Bacterial utilization of humic substances from the Arctic Ocean

The main water exchange between the Arctic and the Atlantic oceans takes pl ace at the Fram Strait. Humic substances (HS), derived from the whole Arcti c, are included within the water entering the Atlantic. Batch experiments w ere set up to investigate the bioavailability of the Arctic HS to bacteria and to characterize any controlling factors. HS were extracted with XAD res ins 2 and 4, and served as the sole carbon source for bacteria. Leucine inc orporation rate, oxygen consumption, bacterial cell counts and dissolved or ganic carbon (DOG) concentrations were monitored for the experiments. Overa ll, 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 temperat ure dependence was observed (Q(10) = 2.9 to 4.8). Comparing the bioavailabi lity of HS from surface water with that of sea water-DOG revealed no signif icant differences, with both showing the same quality as a bacterial carbon source. Additionally, the bioavailability of HS from surface water was com pared with that of HS from deep water. This comparison revealed that surfac e 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.