AUTHOR=Conroy Brandon J. , Steinberg Deborah K. , Song Bongkuen , Kalmbach Andrew , Carpenter Edward J. , Foster Rachel A. 

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

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01436

DOI=10.3389/fmicb.2017.01436

ISSN=1664-302X

ABSTRACT=<p>Diazotrophic cyanobacteria, those capable of fixing di-nitrogen (N<sub>2</sub>), 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 <italic>Trichodesmium</italic> 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: <italic>Richelia</italic> associated with <italic>Rhizosolenia</italic> diatoms (het-1) and <italic>Hemiaulus</italic> diatoms (het-2), and on <italic>Trichodesmium</italic> using highly specific qPCR assays targeting nitrogenase genes (<italic>nifH</italic>). Both DDAs and <italic>Trichodesmium</italic> occurred in zooplankton gut contents, with higher detection of het-2 predominantly in calanoid copepods (2.33–16.76 <italic>nifH</italic> copies organism<sup>-1</sup>). Abundance of <italic>Trichodesmium</italic> was low (2.21–4.03 <italic>nifH</italic> copies organism<sup>-1</sup>), but they were consistently detected at high salinity stations (&gt;35) in calanoid copepods. This suggests direct grazing on DDAs, <italic>Trichodesmium</italic> 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 <italic>Synechococcus</italic> (56%) and <italic>Prochlorococcus</italic> (26%). However, in two separate calanoid copepod samples, two unicellular diazotrophs Candidatus <italic>Atelocyanobacterium thalassa</italic> (UCYN-A) and <italic>Crocosphaera watsonii</italic> (UCYN-B) were present, respectively, as a small component of cyanobacterial assemblages (&lt;2%). This study represents the first evidence of consumption of DDAs, <italic>Trichodesmium</italic>, 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<sub>2</sub> fixation is the main source of new nitrogen.</p>