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Diazotrophic cyanobacteria, those capable of fixing di-nitrogen (N 2 ), are considered one of the major sources of new nitrogen (N) in the oligotrophic tropical ocean, but direct incorporation of diazotrophic N into food webs has not been fully examined. In the Amazon River-influenced western tropical North Atlantic (WTNA), diatom diazotroph associations (DDAs) and the filamentous colonial diazotrophs  Trichodesmium  have seasonally high abundances. We sampled epipelagic mesozooplankton in the Amazon River plume and WTNA in May–June 2010 to investigate direct grazing by mesozooplankton on two DDA populations:  Richelia  associated with  Rhizosolenia  diatoms (het-1) and  Hemiaulus  diatoms (het-2), and on  Trichodesmium  using highly specific qPCR assays targeting nitrogenase genes ( nifH ). Both DDAs and  Trichodesmium  occurred in zooplankton gut contents, with higher detection of het-2 predominantly in calanoid copepods (2.33–16.76  nifH  copies organism -1 ). Abundance of  Trichodesmium  was low (2.21–4.03  nifH  copies organism -1 ), but they were consistently detected at high salinity stations (>35) in calanoid copepods. This suggests direct grazing on DDAs,  Trichodesmium  filaments and colonies, or consumption as part of sinking aggregates, is common. In parallel with the qPCR approach, a next generation sequencing analysis of 16S rRNA genes identified that cyanobacterial assemblage associated with zooplankton guts was dominated by the non-diazotrophic unicellular phylotypes  Synechococcus  (56%) and  Prochlorococcus  (26%). However, in two separate calanoid copepod samples, two unicellular diazotrophs Candidatus  Atelocyanobacterium thalassa  (UCYN-A) and  Crocosphaera watsonii  (UCYN-B) were present, respectively, as a small component of cyanobacterial assemblages (<2%). This study represents the first evidence of consumption of DDAs,  Trichodesmium , and unicellular cyanobacteria by calanoid copepods in an area of the WTNA known for high carbon export. These diazotroph populations are quantitatively important in the global N budget, widespread and hence, the next step is to accurately quantify grazing. Nonetheless, these results highlight a direct pathway of diazotrophic N into the food web and have important implications for biogeochemical cycles, particularly oligotrophic regions where N 2  fixation is the main source of new nitrogen.

Mesozooplankton Graze on Cyanobacteria in the Amazon River Plume and Western Tropical North Atlantic