Nitrogen‐fixing bacteria associated with copepods in coastal waters of the N orth A tlantic O cean

Abstract

Summary The community composition of N 2 ‐fixing microorganisms (diazotrophs) was investigated in copepods (primarily A cartia spp.) in parallel to that of seawater in coastal waters off D enmark ( Ø resund) and N ew E ngland, USA . The unicellular cyanobacterial diazotroph UCYN ‐ A was detected from seawater and full‐gut copepods, suggesting that the new N contributed by UCYN ‐ A is directly transferred to higher trophic levels in these waters. D eltaproteobacterial and C luster 3 nifH sequences were detected in > 1 μm seawater particles and full‐gut copepods, suggesting that they associate with copepods primarily via feeding. The dominant communities in starved copepods were V ibrio spp. and related G ammaproteobacteria , suggesting they represent the most permanent diazotroph associations in the copepods. N 2 fixation rates were up to 3.02 pmol N copepod −1 day −1 . Although at a typical copepod density in estuarine waters, these volumetric rates are low; considering the small size of a copepod, these mesozooplanktonic crustaceans may serve as hotspots of N 2 fixation, at 12.9–71.9 μmol N dm −3 copepod biomass day −1 . Taken together, diazotroph associations range from more permanent attachments to copepod feeding on some groups. Similar diazotroph groups detected on the eastern and western A tlantic O cean suggest that these associations are a general phenomenon and play a role in the coastal N cycles.

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