Symbiotic unicellular cyanobacteria fix nitrogen in the Arctic Ocean.

Biological dinitrogen (N2) fixation is an important source of nitrogen (N) in low-latitude open oceans. The unusual N2-fixing unicellular cyanobacteria (UCYN-A)/haptophyte symbiosis has been found in an increasing number of unexpected environments, including northern waters of the Danish Straight and Bering and Chukchi Seas. We used nanoscale secondary ion mass spectrometry (nanoSIMS) to measure 15N2 uptake into UCYN-A/haptophyte symbiosis and found that UCYN-A strains identical to low-latitude strains are fixing N2 in the Bering and Chukchi Seas, at rates comparable to subtropical waters. These results show definitively that cyanobacterial N2 fixation is not constrained to subtropical waters, challenging paradigms and models of global N2 fixation. The Arctic is particularly sensitive to climate change, and N2 fixation may increase in Arctic waters under future climate scenarios

A comparative study of iron and temperature interactive effects on diatoms and Phaeocystis antarctica from the Ross Sea, Antarctica

In the future, temperature and iron availability are predicted to change in the coastal polynyas of Antarctica, which are the most biologically productive regions of the Southern Ocean. We examined the individual and combined effects of iron addition (+500 nM) and temperature increase (4°C) on Phaeocystis antarctica and several dominant diatom species isolated from the McMurdo Sound sector of the Ross Sea. Iron addition increased growth, carbon fixation, iron uptake rates, cellular carbon quota, and cell size of almost all tested species, while temperature increase only affected certain species. Concurrent increases in temperature and iron synergistically stimulated the growth rates of some species, particularly Pseudo-nitzschia subcurvata. The diversified responses of these phytoplankton to iron and temperature may help explain the current spatial and temporal distributions of diatoms and prymnesiophytes in the Ross Sea. In the future, potential temperature and iron increases may promote the growth of the diatoms Chaetoceros sp., Fragilariopsis cylindrus, and especially P. subcurvata. In contrast, growth rates of P. antarctica did not increase at higher temperatures, suggesting that a shift in community composition toward diatoms may occur under warmer conditions in this biologically and biogeochemically important Southern Ocean polynya region

Nitrogen fixation and release of fixed nitrogen by Trichodesmium spp. in the Gulf of Mexico

During a 3‐yr study in the Gulf of Mexico, we measured dinitrogen (N2) fixation and nitrogen (N) release by Trichodesmium and compared these rates with water column N demand and the estimated N necessary to support blooms of Karenia brevis, a toxic dinoflagellate that severely affects the West Florida shelf. Net and gross N2 fixation rates were compared in simultaneous incubations using δ15N2 uptake and acetylene reduction, respectively. The difference between net and gross N2 fixation is assumed to be an approximation of the rate of N release. Results demonstrate that Trichodesmium in the Gulf of Mexico are fixing N2 at high rates and that an average of 52% of this recently fixed N2 is rapidly released. Calculations suggest that observed densities of Trichodesmium can provide enough N to support moderately sized K. brevis blooms. Based on other studies where δ15N2 and acetylene reduction were compared directly, it appears that N release from Trichodesmium is common but varies in magnitude among environments. In addition, carbon (C) and N‐based doubling times for Trichodesmium vary among studies and environments. Comparing gross N2 fixation and C fixation directly, C‐based doubling times exceeded N‐based doubling times, which suggests an imbalance in elemental turnover or a failure to fully quantify Trichodesmium N use

Marine plankton food webs and climate change

VIMS climate change white papers: Marine plankton food webs and climate chang

Microbial Community Response to Terrestrially Derived Dissolved Organic Matter in the Coastal Arctic

Warming at nearly twice the global rate, higher than average air temperatures are the new \u27normal\u27 for Arctic ecosystems. This rise in temperature has triggered hydrological and geochemical changes that increasingly release carbon-rich water into the coastal ocean via increased riverine discharge, coastal erosion, and the thawing of the semipermanent permafrost ubiquitous in the region. To determine the biogeochemical impacts of terrestrially derived dissolved organic matter (tDOM) on marine ecosystems we compared the nutrient stocks and bacterial communities present under ice-covered and ice-free conditions, assessed the lability of Arctic tDOM to coastal microbial communities from the Chukchi Sea, and identified bacterial taxa that respond to rapid increases in tDOM. Once thought to be predominantly refractory, we found that similar to 7% of dissolved organic carbon and similar to 38% of dissolved organic nitrogen from tDOM was bioavailable to receiving marine microbial communities on short 4 - 6 day time scales. The addition of tDOM shifted bacterial community structure toward more copiotrophic taxa and away from more oligotrophic taxa. Although no single order was found to respond universally (positively or negatively) to the tDOM addition, this study identified 20 indicator species as possible sentinels for increased tDOM. These data suggest the true ecological impact of tDOM will be widespread across many bacterial taxa and that shifts in coastal microbial community composition should be anticipated

Longitudinal variability of size-fractionated N-2 fixation and DON release rates along 24.5 degrees N in the subtropical North Atlantic

Dinitrogen (N-2) fixation and dissolved organic nitrogen (DON) release rates were measured on fractionated samples (\u3e10 mu m and m) along 24.5 degrees N in the subtropical North Atlantic. Net N-2 fixation rates (N-2 assimilation into biomass) ranged from 0.01 to 0.4 nmol N L-1 h(-1), and DON release rates ranged from 0.001 to 0.09 nmol N L-1 h(-1). DON release represented approximate to 14% and approximate to 23% of \u3e10 mu m and (assimilation into biomass plus DON release), respectively. This implies that by overlooking DON release, N-2 fixation rates are underestimated. Net N-2 fixation rates were higher in the east and decreased significantly toward the west (r(s)=-0.487, p=0.002, and r(s)=-0.496, p=0.001, for the \u3e10 mu m and fractions, respectively). The sum of both fractions correlated with aerosol optical depth at 550 nm (AOD 550 nm) (r(s)=0.382, p=0.017) and phosphate (PO43-) concentrations (r(s)=0.453, p=0.018), suggesting an enhancement of diazotrophy as a response to aerosol inputs and phosphorus availability. In contrast, DON release was constant among size fractions and did not correlate with any of these variables. We also compared N-2 fixation rates obtained using the N-15(2) dissolved and bubble methods. The first gave average rates 50% (49% 39) higher than the latter, which supports the finding that previously published N-2 fixation rates are likely underestimated. We suggest that by combining N-2 fixation and DON release measurements using dissolved N-15(2), global N-2 fixation rates could increase enough to balance oceanic fixed nitrogen budget disequilibria

MOLECULAR APPROACHES FOR IN SITU IDENTIFCIATION OF NITRATE UTILIZATION BY MARINE BACTERIA AND PHYTOPLANKTON

Traditionally, the importance of inorganic nitrogen (N) for the nutrition and growth of marine phytoplankton has been recognized, while inorganic N utilization by bacteria has received less attention. Likewise, organic N has been thought to be important for heterotrophic organisms but not for phytoplankton. However, accumulating evidence suggests that bacteria compete with phytoplankton for nitrate (NO3-) and other N species. The consequences of this competition may have a profound effect on the flux of N, and therefore carbon (C), in ocean margins. Because it has been difficult to differentiate between N uptake by heterotrophic bacterioplankton versus autotrophic phytoplankton, the processes that control N utilization, and the consequences of these competitive interactions, have traditionally been difficult to study. Significant bacterial utilization of DIN may have a profound effect on the flux of N and C in the water column because sinks for dissolved N that do not incorporate inorganic C represent mechanisms that reduce the atmospheric CO2 drawdown via the ?biological pump? and limit the flux of POC from the euphotic zone. This project was active over the period of 1998-2007 with support from the DOE Biotechnology Investigations ? Ocean Margins Program (BI-OMP). Over this period we developed a tool kit of molecular methods (PCR, RT-PCR, Q-PCR, QRT-PCR, and TRFLP) and combined isotope mass spectrometry and flow-cytometric approaches that allow selective isolation, characterization, and study of the diversity and genetic expression (mRNA) of the structural gene responsible for the assimilation of NO3- by heterotrophic bacteria (nasA). As a result of these studies we discovered that bacteria capable of assimilating NO3- are ubiquitous in marine waters, that the nasA gene is expressed in these environments, that heterotrophic bacteria can account for a significant fraction of total DIN uptake in different ocean margin systems, that the expression of nasA is differentially regulated in genetically distinct NO3- assimilating bacteria, and that the best predictors of nasA gene expression are either NO3- concentration or NO3- uptake rates. These studies provide convincing evidence of the importance of bacterial utilization of NO3-, insight into controlling processes, and provide a rich dataset that are being used to develop linked C and N modeling components necessary to evaluate the significance of bacterial DIN utilization to global C cycling. Furthermore, as a result of BI-OMP funding we made exciting strides towards institutionalizing a research and education based collaboration between the Skidaway Institute of Oceanography (SkIO) and Savannah State University (SSU), an historically black university within the University System of Georgia with undergraduate and now graduate programs in marine science. The BI-OMP program, in addition to supporting undergraduate (24) graduate (10) and postdoctoral (2) students, contributed to the development of a new graduate program in Marine Sciences at SSU that remains an important legacy of this project. The long-term goals of these collaborations are to increase the capacity for marine biotechnology research and to increase representation of minorities in marine, environmental and biotechnological sciences

Effluent Organic Nitrogen (EON): Bioavailability and Photochemical and Salinity-Mediated Release

The goal of this study was to investigate three potential ways that the soluble organic nitrogen (N) fraction of wastewater treatment plant (WWTP) effluents, termed effluent organic N (EON), could contribute to coastal eutrophication - direct biological removal, photochemical release of labile compounds, and salinity-mediated release of ammonium (NH4+). Effluents from two WWTPs were used in the experiments. For the bioassays, EON was added to water from four salinities (∼0 to 30) collected from the James River (VA) in August 2008, and then concentrations of N and phosphorus compounds were measured periodically over 48 h. Bioassay results, based on changes in DON concentrations, indicate that some fraction of the EON was removed and that the degree of EON removal varied between effluents and with salinity. Further, we caution that bioassay results should be interpreted within a broad context of detailed information on chemical characterization. EON from both WWTPs was also photoreactive, with labile NH4+ and dissolved primary amines released during exposure to sunlight. We also present the first data that demonstrate that when EON is exposed to higher salinities, increasing amounts of NH4+ are released, further facilitating EON use as effluent transits from freshwater through estuaries to the coast

Phytoplankton-Bacterial Interactions Mediate Micronutrient Colimitation at the Coastal Antarctic Sea Ice Edge

Southern Ocean primary productivity plays a key role in global ocean biogeochemistry and climate. At the Southern Ocean sea ice edge in coastal McMurdo Sound, we observed simultaneous cobalamin and iron limitation of surface water phytoplankton communities in late Austral summer. Cobalamin is produced only by bacteria and archaea, suggesting phytoplankton–bacterial interactions must play a role in this limitation. To characterize these interactions and investigate the molecular basis of multiple nutrient limitation, we examined transitions in global gene expression over short time scales, induced by shifts in micronutrient availability. Diatoms, the dominant primary producers, exhibited transcriptional patterns indicative of co-occurring iron and cobalamin deprivation. The major contributor to cobalamin biosynthesis gene expression was a gammaproteobacterial population, Oceanospirillaceae ASP10-02a. This group also contributed significantly to metagenomic cobalamin biosynthesis gene abundance throughout Southern Ocean surface waters. Oceanospirillaceae ASP10-02a displayed elevated expression of organic matter acquisition and cell surface attachment-related genes, consistent with a mutualistic relationship in which they are dependent on phytoplankton growth to fuel cobalamin production. Separate bacterial groups, including Methylophaga, appeared to rely on phytoplankton for carbon and energy sources, but displayed gene expression patterns consistent with iron and cobalamin deprivation. This suggests they also compete with phytoplankton and are important cobalamin consumers. Expression patterns of siderophore- related genes offer evidence for bacterial influences on iron availability as well. The nature and degree of this episodic colimitation appear to be mediated by a series of phytoplankton–bacterial interactions in both positive and negative feedback loops

Global Oceanic Diazotroph Database Version 2 and Elevated Estimate of Global N\u3csub\u3e2\u3c/sub\u3e Fixation

Marine diazotrophs convert dinitrogen (N2) gas into bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the first version of the global oceanic diazotroph database (version 1) was published. Here, we present an updated version of the database (version 2), significantly increasing the number of in situ diazotrophic measurements from 13 565 to 55 286. Data points for N2 fixation rates, diazotrophic cell abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and 809 %, respectively. Version 2 includes two new data sheets for the nifH gene copy abundance of non-cyanobacterial diazotrophs and cell-specific N2 fixation rates. The measurements of N2 fixation rates approximately follow a log-normal distribution in both version 1 and version 2. However, version 2 considerably extends both the left and right tails of the distribution. Consequently, when estimating global oceanic N2 fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43–57 versus 45–63 Tg N yr−1; ranges based on one geometric standard error). In contrast, when using arithmetic means, version 2 suggests a significantly higher rate of 223±30 Tg N yr−1 (mean ± standard error; same hereafter) compared to version 1 (74±7 Tg N yr−1). Specifically, substantial rate increases are estimated for the South Pacific Ocean (88±23 versus 20±2 Tg N yr−1), primarily driven by measurements in the southwestern subtropics, and for the North Atlantic Ocean (40±9 versus 10±2 Tg N yr−1). Moreover, version 2 estimates the N2 fixation rate in the Indian Ocean to be 35±14 Tg N yr−1, which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N2 fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional 15N2 bubble method yields lower rates in 69 % cases compared to the new 15N2 dissolution method. This updated version of the database can facilitate future studies in marine ecology and biogeochemistry. The database is stored at the Figshare repository (https://doi.org/10.6084/m9.figshare.21677687; Shao et al., 2022)