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

Bacteria living in the oligotrophic open ocean have various ways to survive under the pressure of nutrient limitation. Copepods, an abundant portion of the mesozooplankton, release nutrients through excretion and sloppy feeding that can support growth of surrounding bacteria. We conducted incubation experiments in the North Atlantic Subtropical Gyre to investigate the response of bacterial communities in the presence of copepods. Bacterial community composition and abundance measurements indicate that copepods have the potential to influence the microbial communities surrounding and associating with them – their ‘zoosphere’, in two ways. First, copepods may attract and support the growth of copiotrophic bacteria including representatives of Vibrionaceae, Oceanospirillales and Rhodobacteraceae in waters surrounding them. Second, copepods appear to grow specific groups of bacteria in or on the copepod body, particularly Flavobacteriaceae and Pseudoalteromonadaceae, effectively ‘farming’ them and subsequently releasing them. These distinct mechanisms provide a new view into how copepods may shape microbial communities in the open ocean. Microbial processes in the copepod zoosphere may influence estimates of oceanic bacterial biomass and in part control bacterial community composition and distribution in seawater

Chasing after non-cyanobacterial nitrogen fixation in marine pelagic environments

Traditionally, cyanobacterial activity in oceanic photic layers was considered responsible for the marine pelagic dinitrogen (N2) fixation. Other potentially N2-fixing bacteria and archaea have also been detected in the pelagic water column, however, the activity and importance of these non-cyanobacterial diazotrophs (NCDs) remain poorly constrained. In this perspective we summarize the N2 fixation rates from recently published studies on photic and aphotic layers that have been attributed to NCD activity via parallel molecular measurements, and discuss the status, challenges, and data gaps in estimating non-cyanobacterial N2 fixation NCNF in the ocean. Rates attributed to NCNF have generally been near the detection limit thus far (<1 nmol N L−1 d−1). Yet, if considering the large volume of the dark ocean, even low rates of NCNF could make a significant contribution to the new nitrogen input to the ocean. The synthesis here shows that nifH transcription data for NCDs have been reported in only a few studies where N2 fixation rates were detected in the absence of diazotrophic cyanobacteria. In addition, high apparent diversity and regional variability in the NCDs complicate investigations of these communities. Future studies should focus on further investigating impacts of environmental drivers including oxygen, dissolved organic matter, and dissolved inorganic nitrogen on NCNF. Describing the ecology of NCDs and accurately measuring NCNF rates, are critical for a future evaluation of the contribution of NCNF to the marine nitrogen budget

Dissolved organic matter stimulates N₂ fixation and nifH gene expression in Trichodesmium

Mixotrophy, the combination of heterotrophic and autotrophic nutrition modes, is emerging as the rule rather than the exception in marine photosynthetic plankton. Trichodesmium, a prominent diazotroph ubiquitous in the (sub)tropical oceans, is generally considered to obtain energy via autotrophy. While the ability of Trichodesmium to use dissolved organic phosphorus when deprived of inorganic phosphorus sources is well known, the extent to which this important cyanobacterium may benefit from other dissolved organic matter (DOM) sources is unknown. Here we provide evidence of carbon-, nitrogen- and phosphorus-rich DOM molecules enhancing N₂ fixation rates and nifH gene expression in natural Trichodesmium colonies collected at two stations in the western tropical South Pacific. Sampling at a third station located in the oligotrophic South Pacific Gyre revealed no Trichodesmium but showed presence of UCYN-B, although no nifH expression was detected. Our results suggest that Trichodesmium may behave mixotrophically in response to certain environmental conditions, providing them with metabolic plasticity and adding up to the view that mixotrophy is widespread among marine microbes

Longitudinal variability of diazotroph abundances in the subtropical North Atlantic Ocean

Diazotrophy-related studies in the North Atlantic have largely focused on its western tropical area, leaving the subtropics and the east undersampled. We studied the longitudinal distribution of Trichodesmium, UCYN-A, UCYN-B, the putative Gammaproteobacterium g-24774A11 and Richelia (Het1) along 24.58N, using quantitative polymerase chain reaction on different size fractions (10, 10–3 and 3–0.2 mm) and additional filament counts for Trichodesmium. Trichodesmium was the most abundant phylotype, followed by UCYN-A, g-24774A11 and Het1, with maximum abundances of 8.8 ! 105, 2.0 ! 105, 3.3 ! 103 and 3.4 ! 102 nifH copies L21, respectively, whereas UCYN-B was mostly undetected. A clear shift in the diazotroph community was observed at !308W, coinciding with the transition between the North Atlantic Subtropical Gyre boundary and inner core. This transition zone divided the transect into an eastern half dominated by UCYN-A and western half dominated by Trichodesmium and g-24774A11. g-24774A11 was only detected in the 10–3 mm fraction, suggesting their association with larger microbes or aggregates. Our results indicate that typical size fractionation by 10 mm is not optimal for reconciling diazotroph phylotypes to N2 fixation rates and that non-cyanobacterial diazotrophs may contribute importantly to bulk diazotrophic activity in the western subtropical North Atlantic.Consolider-Malaspina (CSD2008-00077), CAIBEX (CTM2007-66408- CO2-02). HOTMIX (CTM2011-30010-CO2-01)Versión del editor1,749

High N2 Fixation in and Near the Gulf Stream Consistent with a Circulation Control on Diazotrophy

The stoichiometry of physical nutrient supply may provide a constraint on the spatial distribution and rate of marine nitrogen (N2) fixation. Yet agreement between the N2 fixation rates inferred from nutrient supply and those directly measured has been lacking. The relative transport of phosphate and nitrate across the Gulf Stream suggests that 3–6 Tg N year−1 must be fixed to maintain steady nutrient stoichiometry in the North Atlantic subtropical gyre. Here we show direct measurements of N2 fixation consistent with these estimates, suggesting elevated N2 fixation in and near the Gulf Stream. At some locations across the Gulf Stream, we measured diazotroph abundances and N2 fixation rates that are 1–3 orders of magnitude greater than previously measured in the central North Atlantic subtropical gyre. In combination, rate measurements and gene abundances suggest that biogeochemical budgets can be a robust predictive tool for N2 fixation hot spots in the global ocean

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

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

A Critical Review of the \u3csup\u3e15\u3c/sup\u3eN\u3csub\u3e2\u3c/sub\u3e Tracer Method to Measure Diazotrophic Production in Pelagic Ecosystems

Dinitrogen (N2) fixation is an important source of biologically reactive nitrogen (N) to the global ocean. The magnitude of this flux, however, remains uncertain, in part because N2 fixation rates have been estimated following divergent protocols and because associated levels of uncertainty are seldom reported—confounding comparison and extrapolation of rate measurements. A growing number of reports of relatively low but potentially significant rates of N2 fixation in regions such as oxygen minimum zones, the mesopelagic water column of the tropical and subtropical oceans, and polar waters further highlights the need for standardized methodological protocols for measurements of N2 fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the 15N2 tracer method used for estimating diazotrophic rates, present results of experiments testing the validity of specific practices, and describe established metrics for reporting detection limits. We put forth a set of recommendations for best practices to estimate N2 fixation rates using 15N2 tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N2 fixation rate estimates intercomparable among studies

No sign of denitrification in a Baltic Sea cyanobacterial bloom

ABSTRACT: Denitrification in sediments is of major importance to the Baltic Sea nitrogen (N) budget. However, little is known about denitrification in the water column in the Baltic. We tested whether denitrification could be active in pelagic cyanobacterial aggregates, commonly found in the Baltic Sea during warm summer months. In these aggregates, anoxic microniches may form. Such microzones suggest a possibility for anaerobic processes, such as denitrification by the heterotrophic bacteria associated with the cyanobacteria. Denitrification and nitrogen (N₂) fixation in a cyanobacterial bloom were measured on a 3 wk cruise in the Baltic Sea in order to determine whether the Baltic Sea cyanobacterial blooms act as sources or sinks of N. Experimental conditions analogous to formation of anoxic microniches within cyanobacterial aggregates did not activate the denitrification process, even when anoxic conditions prevailed for several hours. Only in 3 cases was denitrification, measured using the 15N-isotope pairing method, detectable, giving rates of 0.8 to 1.8 nmol N₂ l–1 h–1. Nitrogen fixation, determined using the acetylene reduction assay, varied from 0.03 to 1.85 μmol N₂ l–1 h–1. According to this study, the blooms of N₂-fixing cyanobacteria in the Baltic Sea must be seen solely as sources, not sinks for N in the marine pelagic environment

Diversity and activity of epiphytic nitrogen-fixers on standing dead stems of the salt marsh grass Spartina alterniflora

Microbial communities growing on the surfaces of standing dead stems of salt marsh grasses contribute to fixation of atmospheric nitrogen (N-2) in these N-limited intertidal systems. Salt marshes in North Carolina, USA, have been shown to exhibit N-2 fixation throughout the growth season, but the composition of these diazotrophic communities is poorly known. This study investigated the diversity of the epiphytic N-2-fixing microbial community on the surfaces of dead stems (shoots) of the cord grass Spartina alterniflora in a salt marsh in North Carolina. Sequencing of the nifH gene and microscopy showed that a diverse diazotrophic community consisting of at least 8 diazotrophic cyanobacterial taxa, as well as an assemblage of alpha-, gamma-, and delta-proteobacteria, was present. Half of the recovered unique nifH sequences fell into the cluster with anaerobes. N-2-fixation rates under natural irradiance ranged from 0.001 to 2.58 nmol C2H4 mu g chl a(-1) h(-1) (0.07 to 206 nmol C2H4 cm(-2) h(-1)) and decreased from spring to fall. Nitrogen-fixation rates had a positive relationship with chlorophyll a, suggesting phototrophs contributed to N-2 fixation. The dominant diazotrophs in this study differed from epiphytic communities described elsewhere in the southeast United States, suggesting regional differences are present in the diversity of these communities