| Climate change is increasing Arctic air and ocean temperatures causing mobilization of terrestrial organic material (tDOM) from permafrost (IPCC, 2007; Peterson et al. 2002). Carbon contained in tDOM is predicted to be released at 10 times the current rate by 2100, ultimately causing higher coastal tDOM concentrations. Marine bacteria are the main organisms which biodegrade DOM in oceans, but uptake of carbon from this material is likely to be stoichiometrically unbalanced with nitrogen uptake due to the high carbon to nitrogen ratio (20--60) of Arctic tDOM and the apparent recalcitrance of nitrogen associated with Arctic tDOM (Tank et al 2012). Since Arctic Ocean bacteria are carbon limited, tDOM flux from melting permafrost could increase bacterial activity by providing carbon substrates resulting in higher demand for exogenous nitrogen. Nitrate is the most likely nitrogen substituent to satisfy new bacterial demand because of high concentrations in the coastal Arctic. However, increased bacterial nitrate uptake could come at the expense of primary production because nitrate is the limiting nutrient during summer phytoplankton blooms.;Another consequence of Arctic warming is the projected decrease in sea-ice extent and thickness on the Arctic Ocean (Belanger et al. 2006). Decreased sea-ice will lead to increased ultra violet (UV) radiation from the sun, which photodegrades DOM and can alter its lability and stoichiometry. The fate and ecological effect of tDOM once it enters the ocean will likely be controlled by a mixture of biological and photochemical processes.;The primary focus of this study was to investigate the hypothesis that tDOM amendments to bioassays containing coastal Arctic bacteria would increase bacterial nitrate demand. Secondarily, this study tested if tDOM photodegradation could cause changes in tDOM lability, % bioavailability, and bacterial nitrate demand.;Phytoplankton and grazer-free Arctic seawater bioassays were conducted in the dark, and were amended with tDOM, tDOM and arginine, photodegraded tDOM, glucose, or were unamended to test these hypotheses. In bioassays amended with tDOM and glucose, partial or complete nitrate assimilation occurred that was coincident with reductions in DOC. Nitrate assimilation did not occur in assays amended with tDOM and arginine or in unamended assays. When photodegraded tDOM was added to bioassays, overall DOC and nitrate utilization decreased compared to assays amended with non-photodegraded tDOM, indicating a decrease in % bioavailability. However, tDOM irradiation increased bacterial growth rates and DOM reactivity.;These results provide evidence that increased tDOM flux to the Arctic Ocean will increase microbial competition for nitrogen. However, the effects of melting permafrost on the Arctic Ocean will be mediated by not only bacterial utilization, but also by photodegradation. Proposed future studies will utilize more complex microbial assemblages utilizing mesocosm-size assays to investigate nitrogen competition between heterotrophs and autotrophs caused by tDOM loading. |
