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

In Situ Diazotroph Population Dynamics Under Different Resource Ratios in the North Pacific Subtropical Gyre.

Major advances in understanding the diversity, distribution, and activity of marine N2-fixing microorganisms (diazotrophs) have been made in the past decades, however, large gaps in knowledge remain about the environmental controls on growth and mortality rates. In order to measure diazotroph net growth rates and microzooplankton grazing rates on diazotrophs, nutrient perturbation experiments and dilution grazing experiments were conducted using free-floating in situ incubation arrays in the vicinity of Station ALOHA in March 2016. Net growth rates for targeted diazotroph taxa as well as Prochlorococcus, Synechococcus and photosynthetic picoeukaryotes were determined under high (H) and low (L) nitrate:phosphate (NP) ratio conditions at four depths in the photic zone (25, 45, 75, and 100 m) using quantitative PCR and flow cytometry. Changes in the prokaryote community composition in response to HNP and LNP treatments were characterized using 16S rRNA variable region tag sequencing. Microzooplankton grazing rates on diazotrophs were measured using a modified dilution technique at two depths in the photic zone (15 and 125 m). Net growth rates for most of the targeted diazotrophs after 48 h were not stimulated as expected by LNP conditions, rather enhanced growth rates were often measured in HNP treatments. Interestingly, net growth rates of the uncultivated prymnesiophyte symbiont UCYN-A1 were stimulated in HNP treatments at 75 and 100 m, suggesting that N used for growth was acquired through continuing to fix N2 in the presence of nitrate. Net growth rates for UCYN-A1, UCYN-C, Crocosphaera sp. (UCYN-B) and the diatom symbiont Richelia (associated with Rhizosolenia) were uniformly high at 45 m (up to 1.6 ± 0.5 d-1), implying that all were growing optimally at the onset of the experiment at that depth. Differences in microzooplankton grazing rates on UCYN-A1 and UCYN-C in 15 m waters indicate that the grazer assemblage preyed preferentially on UCYN-A1. Deeper in the water column (125 m), both diazotrophs were grazed at substantial rates, suggesting grazing pressure may increase with depth in the photic zone. Constraining in situ diazotroph growth and mortality rates are important steps for improving parameterization for diazotrophs in global ecosystem models

Phytoplankton transcriptomic and physiological responses to fixed nitrogen in the California current system

Marine phytoplankton are responsible for approximately half of photosynthesis on Earth. However, their ability to drive ocean productivity depends on critical nutrients, especially bioavailable nitrogen (N) which is scarce over vast areas of the ocean. Phytoplankton differ in their preferences for N substrates as well as uptake efficiencies and minimal N requirements relative to other critical nutrients, including iron (Fe) and phosphorus. In this study, we used the MicroTOOLs high-resolution environmental microarray to examine transcriptomic responses of phytoplankton communities in the California Current System (CCS) transition zone to added urea, ammonium, nitrate, and also Fe in the late summer when N depletion is common. Transcript level changes of photosynthetic, carbon fixation, and nutrient stress genes indicated relief of N limitation in many strains of Prochlorococcus, Synechococcus, and eukaryotic phytoplankton. The transcriptomic responses helped explain shifts in physiological and growth responses observed later. All three phytoplankton groups had increased transcript levels of photosynthesis and/or carbon fixation genes in response to all N substrates. However, only Prochlorococcus had decreased transcript levels of N stress genes and grew substantially, specifically after urea and ammonium additions, suggesting that Prochlorococcus outcompeted other community members in these treatments. Diatom transcript levels of carbon fixation genes increased in response to Fe but not to Fe with N which might have favored phytoplankton that were co-limited by N and Fe. Moreover, transcription patterns of closely related strains indicated variability in N utilization, including nitrate utilization by some high-light adapted Prochlorococcus. Finally, up-regulation of urea transporter genes by both Prochlorococcus and Synechococcus in response to filtered deep water suggested a regulatory mechanism other than classic control via the global N regulator NtcA. This study indicated that co-existing phytoplankton strains experience distinct nutrient stresses in the transition zone of the CCS, an understudied region where oligotrophic and coastal communities naturally mix

Light and depth dependency of nitrogen fixation by the non‐photosynthetic, symbiotic cyanobacterium UCYN‐A

The symbiotic cyanobacterium UCYN-A is one of the most globally abundant marine dinitrogen (N2)-fixers, but cultures have not been available and its biology and ecology are poorly understood. We used cultivation-independent approaches to investigate how UCYN-A single-cell N2 fixation rates (NFRs) and nifH gene expression vary as a function of depth and photoperiod. Twelve-hour day/night incubations showed that UCYN-A only fixed N2 during the day. Experiments conducted using in situ arrays showed a light-dependence of NFRs by the UCYN-A symbiosis, with the highest rates in surface waters (5–45 m) and lower rates at depth (≥ 75 m). Analysis of NFRs versus in situ light intensity yielded a light saturation parameter (Ik) for UCYN-A of 44 μmol quanta m−2 s−1. This is low compared with other marine diazotrophs, suggesting an ecological advantage for the UCYN-A symbiosis under low-light conditions. In contrast to cell-specific NFRs, nifH gene-specific expression levels did not vary with depth, indicating that light regulates N2 fixation by UCYN-A through processes other than transcription, likely including host–symbiont interactions. These results offer new insights into the physiology of the UCYN-A symbiosis in the subtropical North Pacific Ocean and provide clues to the environmental drivers of its global distributions.En prens

Temporal variability of diazotroph community composition in the upwelling region off NW Iberia

Knowledge of the ecology of N 2 -fixing (diazotrophic) plankton is mainly limited to oligotrophic (sub)tropical oceans. However, diazotrophs are widely distributed and active throughout the global ocean. Likewise, relatively little is known about the temporal dynamics of diazotrophs in productive areas. Between February 2014 and December 2015, we carried out 9 one-day samplings in the temperate northwestern Iberian upwelling system to investigate the temporal and vertical variability of the diazotrophic community and its relationship with hydrodynamic forcing. In downwelling conditions, characterized by deeper mixed layers and a homogeneous water column, non-cyanobacterial diazotrophs belonging mainly to nifH clusters 1G (Gammaproteobacteria) and 3 (putative anaerobes) dominated the diazotrophic community. In upwelling and relaxation conditions, affected by enhanced vertical stratification and hydrographic variability, the community was more heterogeneous vertically but less diverse, with prevalence of UCYN-A (unicellular cyanobacteria, subcluster 1B) and non-cyanobacterial diazotrophs from clusters 1G and 3. Oligotyping analysis of UCYN-A phylotype showed that UCYN-A2 sublineage was the most abundant (74%), followed by UCYN-A1 (23%) and UCYN-A4 (2%). UCYN-A1 oligotypes exhibited relatively low frequencies during the three hydrographic conditions, whereas UCYN-A2 showed higher abundances during upwelling and relaxation. Our findings show the presence of a diverse and temporally variable diazotrophic community driven by hydrodynamic forcing in an upwelling system.Xunta de Galicia | Ref. EM2013/021Ministerio de Economía, Industria y Competitividad | Ref. CTM2016-75451-C2-1-RMinisterio de Educación, Cultura y Deporte | Ref. FPU13/01674Ministerio de Educación, Cultura y Deporte | Ref. EST16/00142Universidad de Vigo | Ref. Axudas á investigación 201

Diazotroph diversity in the sea ice, melt ponds and surface waters of the Eurasian Basin of the Central Arctic Ocean

The Eurasian basin of the Central Arctic Ocean is nitrogen limited, but little is known about the presence and role of nitrogen-fixing bacteria. Recent studies have indicated the occurrence of diazotrophs in Arctic coastal waters potentially of riverine origin. Here, we investigated the presence of diazotrophs in ice and surface waters of the Central Arctic Ocean in the summer of 2012. We identified diverse communities of putative diazotrophs through targeted analysis of the nifH gene, which encodes the iron protein of the nitrogenase enzyme. We amplified 529 nifH sequences from 26 samples of Arctic melt ponds, sea ice and surface waters. These sequences resolved into 43 clusters at 92% amino acid sequence identity, most of which were non-cyanobacterial phylotypes from sea ice and water samples. One cyanobacterial phylotype related to Nodularia sp. was retrieved from sea ice, suggesting that this important functional group is rare in the Central Arctic Ocean. The diazotrophic community in sea-ice environments appear distinct from other cold-adapted diazotrophic communities, such as those present in the coastal Canadian Arctic, the Arctic tundra and glacial Antarctic lakes. Molecular fingerprinting of nifH and the intergenic spacer region of the rRNA operon revealed differences between the communities from river-influenced Laptev Sea waters and those from ice-related environments pointing towards a marine origin for sea-ice diazotrophs. Our results provide the first record of diazotrophs in the Central Arctic and suggest that microbial nitrogen fixation may occur north of 77ºN. To assess the significance of nitrogen fixation for the nitrogen budget of the Arctic Ocean and to identify the active nitrogen fixers, further biogeochemical and molecular biological studies are needed

Primer design for the amplification of the ammonium transporter genes from the uncultured haptophyte algal species symbiotic with the marine nitrogen-fixing cyanobacterium UCYN-A1

The multiple symbiotic partnerships between closely related species of the haptophyte algae Braarudosphaera bigelowii and the nitrogen-fixing cyanobacteria Candidatus Atelocyanobacterium thalassa (UCYN-A) contribute importantly to the nitrogen and carbon cycles in vast areas of the ocean. The diversity of the eukaryotic 18S rDNA phylogenetic gene marker has helped to identify some of these symbiotic haptophyte species, yet we still lack a genetic marker to assess its diversity at a finer scale. One of such genes is the ammonium transporter (amt) gene, which encodes the protein that might be involved in the uptake of ammonium from UCYN-A in these symbiotic haptophytes. Here, we designed three specific PCR primer sets targeting the amt gene of the haptophyte species (A1-Host) symbiotic with the open ocean UCYN-A1 sublineage, and tested them in samples collected from open ocean and near-shore environments. Regardless of the primer pair used at Station ALOHA, which is where UCYN-A1 is the pre-dominant UCYN-A sublineage, the most abundant amt amplicon sequence variant (ASV) was taxonomically classified as A1-Host. In addition, two out of the three PCR primer sets revealed the existence of closely-related divergent haptophyte amt ASVs (>95% nucleotide identity). These divergent amt ASVs had higher relative abundances than the haptophyte typically associated with UCYN-A1 in the Bering Sea, or co-occurred with the previously identified A1-Host in the Coral Sea, suggesting the presence of new diversity of closely-related A1-Hosts in polar and temperate waters. Therefore, our study reveals an overlooked diversity of haptophytes species with distinct biogeographic distributions partnering with UCYN-A, and provides new primers that will help to gain new knowledge of the UCYN-A/haptophyte symbiosis

MARTIJANEC-GAMULICA - ANALYSIS OF FINDS FROM 1950

Godine 1950. na tada još neistraženom tumulu Gamulica kod Martijanca ukopan je stup dalekovoda. Tom je prilikom mještanin Martijanca, Martin Štanglin, u tumulu prokopao jamu te otkrio tragove grobne konstrukcije i priloga. Lokalitet je nakon toga obišao prof. Stjepan Vuković, tadašnji kustos Odjela za arheologiju Gradskog muzeja Varaždin, te je pokretne nalaze prikupio, a iskop sanirao. Istraživanje pod vodstvom dr. sc. Zdenka Vinskog kao i detaljna analiza tada prikupljenih arheoloških nalaza, potvrdili su izniman značaj lokaliteta Martijanec-Gamulica, međutim, tom prilikom nisu uzeti u obzir i predmeti koje je 1950. god. prikupio S. Vuković. Dvadeset i sedam keramičkih te jedan brončani predmet čine malu, ali vrijednu skupinu nalaza čija će analiza i konačna objava zaokružiti spoznaje o ukopu u tumulu Gamulica te pridonijeti boljem poznavanju stariježeljeznodobne grupe Martijanec-Kaptol.In 1950, on the then unexcavated tumulus of Gamulica near Martijanec, a transmission line pole was set up. On that occasion Martin Štanglin, a resident of Matrijanec, found a pit in the tumulus which contained traces of a tomb with grave goods. The site was then visited by professor Stjepan Vuković who worked as a curator at the Department of Archaeology of the Varaždin City Museum and who collected movable finds and protected the tomb. Research led by Zdenko Vinski, PhD, as well as a detailed analysis of the finds, confirmed the exceptional status of the Martijanec-Gamulica site. However, the study did not include finds collected in 1950 by S. Vuković. Twenty seven ceramic and one bronze find make up a small but valuable group of finds the analysis of which will unite our knowledge of the tomb inside the Gamulica tumulus and contribute to our understanding of the Iron Age Martijanec-Kaptol group