Bacterial utilization of humic substances from the Arctic Ocean

The main water exchange between the Arctic and the Atlantic oceans takes place at the Fram Strait. Humic substances (HS), derived from the whole Arctic, are included within the water entering the Atlantic. Batch experiments were set up to investigate the bioavailability of the Arctic HS to bacteria and to characterize any controlling factors. HS were extracted with XAD resins 2 and 4, and served as the sole carbon source for bacteria. Leucine incorporation rate, oxygen consumption, bacterial cell counts and dissolved organic carbon (DOG) concentrations were monitored for the experiments. Overall, 11% of the initial carbon was consumed and a turnover time of 220 d was extrapolated for marine HS in polar surface waters and a profound temperature dependence was observed (Q(10) = 2.9 to 4.8). Comparing the bioavailability of HS from surface water with that of sea water-DOG revealed no significant differences, with both showing the same quality as a bacterial carbon source. Additionally, the bioavailability of HS from surface water was compared with that of HS from deep water. This comparison revealed that surface HS are of significantly better bacterial food quality than deep water HS. The experiments indicated that marine HS play an active role in bacterial carbon cycling in the Arctic Ocean.