An official website of the United States government
Nitrogen fixers (diazotrophs) are often an important nitrogen source to phytoplankton nutrient budgets in N-limited marine environments. Diazotrophic symbioses between cyanobacteria and diatoms can dominate nitrogen-fixation regionally, particularly in major river plumes and in open ocean mesoscale blooms. This study reports the successful isolation and growth in monocultures of multiple strains of a diatom-cyanobacteria symbiosis from the Gulf of Mexico using a modified artificial seawater medium. We document the influence of light and nutrients on nitrogen fixation and growth rates of the host diatom Hemiaulus hauckii Grunow together with its diazotrophic endosymbiont Richelia intracellularis Schmidt, as well as less complete results on the Hemiaulus membranaceus - R. intracellularis symbiosis. The symbioses rates reported here are for the joint diatom-cyanobacteria unit. Symbiont diazotrophy was sufficient to support both the host diatom and cyanobacteria symbionts, and the entire symbiosis replicated and grew without added nitrogen. Maximum growth rates of multiple strains of H. hauckii symbioses in N-free medium with N 2 as the sole N source were 0.74–0.93 div d −1 . Growth rates followed light saturation kinetics in H. hauckii symbioses with a growth compensation light intensity (E C ) of 7–16 µmol m −2 s −1 and saturation light level (E K ) of 84–110 µmol m −2 s −1 . Nitrogen fixation rates by the symbiont while within the host followed a diel pattern where rates increased from near-zero in the scotophase to a maximum 4–6 h into the photophase. At the onset of the scotophase, nitrogen-fixation rates declined over several hours to near-zero values. Nitrogen fixation also exhibited light saturation kinetics. Maximum N 2 fixation rates (84 fmol N 2 heterocyst −1 h −1 ) in low light adapted cultures (50 µmol m −2 s − 1) were approximately 40–50% of rates (144–154 fmol N 2 heterocyst −1 h −1 ) in high light (150 and 200 µmol m −2 s −1 ) adapted cultures. Maximum laboratory N 2 fixation rates were ~6 to 8-fold higher than literature-derived field rates of the H. hauckii symbiosis. In contrast to published results on the Rhizosolenia-Richelia symbiosis, the H. hauckii symbiosis did not use nitrate when added, although ammonium was consumed by the H. hauckii symbiosis. Symbiont-free host cell cultures could not be established; however, a symbiont-free H. hauckii strain was isolated directly from the field and grown on a nitrate-based medium that would not support DDA growth. Our observations together with literature reports raise the possibility that the asymbiotic H. hauckii are lines distinct from an obligately symbiotic H. hauckii line. While brief descriptions of successful culture isolation have been published, this report provides the first detailed description of the approaches, handling, and methodologies used for successful culture of this marine symbiosis. These techniques should permit a more widespread laboratory availability of these important marine symbioses.
more »
« less
Overlooked and widespread pennate diatom-diazotroph symbioses in the sea
Abstract Persistent nitrogen depletion in sunlit open ocean waters provides a favorable ecological niche for nitrogen-fixing (diazotrophic) cyanobacteria, some of which associate symbiotically with eukaryotic algae. All known marine examples of these symbioses have involved either centric diatom or haptophyte hosts. We report here the discovery and characterization of two distinct marine pennate diatom-diazotroph symbioses, which until now had only been observed in freshwater environments. Rhopalodiaceae diatomsEpithemia pelagicasp. nov. andEpithemia catenatasp. nov. were isolated repeatedly from the subtropical North Pacific Ocean, and analysis of sequence libraries reveals a global distribution. These symbioses likely escaped attention because the endosymbionts lack fluorescent photopigments, havenifHgene sequences similar to those of free-living unicellular cyanobacteria, and are lost in nitrogen-replete medium. Marine Rhopalodiaceae-diazotroph symbioses are a previously overlooked but widespread source of bioavailable nitrogen in marine habitats and provide new, easily cultured model organisms for the study of organelle evolution.
more »
« less
- PAR ID:
- 10379324
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Dinitrogen (N2) fixation is carried out by specialized microbes, called diazotrophs, and is a major source of nitrogen supporting primary production in oligotrophic oceans. One of the best-characterized diazotroph habitats is the North Pacific Subtropical Gyre (NPSG), where warm, chronically N-limited surface waters promote year-round N2fixation. At Station ALOHA (A Long-Term Oligotrophic Habitat Assessment) in the NPSG, N2fixation is typically ascribed to conspicuous, filamentous cyanobacterial diazotrophs (TrichodesmiumandRichelia), unicellular free-livingCrocosphaera, and the UCYN-A/haptophyte symbiosis, based on using microscopy and quantitative PCR (qPCR). However, the diazotroph community in this ecosystem is diverse and includes non-cyanobacterial diazotrophs (NCDs). We investigated the diversity, depth distributions, and seasonality of diazotroph communities at Stn. ALOHA using high throughput sequencing (HTS) ofnifHgene fragments from samples collected throughout the euphotic zone (0-175 m) at near-monthly intervals from June 2013 to July 2016. The UCYN-A symbioses andTrichodesmiumsp. consistently had the highest relative abundances and seasonal patterns that corroborated qPCR-based analyses. Other prevalent community members included a newCrocosphaera-like species, and several NCDs affiliated with γ- and δ-proteobacteria. Notably, some of the NCDs appear to be stable components of the community at Stn. ALOHA, having also been reported in prior studies. Depth and temporal patterns in microdiversity within two major diazotroph groups (Trichodesmiumand UCYN-A) suggested that sub-populations are adapted to time- and depth-dependent environmental variation. A network analysis of the upper euphotic (0-75 m) HTS data identified two modules that reflect a diazotroph community structure with seasonal turnover between UCYN-A/Gamma A, andTrichodesmium/Crocosphaera. It also reveals the seasonality of several important cyanobacteria and NCDs about which little is known, including a putative δ-proteobacterial phylotype originally discovered at Stn. ALOHA. Collectively, these results underscore the importance of couplingnifHgene HTS with other molecular techniques to obtain a comprehensive view of diazotroph community composition in the marine environment and reveal several understudied diazotroph groups that may contribute to N2fixation in the NPSG.more » « less
-
Abstract The availability of fixed nitrogen (N) is an important factor limiting biological productivity in the oceans. In coastal waters, high dissolved inorganic N concentrations were historically thought to inhibit dinitrogen (N2) fixation, however, recent N2 fixation measurements and the presence of the N2-fixing UCYN-A/haptophyte symbiosis in nearshore waters challenge this paradigm. We characterized the contribution of UCYN-A symbioses to nearshore N2 fixation in the Southern California Current System (SCCS) by measuring bulk community and single-cell N2 fixation rates, as well as diazotroph community composition and abundance. UCYN-A1 and UCYN-A2 symbioses dominated diazotroph communities throughout the region during upwelling and oceanic seasons. Bulk N2 fixation was detected in most surface samples, with rates up to 23.0 ± 3.8 nmol N l−1 d−1, and was often detected at the deep chlorophyll maximum in the presence of nitrate (>1 µM). UCYN-A2 symbiosis N2 fixation rates were higher (151.1 ± 112.7 fmol N cell−1 d−1) than the UCYN-A1 symbiosis (6.6 ± 8.8 fmol N cell−1 d−1). N2 fixation by the UCYN-A1 symbiosis accounted for a majority of the measured bulk rates at two offshore stations, while the UCYN-A2 symbiosis was an important contributor in three nearshore stations. This report of active UCYN-A symbioses and broad mesoscale distribution patterns establishes UCYN-A symbioses as the dominant diazotrophs in the SCCS, where heterocyst-forming and unicellular cyanobacteria are less prevalent, and provides evidence that the two dominant UCYN-A sublineages are separate ecotypes.more » « less
-
Abstract Biological nitrogen fixation is a major important source of nitrogen for low-nutrient surface oceanic waters. Nitrogen-fixing (diazotrophic) cyanobacteria are believed to be the primary contributors to this process, but the contribution of non-cyanobacterial diazotrophic organisms in oxygenated surface water, while hypothesized to be important, has yet to be demonstrated. In this study, we used simultaneous15N-dinitrogen and13C-bicarbonate incubations combined with nanoscale secondary ion mass spectrometry analysis to screen tens of thousands of mostly particle-associated, cell-like regions of interest collected from the North Pacific Subtropical Gyre. These dual isotope incubations allow us to distinguish between non-cyanobacterial and cyanobacterial nitrogen-fixing microorganisms and to measure putative cell-specific nitrogen fixation rates. With this approach, we detect nitrogen fixation by putative non-cyanobacterial diazotrophs in the oxygenated surface ocean, which are associated with organic-rich particles (<210 µm size fraction) at two out of seven locations sampled. When present, up to 4.1% of the analyzed particles contain at least one active putative non-cyanobacterial diazotroph. The putative non-cyanobacterial diazotroph nitrogen fixation rates (0.76 ± 1.60 fmol N cell−1d−1) suggest that these organisms are capable of fixing dinitrogen in oxygenated surface water, at least when attached to particles, and may contribute to oceanic nitrogen fixation.more » « less
-
Glass, Jennifer B (Ed.)ABSTRACT Marine macroalgae produce abundant and diverse polysaccharides, which contribute substantially to the organic matter exported to the deep ocean. Microbial degradation of these polysaccharides plays an important role in the turnover of macroalgal biomass. Various members of thePlanctomycetes-Verrucomicrobia-Chlamydia(PVC) superphylum are degraders of polysaccharides in widespread anoxic environments. In this study, we isolated a novel anaerobic bacterial strain NLcol2Tfrom microbial mats on the surface of marine sediments offshore Santa Barbara, CA, USA. Based on 16S ribosomal RNA (rRNA) gene and phylogenomic analyses, strain NLcol2Trepresents a novel species within thePontiellagenus in theKiritimatiellotaphylum (within the PVC superphylum). Strain NLcol2Tis able to utilize various monosaccharides, disaccharides, and macroalgal polysaccharides such as agar and ɩ-carrageenan. A near-complete genome also revealed an extensive metabolic capacity for anaerobic degradation of sulfated polysaccharides, as evidenced by 202 carbohydrate-active enzymes (CAZymes) and 165 sulfatases. Additionally, its ability of nitrogen fixation was confirmed by nitrogenase activity detected during growth on nitrogen-free medium, and the presence of nitrogenases (nifDKH) encoded in the genome. Based on the physiological and genomic analyses, this strain represents a new species of bacteria that may play an important role in the degradation of macroalgal polysaccharides and with relevance to the biogeochemical cycling of carbon, sulfur, and nitrogen in marine environments. Strain NLcol2T(= DSM 113125T= MCCC 1K08672T) is proposed to be the type strain of a novel species in thePontiellagenus, and the namePontiella agarivoranssp. nov. is proposed.IMPORTANCEGrowth and intentional burial of marine macroalgae is being considered as a carbon dioxide reduction strategy but elicits concerns as to the fate and impacts of this macroalgal carbon in the ocean. Diverse heterotrophic microbial communities in the ocean specialize in these complex polymers such as carrageenan and fucoidan, for example, members of theKiritimatiellotaphylum. However, only four type strains within the phylum have been cultivated and characterized to date, and there is limited knowledge about the metabolic capabilities and functional roles of related organisms in the environment. The new isolate strain NLcol2Texpands the known substrate range of this phylum and further reveals the ability to fix nitrogen during anaerobic growth on macroalgal polysaccharides, thereby informing the issue of macroalgal carbon disposal.more » « less
