Role of mesoscale cyclonic eddies in the distribution and activity of Archaea and Bacteria in the South China Sea

We examined the effect of 2 cold-core cyclonic eddies on the activity of the prokaryotic plankton community in the South China Sea. The abundance of bulk prokaryotes and major prokaryotic groups (Bacteria, marine Crenarchaeota Group I and marine Euryarchaeota Group II) and the number of cells taking up D- vs. L-aspartic acid (Asp) were determined using microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). At all sites, the bulk D-Asp:L-Asp uptake ratio by the prokaryotic community increased with depth. Concurrently, the contribution of marine Crenarchaeota Group I to total prokaryotic abundance and total active cells also increased with depth, while an opposite pattern was observed for Bacteria. Marine Euryarchaeota Group II were generally more dominant in near-surface than in deep waters. Significant differences were observed between sites inside and outside the cyclonic eddies in terms of distribution and activity of prokaryotic communities and the concentration of fluorescent dissolved organic matter (FDOM, an important refractory fraction of DOM). Generally, higher bulk D-Asp:L-Asp uptake ratios by the prokaryotic community and a greater crenarchaeotal. contribution were found in the upper mesopelagic water column inside the cold-core eddies as compared to the outside sites. Taken together, the MICRO-CARD-FISH data and the pattern of FDOM indicate that the higher contribution of refractory DOM induced by upwelled water in the cyclonic eddy may have led to a more prominent role of Crenarchaeota in the organic carbon cycling in the mesopelagic realm of the cold-core eddy than outside the eddy in the South China Sea.