Oceanic subtropical gyres are considered biological deserts because of the extremely low availability of nutrients and thus minimum productivities. The major source of nutrient nitrogen in these ecosystems is N2-fixation. The South Pacific Gyre (SPG) is the largest ocean gyre in the world, but measurements of N2-fixation therein, or identification of microorganisms involved, are scarce. In the 2006/2007 austral summer, we investigated nitrogen and carbon assimilation at 11 stations throughout the SPG. In the ultra-oligotrophic waters of the SPG, the chlorophyll maxima reached as deep as 200 m. Surface primary production seemed limited by nitrogen, as dissolved inorganic carbon uptake was stimulated upon additions of 15N-labeled ammonium and leucine in our incubation experiments. N2-fixation was detectable throughout the upper 200 m at most stations, with rates ranging from 0.001 to 0.19 nM N h−1. N2-fixation in the SPG may account for the production of 8–20% of global oceanic new nitrogen. Interestingly, comparable 15N2-fixation rates were measured under light and dark conditions. Meanwhile, phylogenetic analyses for the functional gene biomarker nifH and its transcripts could not detect any common photoautotrophic diazotrophs, such as, Trichodesmium, but a prevalence of γ-proteobacteria and the unicellular photoheterotrophic Group A cyanobacteria. The dominance of these likely heterotrophic diazotrophs was further verified by quantitative PCR. Hence, our combined results show that the ultra-oligotrophic SPG harbors a hitherto unknown heterotrophic diazotrophic community, clearly distinct from other oceanic gyres previously visited.

/pdf/heterotrophic-organisms-dominate-nitrogen-fixation-in-the-1hrl6dlbw5.pdf

Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre

Although both autotrophic and heterotrophic microorganisms incorporate CO2 in the dark through different metabolic pathways, this process has usually been disregarded in oxic marine environments. We studied the significance and mediators of dark bicarbonate assimilation in dilution cultures inoculated with winter Arctic seawater. At stationary phase, bicarbonate incorporation rates were high (0.5-2.5 mu gC L-1 d(-1)) and correlated with rates of bacterial heterotrophic production, suggesting that most of the incorporation was due to heterotrophs. Accordingly, very few typically chemoautotrophic bacteria were detected by 16S rRNA gene cloning. The genetic analysis of the biotin carboxylase gene accC putatively involved in archaeal CO2 fixation did not yield any archaeal sequence, but amplified a variety of bacterial carboxylases involved in fatty acids biosynthesis, anaplerotic pathways and leucine catabolism. Gammaproteobacteria dominated the seawater cultures (40-70% of cell counts), followed by Betaproteobacteria and Flavobacteria as shown by catalyzed reporter deposition fluorescence in situ hybridization (CARDFISH). Both Beta-and Gammaproteobacteria were active in leucine and bicarbonate uptake, while Flavobacteria did not take up bicarbonate, as measured by microautoradiography combined with CARDFISH. Within Gammaproteobacteria, Pseudoalteromonas-Colwellia and Oleispira were very active in bicarbonate uptake (ca. 30 and 70% of active cells, respectively), while the group Arctic96B-16 did not take up bicarbonate. Our results suggest that, potentially, the incorporation of CO2 can be relevant for the metabolism of specific Arctic heterotrophic phylotypes, promoting the maintenance of their cell activity and/or longer survival under resource depleted conditions.

/pdf/high-bicarbonate-assimilation-in-the-dark-by-arctic-bacteria-t0dfmyiu7j.pdf

High bicarbonate assimilation in the dark by Arctic bacteria

As the only fuel that is not chemically bound to carbon, hydrogen has gained interest as an energy carrier to face the current environmental issues of greenhouse gas emissions and to substitute the depleting non-renewable reserves. In the last years, there has been a significant increase in the number of publications about the bacterium Thermotoga neapolitana that is responsible for production yields of H2 that are among the highest achievements reported in the literature. Here we present an extensive overview of the most recent studies on this hyperthermophilic bacterium together with a critical discussion of the potential of fermentative production by this bacterium. The review article is organized into sections focused on biochemical, microbiological and technical issues, including the effect of substrate, reactor type, gas sparging, temperature, pH, hydraulic retention time and organic loading parameters on rate and yield of gas production.

/pdf/hydrogen-production-by-the-thermophilic-bacterium-thermotoga-33zs35f3qy.pdf

Hydrogen Production by the Thermophilic Bacterium Thermotoga neapolitana

Use of heterotrophic CO2 assimilation as a measure of metabolic activity in planktonic and sessile bacteria.

Summary Sub-seafloor sediments are populated by large numbers of microbial cells but not much is known about their metabolic activities, growth rates and carbon assimilation pathways. Here we introduce a new method enabling the sensitive detection of microbial lipid production and the distinction of autoand heterotrophic carbon assimilation. Application of this approach to anoxic sediments from a Swedish fjord allowed to compare the activity of different functional groups, the growth and turnover times of the bacterial and archaeal communities. The assay involves dual stable isotope probing (SIP) with deuterated water (D2O) and 13 CDIC (dissolved inorganic carbon). Culture experiments confirmed that the D content in newly synthesized lipids is in equilibrium with the D content in labelled water, independent of whether the culture grew hetero- or autotrophically. The ratio of 13 CDIC to D2O incorporation enables distinction between these two carbon pathways in studies of microbial cultures and in environmental communities. Furthermore, D2O-SIP is sufficiently sensitive to detect the formation of few hundred cells per day in a gram of sediment. In anoxic sediments from a Swedish fjord, we found that > 99% of newly formed lipids were attributed to predominantly heterotrophic bacteria. The production rate of bacterial lipids was highest in the top 5 cm and decreased 60-fold below this depth while the production rate of archaeal lipids was rather low throughout the top meter of seabed. The contrasting patterns in the rates of archaeal and bacterial lipid formation indicate that the factors controlling the presence of these two lipid groups must differ fundamentally.

C. H. Werkman

Papers

Heterotrophic Assimilation of Carbon Dioxide