Atelocyanobacterium Research Articles

Distributions and Abundances of Sublineages of the N2-Fixing Cyanobacterium Candidatus Atelocyanobacterium thalassa (UCYN-A) in the New Caledonian Coral Lagoon.

Nitrogen (N2) 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 N2 fixation. In the Noumea lagoon (New Caledonia), high abundances of the unicellular N2-fixing cyanobacteria group A (UCYN-A), coupled with daytime N2 fixation rates associated with the <10 μm size fraction, suggest UCYN-A may be an important diazotroph (N2-fixer) in this region. However, little is known about the seasonal variability and diversity of UCYN-A there. To assess this, surface waters from a 12 km transect from the mouth of the Dumbea River to the Dumbea Pass were sampled monthly between July 2012 and March 2014. UCYN-A abundances for two of the defined sublineages, UCYN-A1 and UCYN-A2, were quantified using qPCR targeting the nifH gene, and the nifH-based diversity of UCYN-A was characterized by identifying oligotypes, alternative taxonomic units defined by nucleotide positions with high variability. UCYN-A abundances were dominated by the UCYN-A1 sublineage, peaked in September and October and could be predicted by a suite of nine environmental parameters. At the sublineage level, UCYN-A1 abundances could be predicted based on lower temperatures (<23°C), nitrate concentrations, precipitation, wind speed, while UCYN-A2 abundances could be predicted based on silica, and chlorophyll a concentrations, wind direction, precipitation, and wind speed. Using UCYN-A nifH oligotyping, similar environmental variables explained the relative abundances of sublineages and their associated oligotypes, with the notable exception of the UCYN-A2 oligotype (oligo43) which had relative abundance patterns distinct from the dominant UCYN-A2 oligotype (oligo3). The results support an emerging pattern that UCYN-A is comprised of a diverse group of strains, with sublineages that may have different ecological niches. By identifying environmental factors that influence the composition and abundance of UCYN-A sublineages, this study helps to explain global UCYN-A abundance patterns, and is important for understanding the significance of N2 fixation at local and global scales.

A Short Comparison of Two Marine Planktonic Diazotrophic Symbioses Highlights an Un-quantified Disparity

Some N2-fixing cyanobacteria form symbiosis with diverse protists. In the plankton two groups of diazotrophic symbioses are described: (1) a collective group of diatoms which associate with heterocystous cyanobacteria (Diatom Diazotroph Associations, DDA), and (2) the microalgal prymnesiophyte Braarudosphaera bigelowii and its relatives which associate with the unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (hereafter as UCYN-A). Both symbiotic systems co-occur, and in both partnerships the symbionts function as a nitrogen (N) source. In this perspective, we provide a brief comparison between the DDAs and the prymnesiophyte- UCYN-A symbioses highlighting similarities and differences in both systems, and present a bias in the attention and current methodology that has led to an under-detection and under-estimation of the DDAs.

The Transcriptional Cycle Is Suited to Daytime N2 Fixation in the Unicellular Cyanobacterium "Candidatus Atelocyanobacterium thalassa" (UCYN-A).

Symbiosis between a marine alga and a N2-fixing cyanobacterium (Cyanobacterium UCYN-A) is geographically widespread in the oceans and is important in the marine N cycle. UCYN-A is uncultivated and is an unusual unicellular cyanobacterium because it lacks many metabolic functions, including oxygenic photosynthesis and carbon fixation, which are typical in cyanobacteria. It is now presumed to be an obligate symbiont of haptophytes closely related to Braarudosphaera bigelowii N2-fixing cyanobacteria use different strategies to avoid inhibition of N2 fixation by the oxygen evolved in photosynthesis. Most unicellular cyanobacteria temporally separate the two incompatible activities by fixing N2 only at night, but, surprisingly, UCYN-A appears to fix N2 during the day. The goal of this study was to determine how the unicellular UCYN-A strain coordinates N2 fixation and general metabolism compared to other marine cyanobacteria. We found that UCYN-A has distinct daily cycles of many genes despite the fact that it lacks two of the three circadian clock genes found in most cyanobacteria. We also found that the transcription patterns in UCYN-A are more similar to those in marine cyanobacteria that are capable of aerobic N2 fixation in the light, such as Trichodesmium and heterocyst-forming cyanobacteria, than to those in Crocosphaera or Cyanothece species, which are more closely related to unicellular marine cyanobacteria evolutionarily. Our findings suggest that the symbiotic interaction has resulted in a shift of transcriptional regulation to coordinate UCYN-A metabolism with that of the phototrophic eukaryotic host, thus allowing efficient coupling of N2 fixation (by the cyanobacterium) to the energy obtained from photosynthesis (by the eukaryotic unicellular alga) in the light.IMPORTANCE The symbiotic N2-fixing cyanobacterium UCYN-A, which is closely related to Braarudosphaera bigelowii, and its eukaryotic algal host have been shown to be globally distributed and important in open-ocean N2 fixation. These unique cyanobacteria have reduced metabolic capabilities, even lacking genes for oxygenic photosynthesis and carbon fixation. Cyanobacteria generally use energy from photosynthesis for nitrogen fixation but require mechanisms for avoiding inactivation of the oxygen-sensitive nitrogenase enzyme by ambient oxygen (O2) or the O2 evolved through photosynthesis. This study showed that symbiosis between the N2-fixing cyanobacterium UCYN-A and its eukaryotic algal host has led to adaptation of its daily gene expression pattern in order to enable daytime aerobic N2 fixation, which is likely more energetically efficient than fixing N2 at night, as found in other unicellular marine cyanobacteria.

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

The classical paradigm about marine N2 fixation establishes that this process is mainly constrained to nitrogen-poor tropical and subtropical regions, and sustained by the colonial cyanobacterium Trichodesmium spp. and diatom-diazotroph symbiosis. However, the application of molecular techniques allowed determining a high phylogenic diversity and a wide distribution of marine diazotrophs, which extends the range of ocean environments where biological N2 fixation may be relevant. Between February 2014 and December 2015, we carried out 10 one-day samplings in the upwelling system off NW Iberia in order to: 1) investigate the seasonal variability in the magnitude of N2 fixation, 2) determine its biogeochemical role as a mechanism of new nitrogen supply, and 3) quantify the main diazotrophs in the region under contrasting hydrographic regimes. Our results indicate that the magnitude of N2 fixation in this region was relatively low (0.001±0.002 – 0.095±0.024 µmol N m-3 d-1), comparable to the lower-end of rates described for the subtropical NE Atlantic. Maximum rates were observed at surface during both upwelling and relaxation conditions. The comparison with nitrate diffusive fluxes revealed the minor role of N2 fixation (2 fixation activity detected in the region. Quantitative PCR targeting the nifH gene revealed the highest abundances of two sublineages of Candidatus Atelocyanobacterium thalassa or UCYN-A (UCYN-A1 and UCYN-A2) mainly at surface waters during upwelling and relaxation conditions, and of Gammaproteobacteria γ-24774A11 at deep waters during downwelling. Maximum abundance for the three groups were up to 6.7 × 102, 1.5 × 103 and 2.4 × 104 nifH copies L-1, respectively. Our findings demonstrate measurable N2 fixation activity and presence of diazotrophs throughout the year in a nitrogen-rich temperate region.

The Green Berry Consortia of the Sippewissett Salt Marsh: Millimeter-Sized Aggregates of Diazotrophic Unicellular Cyanobacteria.

Microbial interactions driving key biogeochemical fluxes often occur within multispecies consortia that form spatially heterogeneous microenvironments. Here, we describe the “green berry” consortia of the Sippewissett salt marsh (Falmouth, MA, United States): millimeter-sized aggregates dominated by an uncultured, diazotrophic unicellular cyanobacterium of the order Chroococcales (termed GB-CYN1). We show that GB-CYN1 is closely related to Crocosphaera watsonii (UCYN-B) and “Candidatus Atelocyanobacterium thalassa” (UCYN-A), two groups of unicellular diazotrophic cyanobacteria that play an important role in marine primary production. Other green berry consortium members include pennate diatoms and putative heterotrophic bacteria from the Alphaproteobacteria and Bacteroidetes. Tight coupling was observed between photosynthetic oxygen production and heterotrophic respiration. When illuminated, the green berries became supersaturated with oxygen. From the metagenome, we observed that GB-CYN1 encodes photosystem II genes and thus has the metabolic potential for oxygen production unlike UCYN-A. In darkness, respiratory activity rapidly depleted oxygen creating anoxia within the aggregates. Metagenomic data revealed a suite of nitrogen fixation genes encoded by GB-CYN1, and nitrogenase activity was confirmed at the whole-aggregate level by acetylene reduction assays. Metagenome reads homologous to marker genes for denitrification were observed and suggest that heterotrophic denitrifiers might co-occur in the green berries, although the physiology and activity of facultative anaerobes in these aggregates remains uncharacterized. Nitrogen fixation in the surface ocean was long thought to be driven by filamentous cyanobacterial aggregates, though recent work has demonstrated the importance of unicellular diazotrophic cyanobacteria (UCYN) from the order Chroococcales. The green berries serve as a useful contrast to studies of open ocean UCYN and may provide a tractable model system to investigate microbial dynamics within phytoplankton aggregates, a phenomenon of global importance to the flux of particulate organic carbon and nitrogen in surface waters.

Mesozooplankton Graze on Cyanobacteria in the Amazon River Plume and Western Tropical North Atlantic.

Diazotrophic cyanobacteria, those capable of fixing di-nitrogen (N2), are considered one of the major sources of new nitrogen (N) in the oligotrophic tropical ocean, but direct incorporation of diazotrophic N into food webs has not been fully examined. In the Amazon River-influenced western tropical North Atlantic (WTNA), diatom diazotroph associations (DDAs) and the filamentous colonial diazotrophs Trichodesmium have seasonally high abundances. We sampled epipelagic mesozooplankton in the Amazon River plume and WTNA in May–June 2010 to investigate direct grazing by mesozooplankton on two DDA populations: Richelia associated with Rhizosolenia diatoms (het-1) and Hemiaulus diatoms (het-2), and on Trichodesmium using highly specific qPCR assays targeting nitrogenase genes (nifH). Both DDAs and Trichodesmium occurred in zooplankton gut contents, with higher detection of het-2 predominantly in calanoid copepods (2.33–16.76 nifH copies organism-1). Abundance of Trichodesmium was low (2.21–4.03 nifH copies organism-1), but they were consistently detected at high salinity stations (>35) in calanoid copepods. This suggests direct grazing on DDAs, Trichodesmium filaments and colonies, or consumption as part of sinking aggregates, is common. In parallel with the qPCR approach, a next generation sequencing analysis of 16S rRNA genes identified that cyanobacterial assemblage associated with zooplankton guts was dominated by the non-diazotrophic unicellular phylotypes Synechococcus (56%) and Prochlorococcus (26%). However, in two separate calanoid copepod samples, two unicellular diazotrophs Candidatus Atelocyanobacterium thalassa (UCYN-A) and Crocosphaera watsonii (UCYN-B) were present, respectively, as a small component of cyanobacterial assemblages (<2%). This study represents the first evidence of consumption of DDAs, Trichodesmium, and unicellular cyanobacteria by calanoid copepods in an area of the WTNA known for high carbon export. These diazotroph populations are quantitatively important in the global N budget, widespread and hence, the next step is to accurately quantify grazing. Nonetheless, these results highlight a direct pathway of diazotrophic N into the food web and have important implications for biogeochemical cycles, particularly oligotrophic regions where N2 fixation is the main source of new nitrogen.

Distinct ecological niches of marine symbiotic N2 -fixing cyanobacterium Candidatus Atelocyanobacterium thalassa sublineages.

A recently described symbiosis between the metabolically streamlined nitrogen-fixing cyanobacterium UCYN-A and a single-celled eukaryote prymnesiophyte alga is widely distributed throughout tropical and subtropical marine waters, and is thought to contribute significantly to nitrogen fixation in these regions. Several UCYN-A sublineages have been defined based on UCYN-A nitrogenase (nifH) sequences. Due to the low abundances of UCYN-A in the global oceans, currently existing molecular techniques are limited for detecting and quantifying these organisms. A targeted approach is needed to adequately characterize the diversity of this important marine cyanobacterium, and to advance understanding of its ecological importance. We present findings on the distribution of UCYN-A sublineages based on high throughput sequencing of UCYN-A nifH PCR amplicons from 78 samples distributed throughout many major oceanic provinces. These UCYN-A nifH fragments were used to define oligotypes, alternative taxonomic units defined by nucleotide positions with high variability. The data set was dominated by a single oligotype associated with the UCYN-A1 sublineage, consistent with previous observations of relatively high abundances in tropical and subtropical regions. However, this analysis also revealed for the first time the widespread distribution of the UCYN-A3 sublineage in oligotrophic waters. Furthermore, distinct assemblages of UCYN-A oligotypes were found in oligotrophic and coastally influenced waters. This unique data set provides a framework for determining the environmental controls on UCYN-A distributions and the ecological importance of the different sublineages.

Cyanobacterial symbionts diverged in the late Cretaceous towards lineage-specific nitrogen fixation factories in single-celled phytoplankton.

The unicellular cyanobacterium UCYN-A, one of the major contributors to nitrogen fixation in the open ocean, lives in symbiosis with single-celled phytoplankton. UCYN-A includes several closely related lineages whose partner fidelity, genome-wide expression and time of evolutionary divergence remain to be resolved. Here we detect and distinguish UCYN-A1 and UCYN-A2 lineages in symbiosis with two distinct prymnesiophyte partners in the South Atlantic Ocean. Both symbiotic systems are lineage specific and differ in the number of UCYN-A cells involved. Our analyses infer a streamlined genome expression towards nitrogen fixation in both UCYN-A lineages. Comparative genomics reveal a strong purifying selection in UCYN-A1 and UCYN-A2 with a diversification process ∼91 Myr ago, in the late Cretaceous, after the low-nutrient regime period occurred during the Jurassic. These findings suggest that UCYN-A diversified in a co-evolutionary process, wherein their prymnesiophyte partners acted as a barrier driving an allopatric speciation of extant UCYN-A lineages.

High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation.

Australia's tropical waters represent predicted 'hotspots' for nitrogen (N2) fixation based on empirical and modelled data. However, the identity, activity and ecology of diazotrophs within this region are virtually unknown. By coupling DNA and cDNA sequencing of nitrogenase genes (nifH) with size-fractionated N2 fixation rate measurements, we elucidated diazotroph dynamics across the shelf region of the Arafura and Timor Seas (ATS) and oceanic Coral Sea during Austral spring and winter. During spring, Trichodesmium dominated ATS assemblages, comprising 60% of nifH DNA sequences, while Candidatus Atelocyanobacterium thalassa (UCYN-A) comprised 42% in the Coral Sea. In contrast, during winter the relative abundance of heterotrophic unicellular diazotrophs (δ-proteobacteria and γ-24774A11) increased in both regions, concomitant with a marked decline in UCYN-A sequences, whereby this clade effectively disappeared in the Coral Sea. Conservative estimates of N2 fixation rates ranged from <1 to 91 nmol l(-1) day(-1), and size fractionation indicated that unicellular organisms dominated N2 fixation during both spring and winter, but average unicellular rates were up to 10-fold higher in winter than in spring. Relative abundances of UCYN-A1 and γ-24774A11 nifH transcripts negatively correlated to silicate and phosphate, suggesting an affinity for oligotrophy. Our results indicate that Australia's tropical waters are indeed hotspots for N2 fixation and that regional physicochemical characteristics drive differential contributions of cyanobacterial and heterotrophic phylotypes to N2 fixation.

Global distribution and vertical patterns of a prymnesiophyte-cyanobacteria obligate symbiosis.

A marine symbiosis has been recently discovered between prymnesiophyte species and the unicellular diazotrophic cyanobacterium UCYN-A. At least two different UCYN-A phylotypes exist, the clade UCYN-A1 in symbiosis with an uncultured small prymnesiophyte and the clade UCYN-A2 in symbiosis with the larger Braarudosphaera bigelowii. We targeted the prymnesiophyte-UCYN-A1 symbiosis by double CARD-FISH (catalyzed reporter deposition-fluorescence in situ hybridization) and analyzed its abundance in surface samples from the MALASPINA circumnavigation expedition. Our use of a specific probe for the prymnesiophyte partner allowed us to verify that this algal species virtually always carried the UCYN-A symbiont, indicating that the association was also obligate for the host. The prymnesiophyte-UCYN-A1 symbiosis was detected in all ocean basins, displaying a patchy distribution with abundances (up to 500 cells ml(-1)) that could vary orders of magnitude. Additional vertical profiles taken at the NE Atlantic showed that this symbiosis occupied the upper water column and disappeared towards the Deep Chlorophyll Maximum, where the biomass of the prymnesiophyte assemblage peaked. Moreover, sequences of both prymnesiophyte partners were searched within a large 18S rDNA metabarcoding data set from the Tara-Oceans expedition around the world. This sequence-based analysis supported the patchy distribution of the UCYN-A1 host observed by CARD-FISH and highlighted an unexpected homogeneous distribution (at low relative abundance) of B. bigelowii in the open ocean. Our results demonstrate that partners are always in symbiosis in nature and show contrasted ecological patterns of the two related lineages.

Prokaryotic and diazotrophic population dynamics within a large oligotrophic inverse estuary

The ecology of microbial assemblages inhabiting classical (positive) estuaries has been well documented. However, we know relatively little about the microbial ecology of inverse (negative) estuaries, which exhibit different physical and hydrodynamic properties, including oligotrophy and hypersalinity. We investigated the dynamics of bacterioplankton communities in Spencer Gulf, an inverse estuary in temperate South Australia. We characterised patterns in the overall diversity and composition of the resident microbial assemblage, and tested the hypothesis that pelagic nitrogen-fixing bacteria (diazotrophs) could be an important functional group in the nutrient limited waters of the region. Prokaryotic and diazotrophic communities were evaluated using 16S ribosomal DNA and nifH amplicon tag pyrosequencing, respectively. Significant heterogeneity in microbial community composition and diazotrophic population structure was observed, which was driven by shifts in the relative importance of temperate vs. subtropical and oceanic vs. coastal ecotypes of Cyanobacteria throughout the inverse estuary. The globally significant unicellular cyanobacterium, UCYN-A 'Candidatus Atelocyano-bacterium thalassa', was the dominant diazotrophic phylotype. Temperature, chlorophyll a and nitrogen availability were all significant drivers of bacterioplankton dynamics within the gulf. These results demonstrate the heterogeneous microbiology of inverse estuaries, indicating that specific abiotic and biotic characteristics select for discrete microbial communities, and that pelagic nitrogen fixation may be important in this temperate oligotrophic system.

Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophyte host.

Symbiotic interactions between nitrogen-fixing prokaryotes and photosynthetic eukaryotes are an integral part of biological nitrogen fixation at a global scale. One of these partnerships involves the cyanobacterium UCYN-A, which has been found in partnership with an uncultivated unicellular prymnesiophyte alga in open-ocean and coastal environments. Phylogenetic analysis of the UCYN-A nitrogenase gene (nifH) showed that the UCYN-A lineage is represented by three distinct clades, referred to herein as UCYN-A1, UCYN-A2 and UCYN-A3, which appear to have overlapping and distinct geographic distributions. The relevance of UCYN-A's genetic diversity to its symbiosis and ecology was explored through combining flow cytometric cell sorting and molecular techniques to determine the host identity, nifH expression patterns and host cell size of one newly discovered clade, UCYN-A2, at a coastal site. UCYN-A2 nifH expression peaked during daylight hours, which is consistent with expression patterns of the UCYN-A1 clade in the open ocean. However, the cell size of the UCYN-A2 host was significantly larger than UCYN-A1 and host, suggesting adaptation to different environmental conditions. Like the UCYN-A1 host, the UCYN-A2 host was closely related to the genus Braarudosphaera; however, the UCYN-A1 and UCYN-A2 host rRNA sequences clustered into two distinct clades suggesting co-evolution of symbiont and host.

Distribution of a consortium between unicellular algae and the N2 fixing cyanobacterium UCYN‐A in the North Atlantic Ocean

The globally abundant, uncultured unicellular cyanobacterium UCYN-A was recently discovered living in association with a eukaryotic cell closely related to a prymnesiophyte. Here, we established a double CAtalysed Reporter Deposition-Fluorescence In Situ Hybridization (CARD-FISH) approach to identify both partners and provided quantitative information on their distribution and abundance across distinct water masses along a transect in the North Atlantic Ocean. The N2 fixation activity coincided with the detection of UCYN-A cells and was only observed in oligotrophic (< 0.067 NO3(-) μM and < 0.04 PO4(3-) μM) and warm (> 18°C) surface waters. Parallel 16S ribosomal RNA gene analyses among unicellular diazotrophs indicated that only UCYN-A cells were present. UCYN-A cells were associated with an algal partner or non-associated using the double CARD-FISH approach. We demonstrated that UCYN-A cells living in association with Haptophyta were the dominant form (87.0 ± 6.1%), whereas non-associated UCYN-A cells represented only a minor fraction (5.2 ± 3.9%). Interestingly, UCYN-A cells were also detected living in association with unknown single-celled eukaryotes in small amounts (7.8 ± 5.2%), presumably Alveolata. The proposed ecological niche of UCYN-A as an oligotrophic, mesophilic and obligate symbiotic nitrogen-fixing microorganism is evident for the North Atlantic Ocean.

Dominance of unicellular cyanobacteria in the diazotrophic community in the Atlantic Ocean

The horizontal and vertical distribution of representatives of diazotrophic unicellular cyanobacteria was investigated in the subtropical northeast Atlantic Ocean (28.87 to 42.00°N; 9.01 to 20.02°W). Samples from stations encompassing different water conditions (from oceanic oligotrophic waters to upwelling areas and a temperature range of 13.1°C to 24.2°C) were size fractionated and analyzed for nifH by a nested polymerase chain reaction (PCR) and by tyramide signal amplification—fluorescence in situ hybridization (TSA‐FISH) using probe Nitro821. In samples from the surface, mixed‐layer depth, and deep chlorophyll maximum waters, most (&gt; 50%) of the nifH recovered was from the 0.2–3 µm fraction and was consistent with TSA‐FISH counts. The &lt; 3 µm Nitro821‐positive cells were more abundant than the larger cells, and the proportion of single cells was larger than that associated with particulate matter or with larger cells. Phylogenetic analysis of representative samples revealed that most of the sequences belong to diazotrophic unicellular cyanobacteria Group A (UCYN‐A or Candidatus Atelocyanobacterium thalassa). N2 fixation in the 0.2–3 µm fraction, putatively representing the activity of UCYN‐A, contributed more than 50% of the total N2 fixation. There was a positive relationship of this putative UCYN‐A abundance and activity with temperature, and a negative relationship with dissolved O2. The dominance of these putative UCYN‐A organisms in nitrate‐rich upwelling filament regions suggests that the activity of this group of organisms may not be strongly controlled by the availability of fixed N.

Discovery of an Endosymbiotic Nitrogen-Fixing Cyanobacterium UCYN-A in Braarudosphaera bigelowii (Prymnesiophyceae)

Braarudosphaera bigelowii (Prymnesiophyceae) is a coastal coccolithophore with a long fossil record, extending back to the late Cretaceous (ca. 100 Ma). A recent study revealed close phylogenetic relationships between B. bigelowii, Chrysochromulina parkeae (Prymnesiophyceae), and a prymnesiophyte that forms a symbiotic association with the nitrogen-fixing cyanobacterium UCYN-A. In order to further examine these relationships, we conducted transmission electron microscopic and molecular phylogenetic studies of B. bigelowii. TEM studies showed that, in addition to organelles, such as the nucleus, chloroplasts and mitochondria, B. bigelowii contains one or two spheroid bodies with internal lamellae. In the 18S rDNA tree of the Prymnesiophyceae, C. parkeae fell within the B. bigelowii clade, and was close to B. bigelowii Genotype III (99.89% similarity). Plastid 16S rDNA sequences obtained from B. bigelowii were close to the unidentified sequences from the oligotrophic SE Pacific Ocean (e.g. HM133411) (99.86% similarity). Bacterial16S rDNA sequences obtained from B. bigelowii were identical to the UCYN-A sequence AY621693 from Arabian Sea, and fell in the UCYN-A clade. From these results, we suggest that; 1) C. parkeae is the alternate life cycle stage of B. bigelowii sensu stricto or that of a sibling species of B. bigelowii, and 2) the spheroid body of B. bigelowii originated from endosymbiosis of the nitrogen-fixing cyanobacterium UCYN-A.

Unicellular Cyanobacterium Symbiotic with a Single-Celled Eukaryotic Alga

Symbioses between nitrogen (N)(2)-fixing prokaryotes and photosynthetic eukaryotes are important for nitrogen acquisition in N-limited environments. Recently, a widely distributed planktonic uncultured nitrogen-fixing cyanobacterium (UCYN-A) was found to have unprecedented genome reduction, including the lack of oxygen-evolving photosystem II and the tricarboxylic acid cycle, which suggested partnership in a symbiosis. We showed that UCYN-A has a symbiotic association with a unicellular prymnesiophyte, closely related to calcifying taxa present in the fossil record. The partnership is mutualistic, because the prymnesiophyte receives fixed N in exchange for transferring fixed carbon to UCYN-A. This unusual partnership between a cyanobacterium and a unicellular alga is a model for symbiosis and is analogous to plastid and organismal evolution, and if calcifying, may have important implications for past and present oceanic N(2) fixation.