Response of under-ice prokaryotes to experimental sea-ice DOM enrichment

Abstract

Marine dissolved organic matter (DOM) plays a key role in the global carbon cycle. In this study we show experimentally that Arctic sea-ice DOM can stimulate prokaryotic activity when added to surface waters. Time-series and dose-response enrichment microcosm experiments were conducted, in which first-year, sea-ice DOM was added to surface waters from Resolute Passage, Canadian Arctic Archipelago. Sea-ice DOM concentrations in this productive region averaged nearly 2000 μmol l-1in May 2011 and 2012. The abundance, activity (high [HNA] versus low [LNA] nucleic acid cells) and apparent size of surface water prokaryotes were quantified along with dissolved organic carbon (DOC) and dissolved nitrogen (DN) concentrations during the experiments. Following a 4 d lag, prokaryotic abundance increased more than 30× in the time-series enrichment experiment and the proportion of HNA cells increased from 60 to <99% of total prokaryote abundance. DOM dose-response experiments conducted in 2011 and 2012 yielded prokaryotic growth rate estimates between 0.35 and 0.67 d-1in response to the addition of sea-ice DOM. On average, 20% of the sea-ice DOC pool was utilized by the surface water prokaryotes and the observed increase in cell abundance and individual cell size indicated a release from carbon limitation of initial in situ conditions. Prokaryotic growth yields ranged from 0.02 to 0.07 cell μmol l-1DOC and 0.01 to 0.06 cell μmol l-1DN and experimental conditions shifted from net autotrophic to net heterotrophic. Heterotrophic activity at the ice-water boundary layer upon the release of labile first-year ice DOM is likely to impact current and future carbon flux estimates as seasonal ice becomes the predominant ice type in the Arctic.