High bicarbonate assimilation in the dark by Arctic bacteria

10 páginas, 4 figuras, 1 tabla.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 μg C 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.This work is a contribution to the International Polar Year – Circumpolar Flaw Lead system study (IPY-CFL 2007/2008) lead by D. Barber (University of Manitoba) supported through grants from the Canadian IPY Federal Program Office, the National Sciences and Engineering Research Council, grant BOREAL (CLG2007-28872-E/ANT) from the Spanish Ministry of Science and Innovation to C.P.-A., and grants from the Swedish Research Council to S.B and L.A.S. L.A.S. was supported by a Marie Curie Intraeuropean Fellowship (CHEMOARC PIEF-GA-2008- 221121), E.O.C by the Spanish grant CGL2009-13318- BOS, and P. E. G by a Marie Curie grant (CRENARC MEIF-CT-2007-040247).Peer reviewe