Nitrogen‐fixing bacteria associated with copepods in coastal waters of the North Atlantic Ocean

Abstract

The community composition of N2 -fixing microorganisms (diazotrophs) was investigated in copepods (primarily Acartia spp.) in parallel to that of seawater in coastal waters off Denmark (Øresund) and New England, 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. Deltaproteobacterial and Cluster 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 Vibrio spp. and related Gammaproteobacteria, suggesting they represent the most permanent diazotroph associations in the copepods. N2 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 N2 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 Atlantic Ocean suggest that these associations are a general phenomenon and play a role in the coastal N cycles.

Fixed-Nitrogen Loss Associated with Sinking Zooplankton Carcasses in a Coastal Oxygen Minimum Zone (Golfo Dulce, Costa Rica)

“…However, individual-specific N 2 production rates measured in the current study were at least three orders of magnitude higher than N 2 fixation rates measured for small copepods in another study (Scavotto et al, 2015). Despite the possible variability of N-cycling associated with mesozooplankton from different habitats, this large difference in rates suggests that carcass-associated microbial communities exhibit net consumption rather than net production of fixed nitrogen in the pelagic zone.…”

“…Recent investigations into copepod microbiomes have revealed a number of recurring bacterial groups, such as Vibrio sp., Bacteroidetes, Firmicutes, Actinobacteria, and Pseudoalteromonas sp., that are associated with different copepod species from different ecosystems (Gerdts et al, 2013;De Corte et al, 2014;Moisander et al, 2015;Scavotto et al, 2015;Shoemaker and Moisander, 2015). Separating out the microbial communities that colonize the gut or the exoskeleton of zooplankton is challenging though (Møller et al, 2007;De Corte et al, 2014;Skovgaard et al, 2015).…”

“…Additionally, the metabolic functions of zooplankton-associated bacteria remain largely unknown, unlike those of microorganisms associated with benthic invertebrates (Dubilier et al, 2001;Martinez-Garcia et al, 2008;Hoffmann et al, 2009;Stief et al, 2009). Notable exceptions are the associations between copepods and N 2 -fixing and denitrifying bacteria (Zehr et al, 1998;Glud et al, 2015;Scavotto et al, 2015).…”

Mesozooplankton structure and functioning in the western tropical South Pacific along the 20° parallel south during the OUTPACE survey (February–April 2015)

“…In this study we did not considered possible association between zooplankton taxa and non-Trichodesmium diazotrophs but Hunt et al (2016) from 15 N 2 labelled grazing experiments provided evidence for direct ingestion and assimilation of UCYN-C-derived N by the zooplankton. Recent observations 10 suggested consumption of UCYN-A and UCYN-B by diverse calanoid copepods (Scavotto et al, 2015;Conroy et al, 2017). respectively) calculated using the allometric relationships of Ikeda (1985), based on individual dry weight estimates of the different zooplankton taxa, and on temperature and chlorophyll levels recorded during OUTPACE, were in the range of literature values for metazooplankton and copepods in the inter-tropical zone (Ikeda, 1985;Mauchline, 25 1998;Hernandez-Leon et al, 2008;McKinnon et al, 2015), thus validating this approach.…”

“…However, up to now, Trichodesmium are thought to be grazed by relatively few mesozooplankton species (Carpenter et al 1999;Conroy et al, 2017), although new techniques to observe diazotroph in zooplankton gut content may extend this list (Scavotto et al, 2015;Azimuddin et al 2016;Conroy et al 2017). Such analysis were not realized during OUTPACE, and we limit our discussion to observed correlation or not of key species distribution with diazotrophs distributions, particularly those among the top 10 species in frequency abundance (Table 4).…”