. 3b).Correlations showed statistically significant relationships between both methanol oxidation and uptake into particles with the numbers of low DNA containing heterotrophic bacteria in the micro-layer samples (n = 11, r = 0.6189, p ≤ 0.05 and Figures

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Full r = 0.7166, p ≤ 0.02, respectively).But for samples from 10 cm, the only statistically significant correlations were found between methanol oxidation and uptake into particles with the numbers of high DNA containing heterotrophic bacteria (n = 9, r = 0.6681, p ≤ 0.05 and r = 0.7518, p ≤ 0.02, respectively).

Depth variability in the top 2 m
High depth resolution sampling in the top 2 m of the water column often revealed significant differences in both methanol oxidation and uptake into particles between samples that were as little as 25 cm depth apart (Fig. 4), illustrating the heterogenic nature of microbial methanol use.Hot spots, or peaks in activity were often seen at depths just below the surface ≤ 50 cm, e.g., 10 July 2007 (Fig. 4d).The average proportion of methanol used for energy compared to growth (E : G) n the top 2 m was 22, in agreement with more traditionally sampled surface (∼ 5 m) values.However, lower E : G values between 3-10 were sometimes observed between depths of 45-100 cm, e.g., 8 July 2007 (N profile, Fig. 4c) indicating that on occasion, relatively more methanol was used for growth.This ratio was more similar to that observed in coastal and shelf sea surface waters (Dixon et al., 2011b).Bacterial leucine incorporation was also compared in surface micro-layer (1-32 pmol l −1 h −1 ), 10 cm (3-34 pmol l −1 h −1 ), 1 m (4-39 pmol l −1 h −1 ), and 2 m (5-20 pmol l −1 h −1 ) depths (Fig. 5).Surface micro-layer samples almost consistently showed values lower than samples 10-100 cm; except for 4-5 July 2007, which showed no statistically significant differences in the top 2 m.

Vertical profiles from 5-1000 m
Depth profiles of methanol oxidation and uptake into cells showed considerable changes with depth, sometimes showing sub surface maxima between 20-30 m (Fig. 6).However, some of the biggest changes were shown between sampling days.For example, methanol oxidation rates generally varied between 0.007-0.09d (Fig. 6a, average 0.02, n = 22) between 5-1000 m, but on 27 June 2007 increased by over an order of magnitude to between 0.25-0.36d −1 (average 0.30, n = 3), with a surface maximum.Likewise incorporation of methanol into cells for growth generally varied between < 0.001-0.0016d −1 (Fig. 6c, average 0.001, n = 28) but were more than one order of magnitude greater on 22 June 2007 with an average of 0.013 d −1 (n = 3).We do not have methanol oxidation rates on this day for which to compare.These sporadic increases in rates are currently not understood.The average E : G over all depths was 106 (n = 19, range 14-305), which decreased to an average of 85 (n = 9, range 14-229) for samples from the top mixed layer which varied between 30-53 m, which is within the range previously reported (Dixon et al., 2011a,b).
For deep samples collected between 500-1000 m on 1 July 2007, methanol oxidation and uptake into particle rate constants were low and varied between 0.008±0.0002and 0.0001 ± 0.0001 d −1 (n = 3), respectively, with an average energy to growth ratio of 70.

Diel variability
The diel experiment was carried out in a patch of water that had been labelled with the inert tracer SF 6 .Surface concentrations of SF 6 started to consistently decrease from midnight to around 04:00 (Fig. 3a).Winds strengthened from 20 to 30 knots during this period and we were unable to navigate back towards the assumed patch centre.Methanol oxidation rate constants varied between 2.8−9.7 × 10 −4 h −1 , showing maximum rates just after dawn (07:00 h), and minimum rates in the middle of the night (03:00 h), although the latter could have been a result of spatial sampling variability, due to station drift.However, there was no consistent trend evident for methanol oxidation rates over the diel cycle, unlike microbial uptake of methanol into the particulate phase, which showed a decreasing trend from 07:00 to 18:00 during daylight hours (Fig. 3d).Methanol cellular incorporation rates were approximately an order or magnitude lower than those for microbial methanol energy use, and varied between 1.3−8.4× 10 −5 h −1 .
Maximum rates were shown at 10:00 h, and minimum rates just before dawn at 05:00 h, both 2-3 h after the respective methanol oxidation rates.The ratio of methanol used for Introduction

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Full microbial energy compared to growth ranged ranged between 5-55 over the diel cycle.Two maximums in this ratio were observed almost 12 h apart at 18:00 and 05:00 h.Numbers of heterotrophic bacteria, picoeukaryotes, Prochlorococcus and Synechococcus cells ranged between 3.8−6.6× 10 5 , 1.1−3.5 × 10 3 , 4.1−8.6 × 10 4 and 2.1−7.3 × 10 3 cells ml −1 , respectively.In general, cell numbers decreased during the day from 07:00 to a minimum at 17:00 h, and subsequently increased during the night (Fig. 3b,c).Bacterial leucine incorporation rates ranged between 3.6−14 pmol leu l −1 h −1 and showed no obvious diel trends, although on average rates were slightly higher at night (10.3 ± 1.6 pmol leu l −1 h −1 ) compared to the day (8.4 ± 2.6 pmol leu l −1 h −1 ).Because we suspect from the SF 6 data (Fig. 3a) that we drifted away from the assumed patch centre during the night, only times sampled during daylight hours (07:00-20:00 h, n = 14) were included in subsequent statistical analysis.The only statistically significant relationship observed was between the uptake of methanol into cellular biomass and the numbers of Prochlorococcus cells (n = 14, r = 0.7289, P ≤ 0.01).

Surface variability in methanol oxidation
Apparent rate constants for methanol oxidation in surface (5-10 m) waters of the NE Atlantic ranged from 0.006-0.39d −1 (average of 0.09 d −1 , n = 17, Fig. 8).The continuous surface fluorometer onboard ship was not working during the cruise, so we do not have in situ chlorophyll-a concentrations.However, surface estimates of < 0.2 and between 0.3-0.6 µ g l −1 for the SF 6 patch area and higher chlorophyll-a sampling regions, respectively, were obtained from a composite Aqua-MODIS image over the duration of the cruise from 16 June to 16 July 2007 (Fig. 1).The average methanol oxidation rate constant in the lower chlorophyll-a sampling region was 0.065 d −1 (n = 13), which more than doubled to 0.17 d −1 (n = 4) in the higher chlorophyll area.Bacterial leucine uptake ranged over an order of magnitude between 3.8-38.5pmol l −1 h −1 (average of 11.6 pmol l −1 h −1 , n = 16, Fig. 8).Interestingly, higher bacterial leucine activities of 11.6 pmol l −1 h −1 (n = 12) were found in the lower chl surface waters cf.Figures

Discussion
The average surface turnover times for methanol in the low chlorophyll a waters was 15 days (n = 13), which compares to 6 days in the higher chlorophyll-a region.These values are within the ranges previously reported (Dixon et al., 2011a,b), but are higher than those sometimes found in ultra-low chlorophyll-a regions (0.003-0.006 µg l −1 , 1-5 days) of the tropical North Atlantic (Dixon et al., 2011a).The majority of methanol (92-99.8%) in surface waters during the spatial survey was used as an energy source, rather than a carbon source for cellular growth, which is typical of more oligotrophic north Atlantic waters (Dixon et al., 2011a).Methanol oxidation rates constants from the diel experiment in this study are on average two orders of magnitude lower compared to those in the subtropical NE Atlantic (Dixon et al., 2011a), which may help explain why a temporal trend was not observed.In contrast, uptake into the particulate phase was comparable, particularly between this study and station 1 in the oligotrophic NE Atlantic (see Table 1 and Dixon et al., 2011a).However for particulate diel of this study, rates decreased during the day in a similar fashion to oxidation rates in Dixon et al. (2011a), which at present we do not fully understand.
The statistically significant relationship observed between the uptake of methanol into cellular biomass and the numbers of Prochlorococcus during the diel experiment could suggest that, this abundant group of marine cyanobacteria, are capable of mixotrophy using methanol as a carbon source for growth.Zubkov and coworkers demonstrated that Prochlorococcus use organic nitrogen compounds, e.g., amino acids (Mary et al., 2008;Zubkov and Tarran, 2005;Zubkov et al., 2003Zubkov et al., , 2004)) whilst sulphur uptake from dimethylsulfoniopropionate has also been demonstrated (Vila-Costa et al., 2006).The uptake of glucose by Prochlorococcus has also more recently been suggested by G ómez-Baena et al. (2008) as the first example of an organic molecule containing only carbon skeletons, i.e., no N, P or Fe being assimilated presumably for cellular structures and/or to obtain reducing power.However, the lack of a statistically significant relationship between methanol oxidation rates (rather than uptake) and the numbers of Prochlorococcus suggests that methanol is being utilised for microbial growth and cellular incorporation, rather than for energy generation.To test the robustness of this relationship further we included the results from three other diel experiments (see Table 1 for data summary) that we have performed in more tropical regions of the north east Atlantic (Fig. 8a, the methanol data has previously been published in Dixon et al. (2011a) but not in combination with the supporting microbial composition data).When the diel data from the present study is combined with station 2, from the subtropical northeast Atlantic, then a statistically significant linear relationship is found between methanol used as a carbon source for cellular growth during light and dark hours, i.e., for particulate methanol and the numbers of Prochlorococcus cyanobacteria y = 0.006x − 0.003, r = 0.776, n = 45 for P = 0.001 (Fig. 8a).However station 1, which is more oligotrophic in nature than station 2 and this study (see Table 1), has a higher particulate methanol uptake rate than expected from this relationship.This could imply that cells other than Prochlorococcus, e.g., methylotrophs are dominating the uptake of methanol at this station, or perhaps microbial resources are more scarce, leading to enhanced reliance and uptake of methanol.Station 3 is located within the Cape Verde archipelago and is the most productive of these stations (see chlorophyll-a concentrations and bacterial production in Table 1).It is thus conceivable that methanol uptake in the particulate phase is dominated by very different microbes in this environment, where numbers of Prochlorococcus cells were very low, and pico-eukaryotic cells were more evident at 6−15 × 10 2 cells ml −1 .The observed relationship between methanol uptake into particles and the numbers of Synechococcus is shown in Fig. 8b which shows a broad, but shallow gradient positive trend Introduction

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Full (y = 0.0007x + 0.0005, r = 0.740, n = 87 for P = 0.001).There is significant scatter in the data evident, particularly for station 3, which shows a positive deviation from the trend during daylight hours.Perhaps the most convincing relationship, in terms of r value, is observed between methanol uptake and bacterial production for all the diel data (Fig. 8c) where y = 6 × 10 −6 x where r = 0.8789, n = 87 for P = 0.001.No statistically significant linear relationships were found between methanol oxidation and bacterial production or cellular numbers.Some very high methanol oxidation rates constants of 0.5-0.6 d −1 were sporadically found in the micro-layer, and immediately underlying waters (10 cm).These values compare favourably to a diel range of 0.3-0.8d −1 reported for surface waters of a very oligotrophic station (chlorophyll-a concentration of 0.06 µg l −1 ) in the tropical North Atlantic (station 1, Dixon et al., 2011b).Methanol oxidation rates in the sea surface microlayer exhibited the greatest heterogeneity of all samples, showing a c. 300 fold difference between minimum and maximum values.This compares with a 3 fold difference shown in surface waters over the diel cycle, from a station with similar background biogeochemical characteristics (Fig. 1, SF 6 patch area), and a 65 fold difference between surface seawater of variable chlorophyll-a concentrations (Fig. 8).Particulate methanol uptake into particles in the micro-layer was generally low at ≤ 0.001 d −1 , but also exhibited more heterogeneity compared to bulk surface (∼ 5 m) seawater over diel (Fig. 7) or spatial trends (Dixon et al., 2011b).This average methanol uptake into particles is lower than previously reported for surface waters (∼ 0.001−0.012d −1 , Dixon et al., 2011b), although within the range reported in the diel experiment of the present study (Fig. 7).These results indicate that microbial use of methanol for incorporation into cellular structures in the micro-layer was generally very low, but on occasion comparable to the lowest estimated reports for surface waters (where chlorophyll-a concentrations were < 0.2 µg l −1 ).Numbers of bacteria, Prochlorococcus and pico-eukaryote cells in the micro-layer and underlying layer (10 cm) were within ranges of our surface diel (Fig. 7) and spatial survey (Fig. 8).Whilst the range in Synechococcus cell numbers in the micro-layer and Introduction

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Full 10 cm layer agreed with surface waters samples over a large spatial scale (Fig. 8), they were on average higher than those reported during our diel experiment (Fig. 7).Higher average microbial rates of methanol oxidation and uptake into particles were determined in surface waters sampled over a wide spatial range (chl-a ranging from < 0.2−0.6 µg l −1 ) compared to diel sampling at one location (chl-a < 0.2 µg l −1 , see Table 2) where for methanol oxidation t ox = 3.03 (t 38 = 2.68 for P = 0.01) and particulate t part = 5.72 (t 33 = 2.75 for P = 0.01).Additionally, greater variability is evident over wider spatial sampling scales compared to diel sampling at one location, particularly for methanol oxidation (cv = 133 % and 29 %, respectively, Table 1) (F ox = 915 where F 16,22 = 2.978 and F part = 25 where F 23,10 = 3.168 and P = 0.01 for spatial compared to diel samples).
The average methanol oxidation and particulate rates determined in the microlayer-10 cm compared to the 0.25-2 m depth were statistically the same (t ox = 1.34, t part = 1.14 and t 46 = 2.75 and P = 0.01), due to the large sample variance.However for methanol oxidation a two-sided F -test (F = 6.55,F 21,27 = 2.78, P = 0.01) suggests that there is more variability in the microlayer/10 cm compared to 0.25-2 m.In contrast, more variability in particulate methanol uptake rates was shown in the 0.25-2 m compared to the microlayer-10 cm sampling depths (F = 256, F 20,26 = 2.78 for P = 0.01).
The average methanol oxidation and particulate rates determined in the 0.25-2 m, 5-25 m and > 25-100 m clearly show no differences, e.g., t ox = 0.638, t part = 0.580 and t 35 = 2.75 and P = 0.01 when comparing 0.25-2 m with 5-25 m rates.Whilst, the only statistically significant difference in variability was shown for the uptake of methanol into the particulate phase between the 0.25-2 m and 5-25 m depths (F = 16, F 26,9 = 3.18 for P = 0.01).Methanol oxidation and uptake rate constants were notably low for deep ocean samples between 500-1000 m compared to depths ≤ 100 m.

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Full

Conclusions
Average surface methanol oxidation rates agree with those previously published, but were relatively higher in more northerly chlorophyll rich regions, compared to low chlorophyll areas of this study.Some relatively high methanol oxidation rates constants of 0.5-0.6 d −1 were sporadically found in the micro-layer and immediately underlying waters (10 cm), although rates were highly variable.Methanol uptake into the particulate phase was exceptionally low in micro-layer samples and at depths between 500-1000 m, suggesting that any methanol utilised by microbes in this environment is for energy generation.The micro-layer and 10 cm depths showed a higher proportion of bacteria with a relatively low DNA content, and bacterial leucine uptake rates in surface micro-layer samples were either less than (for 6 out of 8 occasions), or the same as those in the underlying 10 cm layer.Statistically the average methanol oxidation and particulate rates were the same throughout the water column, despite often demonstrating sub surface peaks, due largely to the data variance.However, particulate methanol uptake showed highest variability in the 0.25-2 m top layer compared to all other depths sampled.The statistically significant relationship between the uptake of methanol into particles and bacterial leucine incorporation suggests that heterotrophic bacteria use methanol as a carbon source for cellular growth, but the lack of relationship observed with methanol oxidation, perhaps suggests that a wider group of marine microbes use methanol for energy generation.We conclude that microbial methanol use, i.e., loss of methanol from seawater is highly variable, particularly close to the seawater surface, which could significantly impact upon seawater concentrations and hence the air-sea flux.Further investigations should focus on identifying what controls this variability, and specifically understanding which marine organisms use methanol for energy and/or for carbon assimilation, we feel could be key to understanding methanol loss patterns, and even the magnitude and direction of the methanol flux across the sea surface.

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Picoeukaryotes
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | litre, Teflon coated steel sprung Niskin bottles were used to routinely sample waters ≥ 5 m during spatial and depth surveys.High resolution sampling in the top 2 m was achieved through the use of a Near Surface Sampling Device (N), which consists of eight 4 l sampling bottles mounted horizontally and spaced at 20-30 cm intervals, Discussion Paper | Discussion Paper | Discussion Paper |

for 3 -
9 h (to ensure sufficient counts).Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Chistoserdova, L.: Modularity of methylotrophy, Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Zubkov, M. V., and Tarran, G. A.: Amino acid uptake of Prochlorococcus spp. in surface waters across the South Atlantic Subtropical Front, Aquat.Microb.
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Table 2 .
Summary of microbial methanol uptake and oxidation.