Copepod-Associated Gammaproteobacteria Respire Nitrate in the Open Ocean Surface Layers

Abstract

Microbial dissimilatory nitrate reduction to nitrite, or nitrate respiration, was detected in association with copepods in the oxygenated water column of the North Atlantic subtropical waters. These unexpected rates correspond to up to 0.09 nmol N copepod−1 d−1 and demonstrate a previously unaccounted nitrogen transformation in the oceanic pelagic surface layers. Genes and transcripts for both the periplasmic and membrane associated dissimilatory nitrate reduction pathways (Nap and Nar, respectively) were detected. The napA genes and transcripts were closely related with sequences from several clades of Vibrio sp., while the closest relatives of the narG sequences were Pseudoalteromonas spp. and Alteromonas spp., many of them representing clades only distantly related to previously described cultivated bacteria. The discovered activity demonstrates a novel Gammaproteobacterial respiratory role in copepod association, presumably providing energy for these facultatively anaerobic bacteria, while supporting a reductive path of nitrogen in the oxygenated water column of the open ocean.

Copepods promote bacterial community changes in surrounding seawater through farming and nutrient enrichment

“…T0 samples were collected for the seawater inoculum and for copepod treatments. Nitrogen and carbonate addition (+N) were done for a parallel study (see Moisander et al ., ) and included 0.5 μM each of NO 2 − , NO 3 − , NH 4 + and CO 3 −2 . Nutrients added included NO 3 − (0.5 μM), NH 4 + (0.5 μM), dextrose (0.5 μM), FeCl (0.5 μM), EDTA (0.5 μM) and PO 4 −3 (1 μM).…”

“…() showed that the dissolved N in the dissolved material derived from Pleuromamma copepods was composed of a 3:1 ratio of ammonium:DON, and rapidly recycled in the euphotic zone. Our previous, parallel study reported that all treatments containing copepods in these experiments resulted in high ammonium concentrations (31.4 ± 13.1 μM), while all seawater treatments, regardless of N addition, contained an average of 1.8 ± 0.99 μM NH 4 + by the end of the incubation (Moisander et al ., ). In this study, bacterial abundances were 5–8 times higher in seawater amended with nutrients or CDM than seawater controls.…”

“…In the same copepod incubations discussed here, dissimilatory nitrate reduction to nitrite was detected in the presence of oxygen in the incubations (Moisander et al ., ). Sequencing and expression data of genes involved in dissimilatory nitrate reduction suggested that Vibrio spp., Alteromonas spp.…”

“…and Pseudoalteromonas spp. were respiring nitrate in the copepod incubations, and this appears to be one of their fitness strategies in copepod associations (Moisander et al, 2018). The sustained nutrient concentrations in the copepod treatments were likely higher than in the seawater samples amended at the beginning of the experiment, which could partially explain the lower competitive fitness of Alteromonadaceae in copepod association than seawater treatments amended with nutrients or CDM (Supporting Information Fig.…”

“…Our findings suggest that copepods likely influence bacterial communities surrounding them via different mechanisms, and these communities form a copepod zoosphere that in turn may be seeding the surrounding seawater beyond. The activities influencing biogeochemical cycles in these communities are likely different from activities in the surrounding free-living organisms (De Corte et al, 2018;Moisander et al, 2018) and merit further study. Given the high abundances and ubiquitous distribution of copepods, the microbial processes occurring within the copepod zoosphere are likely to be globally important.…”