Gradients in Microbial Methanol Uptake in Atlantic Waters

Methanol biogeochemistry is an emerging area of research with importance to understanding marine microbial carbon cycling. Methylotrophic bacteria have been known to exist in oceans for a number of years, but it is only recently that extensive research on them has been carried out. Researchers have begun to establish where methylotrophs are located, often in coastal waters, and understand the significant roles they, and bacteria of similar metabolism, play in carbon cycling, such as the heterotroph SAR11 Alphaproteobacteria’s oxidation of one-carbon compounds representing a source of CO₂ in the upper ocean. This and similar research shows how methanol turnover may be common and be integral in the marine environment.

The researchers (Dixon et al, 2013) in this study investigated the microbial demand for methanol along a gradient of metabolic productivity from highly productive coastal waters to comparatively arid oligotrophic gyres. They collected seawater samples along a transect in the Atlantic ocean during research cruises and started experiments within an hour of sampling.

Microbial methanol uptake was investigated by incubating the seawater samples with ¹⁴C labelled methanol for 5-10 hours at the temperature of their origin. Uptake of this labelled methanol into particulate cell biomass was assessed. Bacterial production rates were determined by measuring the incorporation of ³H-leucine into protein synthesis. ¹⁴C-bicarbonate incorporation was used as a measure of primary production and fluorometric analysis of acetone-extracted pigments was used to determine chlorophyll a concentrations. Flow cytometry on SYBR Green I DNA-stained bacterioplankton cells and unstained Prochlorococcus sp. and Synechococcus sp. cells was used to established community composition and plankton community composition was determined by inverted settlement microscopy. Inorganic phosphate and nitrite concentrations of the samples were also determined. Finally, DNA was extracted from the samples, amplified and analysed using PCR and sequences compared to databases to identify bacteria.

The researchers found that microbial methanol uptake varies between 0.1–24.8 nmol l^-1 d^-1 in coastal upwelling waters and decreased at depths below 20m, though overall methanol uptake was up to 10 times lower in samples from the gyres. Most of the assimilation of methanol in both upwelling waters and gyres was attributed to microbes of 0.2-2.0 mm in size. Leucine uptake rates varied between 190-2279 pmol Leu l^-1 d^-1 in the upwellings, and uptake was 3-7 lower in the gyres. Primary production rates of the upwelling waters decreased at depths below 20m and 99% of primary production in the top 20m of the waters was associated with larger cells such as flagellates and diatoms, the primary production rates of the gyre samples were again lower but were also associated with larger cells. Chlorophyll a concentrations ranged from 6.5 mg m^-3 in recently upwelled waters to 0.5mg m^-3 10 days later and seemed to correlate with methanol uptake. Prochlorococcus sp. were absent from the upwelling waters but Synechococcus was present as were pico-plankton and nano-phytoplankton. All microbes with the exception of pico-plankton (numbers of which were higher in the gyres than the upwellings) were less abundant in the gyres. Inorganic nitrate and phosphate concentrations in the upwelling samples initially ranged between 6.1–7.7 mM and 0.43–0.58 mM but lowered slightly after and were low in the gyre samples.

The results of this study were therefore mostly consistent with expectations, with higher nutrient uptake, primary production and assimilation rates noted in more biochemically active waters than in waters that were less so. Coastal waters showed the highest methanol carbon assimilation rates when compared to northern temperate and equatorial upwelling waters and and in contrast with oligotrophic gyres. Their results also suggested that all surface waters of upwellings may contain a population of methylotrophic microbes, or microbes that use methanol-derived carbon for growth. Preliminary characterization of bacteria in the equatorial waters revealed a variety of methylotrophs supporting this finding. Around 50-60% of the total methanol was assimilated into carbon biomass in upwelling waters, which was in contrast to 97% of methanol being used oligotrophic microbes in gyres as an energy source.

Lastly, the researchers suggested that correlations between methanol uptake and chlorophyll a concentrations in upwelling and coastal waters could be used by climate scientists to infer methanol biological loss rates by imaging chlorophyll a remotely. Leucine uptake was not as stongly correlate and therefore couldn’t offer a similar use. The researchers recognized that further work would be needed to refine a technique that could use chlorophyll a concentrations to effectively estimate methanol loss rates and that further research was needed to narrow down the microbial species which were utilizing methanol.

Reference

Dixon, J.L., Sargeant, S., Nightingale, P.D. and Murrell, J.C. (2013) Gradients in microbial methanol uptake: productive coastal upwelling waters to oligotrophic gyres in the Atlantic Ocean. J ISME 7: 568-580.