Mapping spatial and temporal variation of seafloor organic matter Δ14C and δ13C in the Northern Gulf of Mexico following the Deepwater Horizon Oil Spill.

Following the Deepwater Horizon oil spill of 2010, large amounts of biodegraded oil (petrocarbon) sank to the seafloor. Our objectives were to 1) determine post-spill isotopic values as the sediments approached a new baseline and 2) track the recovery of affected sediments. Sediment organic carbon δ13C and Δ14C reached a post-spill baseline averaging -21.2 ± 0.9‰ (n = 129) and -220 ± 66‰ (n = 95). Spatial variations in seafloor organic carbon baseline isotopic values, 13C and 14C, were influenced by river discharge and hydrocarbon seepage, respectively. Inverse Distance Weighting of surface sediment Δ14C values away from seep sites showed a 50% decrease in the total mass of petrocarbon, from 2010 to 2014. We estimated a rate of loss of -2 × 109 g of petrocarbon-C/year, 2-11% of the degradation rates in surface slicks. Despite the observed recovery in sediments, lingering residual material in the surface sediments was evident seven years following the blowout.

The great Atlantic Sargassum belt.

Pelagic Sargassum is abundant in the Sargasso Sea, but a recurrent great Atlantic Sargassum belt (GASB) has been observed in satellite imagery since 2011, often extending from West Africa to the Gulf of Mexico. In June 2018, the 8850-kilometer GASB contained >20 million metric tons of Sargassum biomass. The spatial distribution of the GASB is mostly driven by ocean circulation. The bloom of 2011 might be a result of Amazon River discharge in previous years, but recent increases and interannual variability after 2011 appear to be driven by upwelling off West Africa during boreal winter and by Amazon River discharge during spring and summer, indicating a possible regime shift and raising the possibility that recurrent blooms in the tropical Atlantic and Caribbean Sea may become the new norm.

Petrocarbon evolution: Ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico.

Hydrocarbons released during the Deepwater Horizon (DWH) oil spill weathered due to exposure to oxygen, light, and microbes. During weathering, the hydrocarbons' reactivity and lability was altered, but it remained identifiable as "petrocarbon" due to its retention of the distinctive isotope signatures (14C and 13C) of petroleum. Relative to the initial estimates of the quantity of oil-residue deposited in Gulf sediments based on 2010-2011 data, the overall coverage and quantity of the fossil carbon on the seafloor has been attenuated. To analyze recovery of oil contaminated deep-sea sediments in the northern Gulf of Mexico we tracked the carbon isotopic composition (13C and 14C, radiocarbon) of bulk sedimentary organic carbon through time at 4 sites. Using ramped pyrolysis/oxidation, we determined the thermochemical stability of sediment organic matter at 5 sites, two of these in time series. There were clear differences between crude oil (which decomposed at a lower temperature during ramped oxidation), natural hydrocarbon seep sediment (decomposing at a higher temperature; Δ14C = -912‰) and our control site (decomposing at a moderate temperature; Δ14C = -189‰), in both the stability (ability to withstand ramped temperatures in oxic conditions) and carbon isotope signatures. We observed recovery toward our control site bulk Δ14C composition at sites further from the wellhead in ~4 years, whereas sites in closer proximity had longer recovery times. The thermographs also indicated temporal changes in the composition of contaminated sediment, with shifts towards higher temperature CO2 evolution over time at a site near the wellhead, and loss of higher temperature CO2 peaks at a more distant site.

Piecewise Structural Equation Model (SEM) Disentangles the Environmental Conditions Favoring Diatom Diazotroph Associations (DDAs) in the Western Tropical North Atlantic (WTNA).

Diatom diazotroph associations (DDAs) are important components in the world's oceans, especially in the western tropical north Atlantic (WTNA), where blooms have a significant impact on carbon and nitrogen cycling. However, drivers of their abundances and distribution patterns remain unknown. Here, we examined abundance and distribution patterns for two DDA populations in relation to the Amazon River (AR) plume in the WTNA. Quantitative PCR assays, targeting two DDAs (het-1 and het-2) by their symbiont's nifH gene, served as input in a piecewise structural equation model (SEM). Collections were made during high (spring 2010) and low (fall 2011) flow discharges of the AR. The distributions of dissolved nutrients, chlorophyll-a, and DDAs showed coherent patterns indicative of areas influenced by the AR. A symbiotic Hemiaulus hauckii-Richelia (het-2) bloom (>106 cells L-1) occurred during higher discharge of the AR and was coincident with mesohaline to oceanic (30-35) sea surface salinities (SSS), and regions devoid of dissolved inorganic nitrogen (DIN), low concentrations of both DIP (>0.1 µmol L-1) and Si (>1.0 µmol L-1). The Richelia (het-1) associated with Rhizosolenia was only present in 2010 and at lower densities (10-1.76 × 105nifH copies L-1) than het-2 and limited to regions of oceanic SSS (>36). The het-2 symbiont detected in 2011 was associated with H. membranaceus (>103nifH copies L-1) and were restricted to regions with mesohaline SSS (31.8-34.3), immeasurable DIN, moderate DIP (0.1-0.60 µmol L-1) and higher Si (4.19-22.1 µmol L-1). The piecewise SEM identified a profound direct negative effect of turbidity on the het-2 abundance in spring 2010, while DIP and water turbidity had a more positive influence in fall 2011, corroborating our observations of DDAs at subsurface maximas. We also found a striking difference in the influence of salinity on DDA symbionts suggesting a niche differentiation and preferences in oceanic and mesohaline salinities by het-1 and het-2, respectively. The use of the piecewise SEM to disentangle the complex and concomitant hydrography of the WTNA acting on two biogeochemically relevant populations was novel and underscores its use to predict conditions favoring abundance and distributions of microbial populations.

Patterns of Transcript Abundance of Eukaryotic Biogeochemically-Relevant Genes in the Amazon River Plume.

The Amazon River has the largest discharge of all rivers on Earth, and its complex plume system fuels a wide array of biogeochemical processes, across a large area of the western tropical North Atlantic. The plume thus stimulates microbial processes affecting carbon sequestration and nutrient cycles at a global scale. Chromosomal gene expression patterns of the 2.0 to 156 µm size-fraction eukaryotic microbial community were investigated in the Amazon River Plume, generating a robust dataset (more than 100 million mRNA sequences) that depicts the metabolic capabilities and interactions among the eukaryotic microbes. Combining classical oceanographic field measurements with metatranscriptomics yielded characterization of the hydrographic conditions simultaneous with a quantification of transcriptional activity and identity of the community. We highlight the patterns of eukaryotic gene expression for 31 biogeochemically significant gene targets hypothesized to be valuable within forecasting models. An advantage to this targeted approach is that the database of reference sequences used to identify the target genes was selectively constructed and highly curated optimizing taxonomic coverage, throughput, and the accuracy of annotations. A coastal diatom bloom highly expressed nitrate transporters and carbonic anhydrase presumably to support high growth rates and enhance uptake of low levels of dissolved nitrate and CO2. Diatom-diazotroph association (DDA: diatoms with nitrogen fixing symbionts) blooms were common when surface salinity was mesohaline and dissolved nitrate concentrations were below detection, and hence did not show evidence of nitrate utilization, suggesting they relied on ammonium transporters to aquire recently fixed nitrogen. These DDA blooms in the outer plume had rapid turnover of the photosystem D1 protein presumably caused by photodegradation under increased light penetration in clearer waters, and increased expression of silicon transporters as silicon became limiting. Expression of these genes, including carbonic anhydrase and transporters for nitrate and phosphate, were found to reflect the physiological status and biogeochemistry of river plume environments. These relatively stable patterns of eukaryotic transcript abundance occurred over modest spatiotemporal scales, with similarity observed in sample duplicates collected up to 2.45 km in space and 120 minutes in time. These results confirm the use of metatranscriptomics as a valuable tool to understand and predict microbial community function.

Upward nitrate transport by phytoplankton in oceanic waters: balancing nutrient budgets in oligotrophic seas.

In oceanic subtropical gyres, primary producers are numerically dominated by small (1-5 µm diameter) pro- and eukaryotic cells that primarily utilize recycled nutrients produced by rapid grazing turnover in a highly efficient microbial loop. Continuous losses of nitrogen (N) to depth by sinking, either as single cells, aggregates or fecal pellets, are balanced by both nitrate inputs at the base of the euphotic zone and N2-fixation. This input of new N to balance export losses (the biological pump) is a fundamental aspect of N cycling and central to understanding carbon fluxes in the ocean. In the Pacific Ocean, detailed N budgets at the time-series station HOT require upward transport of nitrate from the nutricline (80-100 m) into the surface layer (∼0-40 m) to balance productivity and export needs. However, concentration gradients are negligible and cannot support the fluxes. Physical processes can inject nitrate into the base of the euphotic zone, but the mechanisms for transporting this nitrate into the surface layer across many 10s of m in highly stratified systems are unknown. In these seas, vertical migration by the very largest (10(2)-10(3) µm diameter) phytoplankton is common as a survival strategy to obtain N from sub-euphotic zone depths. This vertical migration is driven by buoyancy changes rather than by flagellated movement and can provide upward N transport as nitrate (mM concentrations) in the cells. However, the contribution of vertical migration to nitrate transport has been difficult to quantify over the required basin scales. In this study, we use towed optical systems and isotopic tracers to show that migrating diatom (Rhizosolenia) mats are widespread in the N. Pacific Ocean from 140°W to 175°E and together with other migrating phytoplankton (Ethmodiscus, Halosphaera, Pyrocystis, and solitary Rhizosolenia) can mediate time-averaged transport of N (235 µmol N m(-2) d(-1)) equivalent to eddy nitrate injections (242 µmol NO3 (-) m(-2) d(-1)). This upward biotic transport can close N budgets in the upper 250 m of the central Pacific Ocean and together with diazotrophy creates a surface zone where biological nutrient inputs rather than physical processes dominate the new N flux. In addition to these numerically rare large migrators, there is evidence in the literature of ascending behavior in small phytoplankton that could contribute to upward flux as well. Although passive downward movement has dominated models of phytoplankton flux, there is now sufficient evidence to require a rethinking of this paradigm. Quantifying these fluxes is a challenge for the future and requires a reexamination of individual phytoplankton sinking rates as well as methods for capturing and enumerating ascending phytoplankton in the sea.

The marine nitrogen cycle: recent discoveries, uncertainties and the potential relevance of climate change.

The ocean's nitrogen cycle is driven by complex microbial transformations, including nitrogen fixation, assimilation, nitrification, anammox and denitrification. Dinitrogen is the most abundant form of nitrogen in sea water but only accessible by nitrogen-fixing microbes. Denitrification and nitrification are both regulated by oxygen concentrations and potentially produce nitrous oxide (N2O), a climate-relevant atmospheric trace gas. The world's oceans, including the coastal areas and upwelling areas, contribute about 30 per cent to the atmospheric N2O budget and are, therefore, a major source of this gas to the atmosphere. Human activities now add more nitrogen to the environment than is naturally fixed. More than half of the nitrogen reaches the coastal ocean via river input and atmospheric deposition, of which the latter affects even remote oceanic regions. A nitrogen budget for the coastal and open ocean, where inputs and outputs match rather well, is presented. Furthermore, predicted climate change will impact the expansion of the oceans' oxygen minimum zones, the productivity of surface waters and presumably other microbial processes, with unpredictable consequences for the cycling of nitrogen. Nitrogen cycling is closely intertwined with that of carbon, phosphorous and other biologically important elements via biological stoichiometric requirements. This linkage implies that human alterations of nitrogen cycling are likely to have major consequences for other biogeochemical processes and ecosystem functions and services.

The distribution of thiamin and pyridoxine in the western tropical North Atlantic Amazon River plume.

B-vitamins are recognized as essential organic growth factors for many organisms, although little is known about their abundance and distribution in marine ecosystems. Despite their metabolic functions regulating important enzymatic reactions, the methodology to directly measure different B-vitamins in aquatic environments has only recently been developed. Here, we present the first direct measurements of two B-vitamins, thiamin (B1), and pyridoxine (B6), in the Amazon River plume-influenced western tropical North Atlantic (WTNA) Ocean, an area known to have high productivity, carbon (C) and dinitrogen (N2) fixation, and C sequestration. The vitamins B1 and B6 ranged in concentrations from undetectable to 230 and 40 pM, respectively. Significantly higher concentrations were measured in the surface plume water at some stations and variation with salinity was observed, suggesting a possible riverine influence on those B-vitamins. The influences of vitamins B1 and B6 on biogeochemical processes such as C and N2 fixation were investigated using a linear regression model that indicated the availability of those organic factors could affect these rates in the WTNA. In fact, significant increases in C fixation and N2 fixation were observed with increasing vitamin B1 concentrations at some low and mesohaline stations (stations 9.1 and 1; p value <0.017 and <0.03, respectively). N2 fixation was also found to have a significant positive correlation with B1 concentrations at station 1 (p value 0.029), as well as vitamin B6 at station 9.1 (p value <0.017). This work suggests that there can be a dynamic interplay between essential biogeochemical rates (C and N2 fixation) and B-vitamins, drawing attention to potential roles of B-vitamins in ecosystem dynamics, community structure, and global biogeochemistry.

Nitrogenase (nifH) gene expression in diazotrophic cyanobacteria in the Tropical North Atlantic in response to nutrient amendments.

The Tropical North Atlantic (TNAtl) plays a critical role in the marine nitrogen cycle, as it supports high rates of biological nitrogen (N(2)) fixation, yet it is unclear whether this process is limited by the availability of iron (Fe), phosphate (P) or is co-limited by both. In order to investigate the impact of nutrient limitation on the N(2)-fixing microorganisms (diazotrophs) in the TNAtl, trace metal clean nutrient amendment experiments were conducted, and the expression of nitrogenase (nifH) in cyanobacterial diazotrophs in response to the addition of Fe, P, or Fe+P was measured using quantitative PCR. To provide context, N(2) fixation rates associated with the <10 µm community and diel nifH expression in natural cyanobacterial populations were measured. In the western TNAtl, nifH expression in Crocosphaera, Trichodesmium, and Richelia was stimulated by Fe and Fe+P additions, but not by P, implying that diazotrophs may be Fe-limited in this region. In the eastern TNAtl, nifH expression in unicellular cyanobacteria UCYN-A and Crocosphaera was stimulated by P, implying P-limitation. In equatorial waters, nifH expression in Trichodesmium was highest in Fe+P treatments, implying co-limitation in this region. Nutrient additions did not measurably stimulate N(2) fixation rates in the <10 µm fraction in most of the experiments, even when upregulation of nifH expression was evident. These results demonstrate the utility of using gene expression to investigate the physiological state of natural populations of microorganisms, while underscoring the complexity of nutrient limitation on diazotrophy, and providing evidence that diazotroph populations are slow to respond to the addition of limiting nutrients and may be limited by different nutrients on basin-wide spatial scales. This has important implications for our current understanding of controls on N(2) fixation in the TNAtl and may partially explain why it appears to be intermittently limited by Fe, P, or both.

Analogous nutrient limitations in unicellular diazotrophs and Prochlorococcus in the South Pacific Ocean.

Growth limitation of phytoplankton and unicellular nitrogen (N(2)) fixers (diazotrophs) were investigated in the oligotrophic Western South Pacific Ocean. Based on change in abundances of nifH or 23S rRNA gene copies during nutrient-enrichment experiments, the factors limiting net growth of the unicellular diazotrophs UCYN-A (Group A), Crocosphaera watsonii, γ-Proteobacterium 24774A11, and the non-diazotrophic picocyanobacterium Prochlorococcus, varied within the region. At the westernmost stations, numbers were enhanced by organic carbon added as simple sugars, a combination of iron and an organic chelator, or iron added with phosphate. At stations nearest the equator, the nutrient-limiting growth was not apparent. Maximum net growth rates for UCYN-A, C. watsonii and γ-24774A11 were 0.19, 0.61 and 0.52 d(-1), respectively, which are the first known empirical growth rates reported for the uncultivated UCYN-A and the γ-24774A11. The addition of N enhanced total phytoplankton biomass up to 5-fold, and the non-N(2)-fixing Synechococcus was among the groups that responded favorably to N addition. Nitrogen was the major nutrient-limiting phytoplankton biomass in the Western South Pacific Ocean, while availability of organic carbon or iron and organic chelator appear to limit abundances of unicellular diazotrophs. Lack of phytoplankton response to nutrient additions in the Pacific warm pool waters suggests diazotroph growth in this area is controlled by different factors than in the higher latitudes, which may partially explain previously observed variability in community composition in the region.

Effects of herbivory, nutrients, and reef protection on algal proliferation and coral growth on a tropical reef.

Maintaining coral reef resilience against increasing anthropogenic disturbance is critical for effective reef management. Resilience is partially determined by how processes, such as herbivory and nutrient supply, affect coral recovery versus macroalgal proliferation following disturbances. However, the relative effects of herbivory versus nutrient enrichment on algal proliferation remain debated. Here, we manipulated herbivory and nutrients on a coral-dominated reef protected from fishing, and on an adjacent macroalgal-dominated reef subject to fishing and riverine discharge, over 152 days. On both reefs, herbivore exclusion increased total and upright macroalgal cover by 9-46 times, upright macroalgal biomass by 23-84 times, and cyanobacteria cover by 0-27 times, but decreased cover of encrusting coralline algae by 46-100% and short turf algae by 14-39%. In contrast, nutrient enrichment had no effect on algal proliferation, but suppressed cover of total macroalgae (by 33-42%) and cyanobacteria (by 71% on the protected reef) when herbivores were excluded. Herbivore exclusion, but not nutrient enrichment, also increased sediment accumulation, suggesting a strong link between herbivory, macroalgal growth, and sediment retention. Growth rates of the corals Porites cylindrica and Acropora millepora were 30-35% greater on the protected versus fished reef, but nutrient and herbivore manipulations within a site did not affect coral growth. Cumulatively, these data suggest that herbivory rather than eutrophication plays the dominant role in mediating macroalgal proliferation, that macroalgae trap sediments that may further suppress herbivory and enhance macroalgal dominance, and that corals are relatively resistant to damage from some macroalgae but are significantly impacted by ambient reef condition.

Comment on "A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico".

Kessler et al. (Reports, 21 January 2011, p. 312) reported that methane released from the 2010 Deepwater Horizon blowout, approximately 40% of the total hydrocarbon discharge, was consumed quantitatively by methanotrophic bacteria in Gulf of Mexico deep waters over a 4-month period. We find the evidence explicitly linking observed oxygen anomalies to methane consumption ambiguous and extension of these observations to hydrate-derived methane climate forcing premature.

Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N2 fixation in the tropical Atlantic Ocean.

The abundances of six N2-fixing cyanobacterial phylotypes were profiled at 22 stations across the tropical Atlantic Ocean during June 2006, and used to model the contribution of the diazotrophs to N2 fixation. Diazotroph abundances were measured by targeting the nifH gene of Trichodesmium, unicellular groups A, B, C (UCYN-A, UCYN-B and UCYN-C), and diatom-cyanobiont symbioses Hemiaulus-Richelia, Rhizosolenia-Richelia and Chaetoceros-Calothrix. West to east gradients in temperature, salinity and nutrients [NO3⁻ + NO2⁻, PO4³â», Si(OH)4] showed the influence of the Amazon River plume and its effect on the distributions of the diazotrophs. Trichodesmium accounted for more than 93% of all nifH genes detected, dominated the warmer waters of the western Atlantic, and was the only diazotroph detected at the equatorial upwelling station. UCYN-A was the next most abundant (> 5% of all nifH genes) and dominated the cooler waters of the eastern Atlantic near the Cape Verde Islands. UCYN-C was found at a single depth (200 m) of high salinity and low temperature and nutrients, whereas UCYN-B cells were widespread but in very low abundance (6.1 × 10¹ ± 4.6 × 10² gene copies l⁻¹). The diatom-cyanobionts were observed primarily in the western Atlantic within or near the high Si(OH)4 input of the Amazon River plume. Overall, highest diazotroph abundances were observed at the surface and declined with depth, except for some subsurface peaks in Trichodesmium, UCYN-B and UCYN-A. Modelled contributions of Trichodesmium, UCYN-B and UCYN-A to total N2 fixation suggested that Trichodesmium had the largest input, except for the potential of UCYN-A at the Cape Verde Islands.

Spatial patterns and light-driven variation of microbial population gene expression in surface waters of the oligotrophic open ocean.

Because bacterioplankton production rates do not vary strongly across vast expanses of the ocean, it is unclear how variability in community structure corresponds with functional variability in the open ocean. We surveyed community transcript functional profiles at eight locations in the open ocean, in both the light and in the dark, using the genomic subsystems approach, to understand variability in gene expression patterns in surface waters. Metatranscriptomes from geographically distinct areas and collected during the day and night shared a large proportion of metabolic functional similarity (74%) at the finest metabolic resolution possible. The variability between metatranscriptomes could be explained by phylogenetic differences between libraries (Mantel test, P < 0.0001). Several key gene expression pathways, including Photosystem I, Photosystem II and ammonium uptake, demonstrated the most variability both geographically and between light and dark. Libraries were dominated by transcripts of the cyanobacterium Prochlorocococcus marinus, where most geographical and diel variability between metatranscriptomes reflected between-station differences in cyanobacterial phototrophic metabolism. Our results demonstrate that active genetic machinery in surface waters of the ocean is dominated by photosynthetic microorganisms and their site-to-site variability, while variability in the remainder of assemblages is dependent on local taxonomic composition.

Unicellular cyanobacterial distributions broaden the oceanic N2 fixation domain.

Nitrogen (N2)-fixing microorganisms (diazotrophs) are an important source of biologically available fixed N in terrestrial and aquatic ecosystems and control the productivity of oligotrophic ocean ecosystems. We found that two major groups of unicellular N2-fixing cyanobacteria (UCYN) have distinct spatial distributions that differ from those of Trichodesmium, the N2-fixing cyanobacterium previously considered to be the most important contributor to open-ocean N2 fixation. The distributions and activity of the two UCYN groups were separated as a function of depth, temperature, and water column density structure along an 8000-kilometer transect in the South Pacific Ocean. UCYN group A can be found at high abundances at substantially higher latitudes and deeper in subsurface ocean waters than Trichodesmium. These findings have implications for the geographic extent and magnitude of basin-scale oceanic N2 fixation rates.

Microbial community gene expression within colonies of the diazotroph, Trichodesmium, from the Southwest Pacific Ocean.

Trichodesmium are responsible for a large fraction of open ocean nitrogen fixation, and are often found in complex consortia of other microorganisms, including viruses, prokaryotes, microbial eukaryotes and metazoa. We applied a community gene expression (metatranscriptomic) approach to study the patterns of microbial gene utilization within colonies of Trichodesmium collected during a bloom in the Southwest Pacific Ocean in April 2007. The survey generated 5711-day and 5385-night putative mRNA reads. The majority of mRNAs were from the co-occurring microorganisms and not Trichodesmium, including other cyanobacteria, heterotrophic bacteria, eukaryotes and phage. Most transcripts did not share homology with proteins from cultivated microorganisms, but were similar to shotgun sequences and unannotated proteins from open ocean metagenomic surveys. Trichodesmium transcripts were mostly expressed photosynthesis, N(2) fixation and S-metabolism genes, whereas those in the co-occurring microorganisms were mostly involved in genetic information storage and processing. Detection of Trichodesmium genes involved in P uptake and As detoxification suggest that local enrichment of N through N(2) fixation may lead to a P-stress response. Although containing similar dominant transcripts to open ocean metatranscriptomes, the overall pattern of gene expression in Trichodesmium colonies was distinct from free-living pelagic assemblages. The identifiable genes expressed by Trichodesmium and closely associated microorganisms reflect the constraints of life in well-lit and nutrient-poor waters, with biosynthetic investment in nutrient acquisition and cell maintenance, which is in contrast to gene transcription by soil and coastal seawater microbial assemblages. The results provide insight into aggregate microbial communities in contrast to planktonic free-living assemblages that are the focus of other studies.

In situ transcriptomic analysis of the globally important keystone N2-fixing taxon Crocosphaera watsonii.

The diazotrophic cyanobacterium Crocosphaera watsonii supplies fixed nitrogen (N) to N-depleted surface waters of the tropical oceans, but the factors that determine its distribution and contribution to global N(2) fixation are not well constrained for natural populations. Despite the heterogeneity of the marine environment, the genome of C. watsonii is highly conserved in nucleotide sequence in contrast to sympatric planktonic cyanobacteria. We applied a whole assemblage shotgun transcript sequencing approach to samples collected from a bloom of C. watsonii observed in the South Pacific to understand the genomic mechanisms that may lead to high population densities. We obtained 999 C. watsonii transcript reads from two metatranscriptomes prepared from mixed assemblage RNA collected in the day and at night. The C. watsonii population had unexpectedly high transcription of hypothetical protein genes (31% of protein-encoding genes) and transposases (12%). Furthermore, genes were expressed that are necessary for living in the oligotrophic ocean, including the nitrogenase cluster and the iron-stress-induced protein A (isiA) that functions to protect photosystem I from high-light-induced damage. C. watsonii transcripts retrieved from metatranscriptomes at other locations in the southwest Pacific Ocean, station ALOHA and the equatorial Atlantic Ocean were similar in composition to those recovered in the enriched population. Quantitative PCR and quantitative reverse transcriptase PCR were used to confirm the high expression of these genes within the bloom, but transcription patterns varied at shallower and deeper horizons. These data represent the first transcript study of a rare individual microorganism in situ and provide insight into the mechanisms of genome diversification and the ecophysiology of natural populations of keystone organisms that are important in global nitrogen cycling.

Phylogenetic diversity of cyanobacterial narB genes from various marine habitats.

Nitrate, the most abundant combined, dissolved form of inorganic nitrogen in global oceans, is a common source of nitrogen (N) for phytoplankton including cyanobacteria. Using a nested polymerase chain reaction (PCR) method, the diversity of the cyanobacterial nitrate reductase gene, narB, was examined in plankton samples from a variety of marine habitats. A total of 480 narB gene fragment sequences were obtained from a coastal coral reef (Heron Island, Australia), open-ocean tropical and subtropical oceanic waters (Atlantic and Pacific Oceans) and a temperate N. Pacific Ocean site (34 degrees N, 129 degrees W). Phylogenetic analyses distinguished eight picocyanobacterial narB clades comprised of DNA sequences derived from the nutrient-replete coastal, nutrient-deplete pelagic and tidally influenced coral reef habitats. The phylogeny of recovered narB gene sequences was consistent with 16S rRNA and ITS sequence phylogenies, suggesting minimal horizontal gene transfer of the narB gene. Depending on sampled habitat, environmental narB sequence types segregated into three divisions: non-picocyanobacterial, coastal picocyanobacterial and open-ocean picocyanobacterial sequences. Using a reverse transcription PCR method, narB mRNA sequences were amplified from Heron Island samples, indicating that narB expression can be detected in environmental samples.