Isolation and biogeography of the oligotrophic ocean diazotroph, Crocosphaera waterburyi nov. sp.

Marine N2-fixing cyanobacteria, including the unicellular genus Crocosphaera, are considered keystone species in marine food webs. Crocosphaera are globally distributed and provide new sources of nitrogen and carbon, which fuel oligotrophic microbial communities and upper trophic levels. Despite their ecosystem importance, only one pelagic, oligotrophic, phycoerythrin-rich species, Crocosphaera watsonii, has ever been identified and characterized as widespread.

Phagotrophy in the nitrogen-fixing haptophyte Braarudosphaera bigelowii.

Biological nitrogen fixation provides fixed nitrogen for microbes living in the oligotrophic open ocean. UCYN-A2, the previously known symbiont of Braarudosphaera bigelowii, now believed to be an early-stage B. bigelowii organelle that exchanges fixed nitrogen for fixed carbon, is globally distributed.

The genome sequences of the marine diatom strain UHM3201 (Schvarcz, Stancheva & Steward, 2022) and its nitrogen-fixing, endosymbiotic cyanobacterium.

We present the genome assembly of the pennate diatom strain UHM3201 (Ochrophyta; Bacillariophyceae; Rhopalodiales; Rhopalodiaceae) and that of its cyanobacterial endosymbiont (Chroococcales: Aphanothecaceae). The genome sequence of the diatom is 60.3 megabases in span, and the cyanobacterial genome has a length of 2.

Nitrogen-fixing organelle in a marine alga.

Symbiotic interactions were key to the evolution of chloroplast and mitochondria organelles, which mediate carbon and energy metabolism in eukaryotes. Biological nitrogen fixation, the reduction of abundant atmospheric nitrogen gas (N) to biologically available ammonia, is a key metabolic process performed exclusively by prokaryotes. Atelocyanobacterium thalassa, or UCYN-A, is a metabolically streamlined N-fixing cyanobacterium previously reported to be an endosymbiont of a marine unicellular alga.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO.

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO, temperature, and nutrient enrichment on cyanobacterial and N-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively.

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 and the nitrogen-fixing cyanobacteria 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 () gene, which encodes the protein that might be involved in the uptake of ammonium from UCYN-A in these symbiotic haptophytes.

Non-cyanobacterial diazotrophs: global diversity, distribution, ecophysiology, and activity in marine waters.

Biological dinitrogen (N2) fixation supplies nitrogen to the oceans, supporting primary productivity, and is carried out by some bacteria and archaea referred to as diazotrophs. Cyanobacteria are conventionally considered to be the major contributors to marine N2 fixation, but non-cyanobacterial diazotrophs (NCDs) have been shown to be distributed throughout ocean ecosystems. However, the biogeochemical significance of marine NCDs has not been demonstrated.

Cell-specific measurements show nitrogen fixation by particle-attached putative non-cyanobacterial diazotrophs in the North Pacific Subtropical Gyre.

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 simultaneous N-dinitrogen and C-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.

Overlooked and widespread pennate diatom-diazotroph symbioses in the sea.

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.

UCYN-A/haptophyte symbioses dominate N fixation in the Southern California Current System.

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 (N) fixation, however, recent N fixation measurements and the presence of the N-fixing UCYN-A/haptophyte symbiosis in nearshore waters challenge this paradigm. We characterized the contribution of UCYN-A symbioses to nearshore N fixation in the Southern California Current System (SCCS) by measuring bulk community and single-cell N fixation rates, as well as diazotroph community composition and abundance.

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 (N )-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 N 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 N during the day.

Critical Role of Light in the Growth and Activity of the Marine N-Fixing UCYN-A Symbiosis.

The unicellular N-fixing cyanobacteria UCYN-A live in symbiosis with haptophytes in the lineage. Maintaining N-fixing symbioses between two unicellular partners requires tight coordination of multiple biological processes including cell growth and division and, in the case of the UCYN-A symbiosis, N fixation of the symbiont and photosynthesis of the host. In this system, it is thought that the host photosynthesis supports the high energetic cost of N fixation, and both processes occur during the light period.

Cell sorting reveals few novel prokaryote and photosynthetic picoeukaryote associations in the oligotrophic ocean.

Close associations between single-celled marine organisms can have a central role in biogeochemical processes and are of great interest for understanding the evolution of organisms. The global significance of such associations raises the question of whether unidentified associations are yet to be discovered. In this study, fluorescence-activated cell sorted photosynthetic picoeukayote (PPE) populations and single cells were analysed by sequencing of 16S rRNA genes in the oligotrophic North Pacific Subtropical Gyre.

Unexpected presence of the nitrogen-fixing symbiotic cyanobacterium UCYN-A in Monterey Bay, California.

In the last decade, the known biogeography of nitrogen fixation in the ocean has been expanded to colder and nitrogen-rich coastal environments. The symbiotic nitrogen-fixing cyanobacteria group A (UCYN-A) has been revealed as one of the most abundant and widespread nitrogen-fixers, and includes several sublineages that live associated with genetically distinct but closely related prymnesiophyte hosts. The UCYN-A1 sublineage is associated with an open ocean picoplanktonic prymnesiophyte, whereas UCYN-A2 is associated with the coastal nanoplanktonic coccolithophore Braarudosphaera bigelowii, suggesting that different sublineages may be adapted to different environments.

Unusual marine cyanobacteria/haptophyte symbiosis relies on N fixation even in N-rich environments.

The microbial fixation of N is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N fixation to DIN is unknown.

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.

Kīlauea lava fuels phytoplankton bloom in the North Pacific Ocean.

From June to August 2018, the eruption of Kīlauea volcano on the island of Hawai'i injected millions of cubic meters of molten lava into the nutrient-poor waters of the North Pacific Subtropical Gyre. The lava-impacted seawater was characterized by high concentrations of metals and nutrients that stimulated phytoplankton growth, resulting in an extensive plume of chlorophyll a that was detectable by satellite. Chemical and molecular evidence revealed that this biological response hinged on unexpectedly high concentrations of nitrate, despite the negligible quantities of nitrogen in basaltic lava.

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

Knowledge of the ecology of N-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.

UCYN-A3, a newly characterized open ocean sublineage of the symbiotic N -fixing cyanobacterium Candidatus Atelocyanobacterium thalassa.

The symbiotic unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A) is one of the most abundant and widespread nitrogen (N )-fixing cyanobacteria in the ocean. Although it remains uncultivated, multiple sublineages have been detected based on partial nitrogenase (nifH) gene sequences, including the four most commonly detected sublineages UCYN-A1, UCYN-A2, UCYN-A3 and UCYN-A4. However, very little is known about UCYN-A3 beyond the nifH sequences from nifH gene diversity surveys.

Diverse diazotrophs are present on sinking particles in the North Pacific Subtropical Gyre.

Sinking particles transport carbon and nutrients from the surface ocean into the deep sea and are considered hot spots for bacterial diversity and activity. In the oligotrophic oceans, nitrogen (N)-fixing organisms (diazotrophs) are an important source of new N but the extent to which these organisms are present and exported on sinking particles is not well known. Sinking particles were collected every 6 h over a 2-day period using net traps deployed at 150 m in the North Pacific Subtropical Gyre.

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

Major advances in understanding the diversity, distribution, and activity of marine N-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 incubation arrays in the vicinity of Station ALOHA in March 2016. Net growth rates for targeted diazotroph taxa as well as 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.

Distributions and Abundances of Sublineages of the N-Fixing Cyanobacterium Atelocyanobacterium thalassa (UCYN-A) in the New Caledonian Coral Lagoon.

Nitrogen (N) fixation is a major source of nitrogen that supports primary production in the vast oligotrophic areas of the world's oceans. The Western Tropical South Pacific has recently been identified as a hotspot for N fixation. In the Noumea lagoon (New Caledonia), high abundances of the unicellular N-fixing cyanobacteria group A (UCYN-A), coupled with daytime N fixation rates associated with the <10 μm size fraction, suggest UCYN-A may be an important diazotroph (N-fixer) in this region.

Coordinated regulation of growth, activity and transcription in natural populations of the unicellular nitrogen-fixing cyanobacterium Crocosphaera.

The temporal dynamics of phytoplankton growth and activity have large impacts on fluxes of matter and energy, yet obtaining in situ metabolic measurements of sufficient resolution for even dominant microorganisms remains a considerable challenge. We performed Lagrangian diel sampling with synoptic measurements of population abundances, dinitrogen (N) fixation, mortality, productivity, export and transcription in a bloom of Crocosphaera over eight days in the North Pacific Subtropical Gyre (NPSG). Quantitative transcriptomic analyses revealed clear diel oscillations in transcript abundances for 34% of Crocosphaera genes identified, reflecting a systematic progression of gene expression in diverse metabolic pathways.