Modeling Selective Pressures on Phytoplankton in the Global Ocean

Our view of marine microbes is transforming, as culture-independent methods facilitate rapid characterization of microbial diversity. It is difficult to assimilate this information into our understanding of marine microbe ecology and evolution, because their distributions, traits, and genomes are shaped by forces that are complex and dynamic. Here we incorporate diverse forces—physical, biogeochemical, ecological, and mutational—into a global ocean model to study selective pressures on a simple trait in a widely distributed lineage of picophytoplankton: the nitrogen use abilities of Synechococcus and Prochlorococcus cyanobacteria. Some Prochlorococcus ecotypes have lost the ability to use nitrate, whereas their close relatives, marine Synechococcus, typically retain it. We impose mutations for the loss of nitrogen use abilities in modeled picophytoplankton, and ask: in which parts of the ocean are mutants most disadvantaged by losing the ability to use nitrate, and in which parts are they least disadvantaged? Our model predicts that this selective disadvantage is smallest for picophytoplankton that live in tropical regions where Prochlorococcus are abundant in the real ocean. Conversely, the selective disadvantage of losing the ability to use nitrate is larger for modeled picophytoplankton that live at higher latitudes, where Synechococcus are abundant. In regions where we expect Prochlorococcus and Synechococcus populations to cycle seasonally in the real ocean, we find that model ecotypes with seasonal population dynamics similar to Prochlorococcus are less disadvantaged by losing the ability to use nitrate than model ecotypes with seasonal population dynamics similar to Synechococcus. The model predictions for the selective advantage associated with nitrate use are broadly consistent with the distribution of this ability among marine picocyanobacteria, and at finer scales, can provide insights into interactions between temporally varying ocean processes and selective pressures that may be difficult or impossible to study by other means. More generally, and perhaps more importantly, this study introduces an approach for testing hypotheses about the processes that underlie genetic variation among marine microbes, embedded in the dynamic physical, chemical, and biological forces that generate and shape this diversity.     

Copper toxicity response influences mesotrophic Synechococcus community structure

Picocyanobacteria from the genus Synechococcus are ubiquitous in ocean waters. Their phylogenetic and genomic diversity suggests ecological niche differentiation, but the selective forces influencing this are not well defined. Marine picocyanobacteria are sensitive to Cu toxicity, so adaptations to this stress could represent a selective force within, and between, ‘species’, also known as clades. Here, we compared Cu stress responses in cultures and natural populations of marine Synechococcus from two co‐occurring major mesotrophic clades (I and IV). Using custom microarrays and proteomics to characterize expression responses to Cu in the lab and field, we found evidence for a general stress regulon in marine Synechococcus. However, the two clades also exhibited distinct responses to copper. The Clade I representative induced expression of genomic island genes in cultures and Southern California Bight populations, while the Clade IV representative downregulated Fe‐limitation proteins. Copper incubation experiments suggest that Clade IV populations may harbour stress‐tolerant subgroups, and thus fitness tradeoffs may govern Cu‐tolerant strain distributions. This work demonstrates that Synechococcus has distinct adaptive strategies to deal with Cu toxicity at both the clade and subclade level, implying that metal toxicity and stress response adaptations represent an important selective force for influencing diversity within marine Synechococcus populations.     

Genomic island genes in a coastal marine Synechococcus strain confer enhanced tolerance to copper and oxidative stress

Highly variable regions called genomic islands are found in the genomes of marine picocyanobacteria, and have been predicted to be involved in niche adaptation and the ecological success of these microbes. These picocyanobacteria are typically highly sensitive to copper stress and thus, increased copper tolerance could confer a selective advantage under some conditions seen in the marine environment. Through targeted gene inactivation of genomic island genes that were known to be upregulated in response to copper stress in Synechococcus sp. strain CC9311, we found two genes (sync_1495 and sync_1217) conferred tolerance to both methyl viologen and copper stress in culture. The prevalence of one gene, sync_1495, was then investigated in natural samples, and had a predictable temporal variability in abundance at a coastal monitoring site with higher abundance in winter months. Together, this shows that genomic island genes can confer an adaptive advantage to specific stresses in marine Synechococcus, and may help structure their population diversity.     

Diversity of Synechococcus at the Martha’s Vineyard Coastal Observatory: Insights from Culture Isolations,Clone Libraries,and Flow Cytometry

The cyanobacterium Synechococcus is a ubiquitous, important phytoplankter across the world’s oceans. A high degree of genetic diversity exists within the marine group, which likely contributes to its global success. Over 20 clades with different distribution patterns have been identified. However, we do not fully understand the environmental factors that control clade distributions. These factors are likely to change seasonally, especially in dynamic coastal systems. To investigate how coastal Synechococcus assemblages change temporally, we assessed the diversity of Synechococcus at the Martha’s Vineyard Coastal Observatory (MVCO) over three annual cycles with culture-dependent and independent approaches. We further investigated the abundance of both phycoerythrin (PE)-containing and phycocyanin (PC)-only Synechococcus with a flow cytometric setup that distinguishes PC-only Synechococcus from picoeukaryotes. We found that the Synechococcus assemblage at MVCO is diverse (13 different clades identified), but dominated by clade I representatives. Many clades were only isolated during late summer and fall, suggesting more favorable conditions for isolation at this time. PC-only strains from four different clades were isolated, but these cells were only detected by flow cytometry in a few samples over the time series, suggesting they are rare at this site. Within clade I, we identified four distinct subclades. The relative abundances of each subclade varied over the seasonal cycle, and the high Synechococcus cell concentration at MVCO may be maintained by the diversity found within this clade. This study highlights the need to understand how temporal aspects of the environment affect Synechococcus community structure and cell abundance.     

The microbiome associated with two Synechococcus ribotypes at different levels of ecological interaction

Planktonic cyanobacteria belonging to the genus Synechococcus are ubiquitously distributed in marine and fresh waters, substantially contributing to total carbon fixation on a global scale. While their ecological relevance is acknowledged, increasing resolution in molecular techniques allows disentangling cyanobacteria's role at the micro‐scale, where complex microbial interactions may drive the overall community assembly. The interplay between phylogenetically different Synechococcus clades and their associated bacterial communities can affect their ecological fate and susceptibility to protistan predation. In this study, we experimentally promoted different levels of ecological interaction by mixing two Synechococcus ribotypes (MW101C3 and LL) and their associated bacteria, with and without a nanoflagellate grazer (Poterioochromonas sp.) in laboratory cultures. The beta‐diversity of the Synechococcus‐associated microbiome in laboratory cultures indicated that the presence of the LL ribotype was the main factor determining community composition changes (41% of total variance), and prevailed over the effect of protistan predation (18% of total variance). Our outcomes also showed that species coexistence and predation may promote microbial diversity, thus highlighting the underrated ecological relevance of such micro‐scale factors.     

Phylogeography and pigment type diversity of Synechococcus cyanobacteria in surface waters of the northwestern pacific ocean

The widespread unicellular cyanobacteria Synechococcus are major contributors to global marine primary production. Here, we report their abundance, phylogenetic diversity (as assessed using the RNA polymerase gamma subunit gene rpoC1) and pigment diversity (as indirectly assessed using the laterally transferred cpeBA genes, encoding phycoerythrin‐I) in surface waters of the northwestern Pacific Ocean, sampled over nine distinct cruises (2008–2015). Abundance of Synechococcus was low in the subarctic ocean and South China Sea, intermediate in the western subtropical Pacific Ocean, and the highest in the Japan and East China seas. Clades I and II were by far the most abundant Synechococcus lineages, the former dominating in temperate cold waters and the latter in (sub)tropical waters. Clades III and VI were also fairly abundant in warm waters, but with a narrower distribution than clade II. One type of chromatic acclimater (3dA) largely dominated the Synechococcus communities in the subarctic ocean, while another (3dB) and/or cells with a fixed high phycourobilin to phycoerythrobilin ratio (pigment type 3c) predominated at mid and low latitudes. Altogether, our results suggest that the variety of pigment content found in most Synechococcus clades considerably extends the niches that they can colonize and therefore the whole genus habitat.     

Synechococcus: 3 billion years of global dominance

Cyanobacteria are among the most important primary producers on the Earth. However, the evolutionary forces driving cyanobacterial species diversity remain largely enigmatic due to both their distinction from macro‐organisms and an undersampling of sequenced genomes. Thus, we present a new genome of a Synechococcus‐like cyanobacterium from a novel evolutionary lineage. Further, we analyse all existing 16S rRNA sequences and genomes of Synechococcus‐like cyanobacteria. Chronograms showed extremely polyphyletic relationships in Synechococcus, which has not been observed in any other cyanobacteria. Moreover, most Synechococcus lineages bifurcated after the Great Oxidation Event, including the most abundant marine picoplankton lineage. Quantification of horizontal gene transfer among 70 cyanobacterial genomes revealed significant differences among studied genomes. Horizontal gene transfer levels were not correlated with ecology, genome size or phenotype, but were correlated with the age of divergence. All findings were synthetized into a novel model of cyanobacterial evolution, characterized by serial convergence of the features, that is multicellularity and ecology.     

Selection and Characterization of Cyanophage Resistance in Marine Synechococcus Strains

Marine viruses are an important component of the microbial food web, influencing microbial diversity and contributing to bacterial mortality rates. Resistance to cooccurring cyanophages has been reported for natural communities of Synechococcus spp.; however, little is known about the nature of this resistance. This study examined the patterns of infectivity among cyanophage isolates and unicellular marine cyanobacteria (Synechococcus spp.). We selected for phage-resistant Synechococcus mutants, examined the mechanisms of phage resistance, and determined the extent of cross-resistance to other phages. Four strains of Synechococcus spp. (WH7803, WH8018, WH8012, and WH8101) and 32 previously isolated cyanomyophages were used to select for phage resistance. Phage-resistant Synechococcus mutants were recovered from 50 of the 101 susceptible phage-host pairs, and 23 of these strains were further characterized. Adsorption kinetic assays indicate that resistance is likely due to changes in host receptor sites that limit viral attachment. Our results also suggest that receptor mutations conferring this resistance are diverse. Nevertheless, selection for resistance to one phage frequently resulted in cross-resistance to other phages. On average, phage-resistant Synechococcus strains became resistant to eight other cyanophages; however, there was no significant correlation between the genetic similarity of the phages (based on g20 sequences) and cross-resistance. Likewise, host Synechococcus DNA-dependent RNA polymerase (rpoC1) genotypes could not be used to predict sensitivities to phages. The potential for the rapid evolution of multiple phage resistance may influence the population dynamics and diversity of both Synechococcus and cyanophages in marine waters.     

The diversity of cyanomyovirus populations along a North–South Atlantic Ocean transect

Viruses that infect the marine cyanobacterium Prochlorococcus have the potential to impact the growth, productivity, diversity and abundance of their hosts. In this study, changes in the microdiversity of cyanomyoviruses were investigated in 10 environmental samples taken along a North–South Atlantic Ocean transect using a myoviral-specific PCR-sequencing approach. Phylogenetic analyses of 630 viral g20 clones from this study, with 786 published g20 sequences, revealed that myoviral populations in the Atlantic Ocean had higher diversity than previously reported, with several novel putative g20 clades. Some of these clades were detected throughout the Atlantic Ocean. Multivariate statistical analyses did not reveal any significant correlations between myoviral diversity and environmental parameters, although myoviral diversity appeared to be lowest in samples collected from the north and south of the transect where Prochlorococcus diversity was also lowest. The results were correlated to the abundance and diversity of the co-occurring Prochlorococcus and Synechococcus populations, but revealed no significant correlations to either of the two potential host genera. This study provides evidence that cyanophages have extremely high and variable diversity and are distributed over large areas of the Atlantic Ocean.     

Mechanisms of dominance: coexistence of picocyanobacterial genotypes in a freshwater ecosystem

The autotrophic picoplankton of the pelagic zone of the mesotrophic Lake Constance is dominated by phycoerythrin-rich unicellular cyanobacteria phylogenetically related to the marine Synechococcus and Prochlorococcus cluster. In Lake Constance, the abundance of picocyanobacteria shows a recurrent pattern of seasonal variations. Evidence of diverse subpopulations was obtained by electron-microscopic examination of natural water samples and isolated strains that unveiled different surface structures of picocyanobacteria. Further evidence was obtained by DNA analysis of 26 clonal isolates representing 12 different genotypes. Variations in light and nutrient supply revealed distinct abilities of the genetically different strains to cope with these stress situations. Furthermore, cultured heterotrophic nanoflagellates exhibited differential feeding preferences for certain Synechococcus strains. The findings imply that growth and loss rates of the natural cyanobacterial community may be influenced by its genetic composition. Phylogenetic analyses of isolated strains indicated that the physiological diversification of pelagic Synechococcus spp. has occurred during a recent adaptive radiation. An example for genetic mechanisms underlying physiological diversification is indicated by mobile DNA elements found in a Synechocystis strain also isolated from the pelagic zone of Lake Constance. The observations suggest that dominance of Synechococcus spp. was achieved by evolutionary adaptation and coexistence of numerous genotypes generating a physiologically highly diversified population. Received: 15 April 1999 / Accepted: 26 May 1999     

Marine Synechococcus isolates representing globally abundant genomic lineages demonstrate a unique evolutionary path of genome reduction without a decrease in GC content

Synechococcus, a genus of unicellular cyanobacteria found throughout the global surface ocean, is a large driver of Earth's carbon cycle. Developing a better understanding of its diversity and distributions is an ongoing effort in biological oceanography. Here, we introduce 12 new draft genomes of marine Synechococcus isolates spanning five clades and utilize ~100 environmental metagenomes largely sourced from the TARA Oceans project to assess the global distributions of the genomic lineages they and other reference genomes represent. We show that five newly provided clade-II isolates are by far the most representative of the recovered in situ populations (most ‘abundant’) and have biogeographic distributions distinct from previously available clade-II references. Additionally, these isolates form a subclade possessing the smallest genomes yet identified of the genus (2.14 ± 0.05Mbps; mean ± 1SD) while concurrently hosting some of the highest GC contents (60.67 ± 0.16%). This is in direct opposition to the pattern in Synechococcus’s nearest relative, Prochlorococcus – wherein decreasing genome size has coincided with a strong decrease in GC content – suggesting this new subclade of Synechococcus appears to have convergently undergone genomic reduction relative to the rest of the genus, but along a fundamentally different evolutionary trajectory.     

Examination of Genetic Relatedness of Marine Synechococcus spp. by Using Restriction Fragment Length Polymorphisms

The relatedness of several marine Synechococcus spp. was estimated by DNA hybridization. Strains isolated from various geographical locations and representing a diversity of DNA base compositions and phycobiliprotein profiles were compared by restriction fragment length polymorphisms for a number of genes. DNAs from two marine red algae and a cryptomonad alga (which exhibit a phycobiliprotein composition similar to that of the marine Synechococcus spp.) and Synechococcus strain PCC6301 (Anacystis nidulans) were also included in the comparison. Strains WH8008, WH8018, and WH7805 were shown to be very similar to one another, as were strains WH7802 and WH7803. Strains WH8110 and WH5701 were clearly unrelated to any of the other strains, and no marine Synechococcus isolate showed any similarity to the freshwater Synechococcus strain PCC6301 or the eucaryotic algae. The method is relatively straightforward and sensitive and uses a variety of basic molecular biology techniques. Its utility in ascertaining the genetic relatedness and diversity of marine Synechococcus spp. and possible extension to field studies are discussed.     

Rapid Diversification of Marine Picophytoplankton with Dissimilar Light-Harvesting Structures Inferred from Sequences of Prochlorococcus and Synechococcus (Cyanobacteria)

Cultured isolates of the unicellular planktonic cyanobacteria Prochlorococcus and marine Synechococcus belong to a single marine picophytoplankton clade. Within this clade, two deeply branching lineages of Prochlorococcus, two lineages of marine A Synechococcus and one lineage of marine B Synechococcus exhibit closely spaced divergence points with low bootstrap support. This pattern is consistent with a near-simultaneous diversification of marine lineages with divinyl chlorophyll b and phycobilisomes as photosynthetic antennae. Inferences from 16S ribosomal RNA sequences including data for 18 marine picophytoplankton clade members were congruent with results of psbB and petB and D sequence analyses focusing on five strains of Prochlorococcus and one strain of marine A Synechococcus. Third codon position and intergenic region nucleotide frequencies vary widely among members of the marine picophytoplankton group, suggesting that substitution biases differ among the lineages. Nonetheless, standard phylogenetic methods and newer algorithms insensitive to such biases did not recover different branching patterns within the group, and failed to cluster Prochlorococcus with chloroplasts or other chlorophyll b-containing prokaryotes. Prochlorococcus isolated from surface waters of stratified, oligotrophic ocean provinces predominate in a lineage exhibiting low G + C nucleotide frequencies at highly variable positions. Received: 18 January 1997 / Accepted: 18 May 1997     

East Tibetan Lakes Harbour Novel Clusters of Picocyanobacteria as Inferred from the 16S–23S rRNA Internal Transcribed Spacer Sequences

Planktonic picocyanobacteria abundance and diversity were investigated in nine lakes on the East Tibetan Plateau spanning a salinity gradient of 0.4–22.6 g l⁻¹. The investigation was conducted using epifluorescence microscopy (EFM) and terminal restriction fragment polymorphism analysis of 16S–23S rRNA internal transcribed spacer (ITS) PCR amplicons followed by sequence analyses of large ITS clone libraries of seven selected samples. EFM showed that picocyanobacteria comprised 7–19% of the total prokaryotic cells found in surface water. Most of the clones were classified into six clusters and grouped within the “picocyanobacterial clade”, which consists exclusively of freshwater Synechococcus. Four new phylogenetic clusters and one new subcluster of Synechococcus spp. were found, none of which are members of any known picocyanobacterial clusters. The new clusters and subcluster were the most abundant picocyanobacteria (about 96% of the sequences) in the samples collected. Sequence analyses indicated that members of the four new Synechococcus groups were only found in freshwater lakes (<1.0 g l⁻¹ of total dissolved solid), while members of the new subcluster were found in all the investigated Tibetan lakes, over a large salinity gradient of 0.4–22.6 g l ⁻¹. This suggests that there is ecologically significant microdiversity within the observed Synechococcus group as defined by ITS sequences. Collectively our study demonstrated abundant and potentially novel Synechococcus in East Tibetan lakes that are likely the result of evolutionary adaptations to regional conditions.     

Clade-Specific 16S Ribosomal DNA Oligonucleotides Reveal the Predominance of a Single Marine Synechococcus Clade throughout a Stratified Water Column in the Red Sea

Phylogenetic relationships among members of the marine Synechococcus genus were determined following sequencing of the 16S ribosomal DNA (rDNA) from 31 novel cultured isolates from the Red Sea and several other oceanic environments. This revealed a large genetic diversity within the marine Synechococcus cluster consistent with earlier work but also identified three novel clades not previously recognized. Phylogenetic analyses showed one clade, containing halotolerant isolates lacking phycoerythrin (PE) and including strains capable, or not, of utilizing nitrate as the sole N source, which clustered within the MC-A (Synechococcus subcluster 5.1) lineage. Two copies of the 16S rRNA gene are present in marine Synechococcus genomes, and cloning and sequencing of these copies from Synechococcus sp. strain WH 7803 and genomic information from Synechococcus sp. strain WH 8102 reveal these to be identical. Based on the 16S rDNA sequence information, clade-specific oligonucleotides for the marine Synechococcus genus were designed and their specificity was optimized. Using dot blot hybridization technology, these probes were used to determine the in situ community structure of marine Synechococcus populations in the Red Sea at the time of a Synechococcus maximum during April 1999. A predominance of genotypes representative of a single clade was found, and these genotypes were common among strains isolated into culture. Conversely, strains lacking PE, which were also relatively easily isolated into culture, represented only a minor component of the Synechococcus population. Genotypes corresponding to well-studied laboratory strains also appeared to be poorly represented in this stratified water column in the Red Sea.     

Biogeography of Photosynthetic Light-Harvesting Genes in Marine Phytoplankton

Background

Photosynthetic light-harvesting proteins are the mechanism by which energy enters the marine ecosystem. The dominant prokaryotic photoautotrophs are the cyanobacterial genera Prochlorococcus and Synechococcus that are defined by two distinct light-harvesting systems, chlorophyll-bound protein complexes or phycobilin-bound protein complexes, respectively. Here, we use the Global Ocean Sampling (GOS) Project as a unique and powerful tool to analyze the environmental diversity of photosynthetic light-harvesting genes in relation to available metadata including geographical location and physical and chemical environmental parameters.

Methods

All light-harvesting gene fragments and their metadata were obtained from the GOS database, aligned using ClustalX and classified phylogenetically. Each sequence has a name indicative of its geographic location; subsequent biogeographical analysis was performed by correlating light-harvesting gene budgets for each GOS station with surface chlorophyll concentration.

Conclusion/Significance

Using the GOS data, we have mapped the biogeography of light-harvesting genes in marine cyanobacteria on ocean-basin scales and show that an environmental gradient exists in which chlorophyll concentration is correlated to diversity of light-harvesting systems. Three functionally distinct types of light-harvesting genes are defined: (1) the phycobilisome (PBS) genes of Synechococcus; (2) the pcb genes of Prochlorococcus; and (3) the iron-stress-induced (isiA) genes present in some marine Synechococcus. At low chlorophyll concentrations, where nutrients are limited, the Pcb-type light-harvesting system shows greater genetic diversity; whereas at high chlorophyll concentrations, where nutrients are abundant, the PBS-type light-harvesting system shows higher genetic diversity. We interpret this as an environmental selection of specific photosynthetic strategy. Importantly, the unique light-harvesting system isiA is found in the iron-limited, high-nutrient low-chlorophyll region of the equatorial Pacific. This observation demonstrates the ecological importance of isiA genes in enabling marine Synechococcus to acclimate to iron limitation and suggests that the presence of this gene can be a natural biomarker for iron limitation in oceanic environments.     

聚球藻硅质化作用初探

硅元素是全球生地化循环的重要组成成分之一,对海洋生态系统中以浮游植物主导的初级生产力和硅碳循环具有重要的意义。普遍认为硅藻主导着全球海洋的硅循环,成为海洋硅循环和碳循环交互作用的重要桥梁。海洋单细胞聚球藻对海洋食物网和能量流具有关键启动和支撑作用,是全球碳循环中固碳过程的主要贡献者,近年又被发现其具有重要的硅质化作用,为我们提供了一个在海洋中(特别是寡营养海域),除硅藻之外,连接硅碳循环交互作用的新视角,对硅藻在全球海洋硅碳循环的绝对地位具有重要的挑战意义。面对聚球藻在大洋中如此巨大的生物量,甚至高于硅藻,有必要弄清楚其碳沉降机制以及准确的模拟其硅循环,然而关于其在海洋硅循环的研究极少,硅质化作用的吸收和储存机理以及环境调节机制也不清楚;另外,其对世界海洋硅碳循环的调节作用也未见报道。为此,通过前人对海洋单细胞聚球藻硅质化作用研究的基础上进行有针对性的探讨,可望对海洋单细胞聚球藻硅质化作用及其对硅碳循环的调控机制有一个基本的认识,为深入研究聚球藻在全球海洋硅循环中的作用提供基础。     

Strong Seasonality and Interannual Recurrence in Marine Myovirus Communities

The temporal community dynamics and persistence of different viral types in the marine environment are still mostly obscure. Polymorphism of the major capsid protein gene, g23, was used to investigate the community composition dynamics of T4-like myoviruses in a North Atlantic fjord for a period of 2 years. A total of 160 unique operational taxonomic units (OTUs) were identified by terminal restriction fragment length polymorphism (TRFLP) of the gene g23. Three major community profiles were identified (winter-spring, summer, and autumn), which resulted in a clear seasonal succession pattern. These seasonal transitions were recurrent over the 2 years and significantly correlated with progression of seawater temperature, Synechococcus abundance, and turbidity. The appearance of the autumn viral communities was concomitant with the occurrence of prominent Synechococcus blooms. As a whole, we found a highly dynamic T4-like viral community with strong seasonality and recurrence patterns. These communities were unexpectedly dominated by a group of persistently abundant viruses.     

Culture Isolation and Culture-Independent Clone Libraries Reveal New Marine Synechococcus Ecotypes with Distinctive Light and N Physiologies

Marine microbial communities often contain multiple closely related phylogenetic clades, but in many cases, it is still unclear what physiological traits differentiate these putative ecotypes. The numerically abundant marine cyanobacterium Synechococcus can be divided into at least 14 clades. In order to better understand ecotype differentiation in this genus, we assessed the diversity of a Synechococcus community from a well-mixed water column in the Sargasso Sea during March 2002, a time of year when this genus typically reaches its annual peak in abundance. Diversity was estimated from water sampled at three depths (approximately 5, 70, and 170 m) using both culture isolation and construction of cyanobacterial 16S-23S rRNA internal transcribed sequence clone libraries. Clonal isolates were obtained by enrichment with ammonium, nitrite, or nitrate as the sole N source, followed by pour plating. Each method sampled the in situ diversity differently. The combined methods revealed a total of seven Synechococcus phylotypes including two new putative ecotypes, labeled XV and XVI. Although most other isolates grow on nitrate, clade XV exhibited a reduced efficiency in nitrate utilization, and both clade XV and XVI are capable of chromatic adaptation, demonstrating that this trait is more widely distributed among Synechococcus strains than previously known. Thus, as in its sister genus Prochlorococcus, light and nitrogen utilization are important factors in ecotype differentiation in the marine Synechococcus lineage.     

CHARACTERIZATION OF A FUNCTIONAL VANADIUM‐DEPENDENT BROMOPEROXIDASE IN THE MARINE CYANOBACTERIUM SYNECHOCOCCUS SP. CC93111

Vanadium‐dependent bromoperoxidases (VBPOs) are characterized by the ability to oxidize halides using hydrogen peroxide. These enzymes are well‐studied in eukaryotic macroalgae and are known to produce a variety of brominated secondary metabolites. Though genes have been annotated as VBPO in multiple prokaryotic genomes, they remain uncharacterized. The genome of the coastal marine cyanobacterium Synechococcus sp. CC9311 encodes a predicted VBPO (YP_731869.1, sync_2681), and in this study, we show that protein extracts from axenic cultures of Synechococcus possess bromoperoxidase activity, oxidizing bromide and iodide, but not chloride. In‐gel activity assays of Synechococcus proteins separated using PAGE reveal a single band having VBPO activity. When sequenced via liquid chromatography/mass spectrometry/mass spectrometry (LC/MS/MS), peptides from the band aligned to the VBPO sequence predicted by the open reading frame (ORF) sync_2681. We show that a VBPO gene is present in a closely related strain, Synechococcus sp. WH8020, but not other clade I Synechococcus strains, consistent with recent horizontal transfer of the gene into Synechococcus. Diverse cyanobacterial‐like VBPO genes were detected in a pelagic environment off the California coast using PCR. Investigation of functional VBPOs in unicellular cyanobacteria may lead to discovery of novel halogenated molecules and a better understanding of these organisms’ chemical ecology and physiology.