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     

Ocean time-series reveals recurring seasonal patterns of virioplankton dynamics in the northwestern Sargasso Sea

There are an estimated 10³⁰ virioplankton in the world oceans, the majority of which are phages (viruses that infect bacteria). Marine phages encompass enormous genetic diversity, affect biogeochemical cycling of elements, and partially control aspects of prokaryotic production and diversity. Despite their importance, there is a paucity of data describing virioplankton distributions over time and depth in oceanic systems. A decade of high-resolution time-series data collected from the upper 300 m in the northwestern Sargasso Sea revealed recurring temporal and vertical patterns of virioplankton abundance in unprecedented detail. An annual virioplankton maximum developed between 60 and 100 m during periods of summer stratification and eroded during winter convective mixing. The timing and vertical positioning of this seasonal pattern was related to variability in water column stability and the dynamics of specific picophytoplankton and heterotrophic bacterioplankton lineages. Between 60 and 100 m, virioplankton abundance was negatively correlated to the dominant heterotrophic bacterioplankton lineage SAR11, as well as the less abundant picophytoplankton, Synechococcus. In contrast, virioplankton abundance was positively correlated to the dominant picophytoplankton lineage Prochlorococcus, and the less abundant alpha-proteobacteria, Rhodobacteraceae. Seasonally, virioplankton abundances were highly synchronous with Prochlorococcus distributions and the virioplankton to Prochlorococcus ratio remained remarkably constant during periods of water column stratification. The data suggest that a significant fraction of viruses in the mid-euphotic zone of the subtropical gyres may be cyanophages and patterns in their abundance are largely determined by Prochlorococcus dynamics in response to water column stability. This high-resolution, decadal survey of virioplankton abundance provides insight into the possible controls of virioplankton dynamics in the open ocean.     

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.     

Ecological Genomics of Marine Picocyanobacteria

Summary: Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus numerically dominate the picophytoplankton of the world ocean, making a key contribution to global primary production. Prochlorococcus was isolated around 20 years ago and is probably the most abundant photosynthetic organism on Earth. The genus comprises specific ecotypes which are phylogenetically distinct and differ markedly in their photophysiology, allowing growth over a broad range of light and nutrient conditions within the 45°N to 40°S latitudinal belt that they occupy. Synechococcus and Prochlorococcus are closely related, together forming a discrete picophytoplankton clade, but are distinguishable by their possession of dissimilar light-harvesting apparatuses and differences in cell size and elemental composition. Synechococcus strains have a ubiquitous oceanic distribution compared to that of Prochlorococcus strains and are characterized by phylogenetically discrete lineages with a wide range of pigmentation. In this review, we put our current knowledge of marine picocyanobacterial genomics into an environmental context and present previously unpublished genomic information arising from extensive genomic comparisons in order to provide insights into the adaptations of these marine microbes to their environment and how they are reflected at the genomic level.     

EFFECTS OF INCREASED TEMPERATURE AND CO2 ON PHOTOSYNTHESIS,GROWTH, AND ELEMENTAL RATIOS IN MARINE SYNECHOCOCCUS AND PROCHLOROCOCCUS (CYANOBACTERIA)1

Little is known about the combined impacts of future CO₂ and temperature increases on the growth and physiology of marine picocyanobacteria. We incubated Synechococcus and Prochlorococcus under present‐day (380 ppm) or predicted year‐2100 CO₂ levels (750 ppm), and under normal versus elevated temperatures (+4°C) in semicontinuous cultures. Increased temperature stimulated the cell division rates of Synechococcus but not Prochlorococcus. Doubled CO₂ combined with elevated temperature increased maximum chl a–normalized photosynthetic rates of Synechococcus four times relative to controls. Temperature also altered other photosynthetic parameters (α, Φ_max, E_k, and ) in Synechococcus, but these changes were not observed for Prochlorococcus. Both increased CO₂ and temperature raised the phycobilin and chl a content of Synechococcus, while only elevated temperature increased divinyl chl a in Prochlorococcus. Cellular carbon (C) and nitrogen (N) quotas, but not phosphorus (P) quotas, increased with elevated CO₂ in Synechococcus, leading to ～20% higher C:P and N:P ratios. In contrast, Prochlorococcus elemental composition remained unaffected by CO₂, but cell volume and elemental quotas doubled with increasing temperature while maintaining constant stoichiometry. Synechococcus showed a much greater response to CO₂ and temperature increases for most parameters measured, compared with Prochlorococcus. Our results suggest that global change could influence the dominance of Synechococcus and Prochlorococcus ecotypes, with likely effects on oligotrophic food‐web structure. However, individual picocyanobacteria strains may respond quite differently to future CO₂ and temperature increases, and caution is needed when generalizing their responses to global change in the ocean.     

Picophytoplankton, nanophytoplankton, heterotrohpic bacteria and viruses in the Changjiang Estuary and adjacent coastal waters

Flow cytometry (FCM) was used to examine the abundances anddistributions of different picophytoplankton groups (i.e. Synechococcus,Prochlorococcus and picoeukaryotes), nanophytoplankton, heterotrophicbacteria and viruses were examined in the Changjiang Estuary,China and adjacent coastal waters during autumn 2004. Watertemperature and light availability were found to be criticalfactors for picophytoplankton growth. Positive correlationswere found between picophytoplankton, heterotrophic bacteriaand viruses, and a seaward-increasing trend in the V-I (thegroup yielding high green fluorescence according to FCM) populationwithin viruses was detected. The importance of nanophytoplanktonis progressively usurped by picophytoplankton with increasingdistance offshore. Picoeukaryotes are the most successful groupamong picophytoplankton in near-shore eutrophic waters, whereasProchlorococcus surpasses other groups within the pico- andnanophytoplankton community in offshore oligotrophic regionsof the East China Sea Shelf.     

Picophytoplankton biomass, community structure and productivity in the Great Astrolabe Lagoon, Fiji

 Phytoplankton biomass, community structure and productivity of the Great Astrolabe lagoon and surrounding ocean were studied using measurements of chlorophyll concentration and carbon uptake. The contribution of picophytoplankton to biomass, productivity and community structure was estimated by size fractionation, ¹⁴C-incubation and flow cytometry analysis. Picoplankton red fluorescence was demonstrated to be a proxy for chlorophyll <3 μm. Consequently, the percentage contribution to chl a<3 μm from each picoplankton group could be calculated using regression estimated values of ψ _i (fg chl a per unit of red fluorescence). In the lagoon, average chlorophyll concentration was 0.8 mg m^-3 with 45% of phytoplankton <3 μm. Primary production reached 1.3 g C m^-2 day^-1 with 53% due to phytoplankton <3 μm. Synechococcus was the most abundant group at all stations, followed by Prochlorococcus and picoeukaryotes. At all stations, Prochlorococcus represented less than 4% of the chl a <3 μm, Synechococcus between 85 and 95%, and Picoeukaryotes between 5 and 10%. In the upper 40 m of surrounding oceanic waters, phytoplankton biomass was dominated by the >3 μm size fraction. In deeper water, the <1 μm size fraction dominated. Prochlorococcus was the most abundant picoplankton group and their contributions to the chlorophyll a<3 μm were close to that of the picoeukaryotes (50% each). Accepted: 27 May 1999     

Distribution of microbial populations and their relationship with environmental parameters in the coastal waters of Qingdao,China

In order to understand the large‐scale distribution of microbial populations simultaneously and their relationship with environmental parameters, flow cytometry was used to analyse samples collected from 46 stations in the coastal waters of Qingdao in spring, 2007. The distribution of virus was significantly and positively correlated with heterotrophic bacteria. Two groups of picophytoplankton (Synechococcus and picoeukaryotes) were detected; however, Prochlorococcus was not found. Picoeukaryotes and nanophytoplankton were abundant in the near‐shore waters, whereas Synechococcus was abundant in the off‐shore areas. No variation was found in vertical distribution of virus, heterotrophic bacteria, Synechococcus and nanophytoplankton abundances, except picoeukaryotes abundance in the bottom layer was dramatically lower than that in the upper layers. Correlation analyses indicated that the relationship between abiotic variables and heterotrophic bacteria, pico‐ and nanophytoplankton was closer than that between abiotic variables and virioplankton. Temperature and nutrients were the synchronous factors controlling the growth of heterotrophic bacteria, pico‐ and nanophytoplankton in the coastal waters of Qingdao in spring. The results suggested that synergistic and antagonistic effects existed among microbial groups.     

DIEL IN SITU PICOPHYTOPLANKTON CELL DEATH CYCLES COUPLED WITH CELL DIVISION1

The diel variability in picophytoplankton cell death was analyzed by quantifying the proportion of dead cyanobacteria Prochlorococcus and Synechococcus cells along several in situ diel cycles in the open Mediterranean Sea. During the diel cycle, total cell abundance varied on average 2.8 ± 0.6 and 2.6 ± 0.4 times for Synechococcus and Prochlorococcus populations, respectively. Increasing percentages of dead cells of Prochlorococcus and Synechococcus were observed during the course of the day reaching the highest values around dusk and decreasing as the night progressed, indicating a clear pattern of diel variation in the cell mortality of both cyanobacteria. Diel cycles of cell division were also monitored. The maximum percentage of dead cells (Max % DC) and the G2 + M phase of the cell division occurred within a period of 2 h for Synechoccoccus and 4.5 h for Prochlorococcus, and the lowest fraction of dead cells occurred at early morning, when the maximum number of cells in G1 phase were also observed. The G1 maximum corresponded with the maximal increase in newly divided cells (minimum % dead cells), and the subsequent exposure of healthy daughter cells to environmental stresses during the day resulted in the progressive increase in dying cells, with the loss of these cells from the population when cell division takes place. The discovery of diel patterns in cell death observed revealed the intense dynamics of picocyanobacterial populations in nature.     

The Repertoire and Evolution of ATP-Binding Cassette Systems in <Emphasis Type="Italic">Synechococcus</Emphasis> and <Emphasis Type="Italic">Prochlorococcus</Emphasis>

Synechococcus and Prochlorococcus have made great contributions to earth’s photosynthetic biomass. ATP-binding cassette (ABC) protein systems have been characterized to play important roles in various physiological functions, including carbon fixation, phosphate assimilation, and vitamin B₁₂ metabolism. In this study, the repertoire and domain architectures of ABC systems in Synechococcus and Prochlorococcus, as well as their potential evolutionary mechanism, have been surveyed extensively. Comparative analysis revealed an uneven phylogenetic distribution of the ABC systems in these organisms, and in particular that fresh-water Synechococcus strains contain more ABC systems than those of marine ones. Phylogenetic analysis indicated that lineage-specific gene expansion and duplication may be the important forces driving the variability of ABC systems in fresh-water Synechococcus and such an expansion was likely to be relevant to their ecological tolerance. At the domain level, ATP-binding domains in several ABC systems were found to fuse with many additional domains after the divergence from their common ancestor, indicating the versatile functions of ABC systems in cyanobacteria. Subsequently, 19 ABC system families were deduced to be the core set of ABC systems conserved in all marine-living Synechococcus and Prochlorococcus. In conclusion, the comprehensive survey of ABC systems in Synechococcus and Prochlorococcus provides novel insights into their potential evolutionary mechanism and the basis for further investigation of their physiological roles.     

GROWTH REGULATION OF rRNA CONTENT IN PROCHLOROCOCCUS AND SYNECHOCOCCUS (MARINE CYANOBACTERIA) MEASURED BY WHOLE‐CELL HYBRIDIZATION OF rRNA‐TARGETED PEPTIDE NUCLEIC ACIDS1

The cyanobacteria Synechococcus and Prochlorococcus are important primary producers in marine ecosystems. Because currently available approaches for estimating microbial growth rates can be difficult to apply in the field, we have been exploring the feasibility of using quantitative rRNA measurements as the basis for making such estimates. In this study we examined the relationship between rRNA and growth rate in several Synechococcus and Prochlorococcus strains over a range of light‐regulated growth rates. Whole‐cell hybridization with fluorescently labeled peptide nucleic acid (PNA) probes was used in conjunction with flow cytometry to quantify rRNA on a per cell basis. This PNA probing technique allowed rRNA analysis in a phycoerythrin‐containing Synechococcus strain (WH7803) and in a non–phycoerythrin‐containing strain and in Prochlorococcus. All the strains showed a qualitatively similar tri‐phasic relationship between rRNA·cell^?1 and growth rate, involving relatively little change in rRNA·cell^?1 at low growth rates, linear increase at intermediate growth rates, and a plateau and/or decrease at the highest growth rates. The onset of each phase was associated with the relative, rather than absolute, growth rate of each strain. In the Synechococcus strains, rRNA normalized to flow cytometrically measured forward angle light scatter (an indicator of size) was well‐correlated with growth rate across strains. These findings support the idea that cellular rRNA may be useful as an indicator of in situ growth rate in natural Synechococcus and Prochlorococcus populations.     

Novel lineages of Prochlorococcus and Synechococcus in the global oceans

Picocyanobacteria represented by Prochlorococcus and Synechococcus have an important role in oceanic carbon fixation and nutrient cycling. In this study, we compared the community composition of picocyanobacteria from diverse marine ecosystems ranging from estuary to open oceans, tropical to polar oceans and surface to deep water, based on the sequences of 16S-23S rRNA internal transcribed spacer (ITS). A total of 1339 ITS sequences recovered from 20 samples unveiled diverse and several previously unknown clades of Prochlorococcus and Synechococcus. Six high-light (HL)-adapted Prochlorococcus clades were identified, among which clade HLVI had not been described previously. Prochlorococcus clades HLIII, HLIV and HLV, detected in the Equatorial Pacific samples, could be related to the HNLC clades recently found in the high-nutrient, low-chlorophyll (HNLC), iron-depleted tropical oceans. At least four novel Synechococcus clades (out of six clades in total) in subcluster 5.3 were found in subtropical open oceans and the South China Sea. A niche partitioning with depth was observed in the Synechococcus subcluster 5.3. Members of Synechococcus subcluster 5.2 were dominant in the high-latitude waters (northern Bering Sea and Chukchi Sea), suggesting a possible cold-adaptation of some marine Synechococcus in this subcluster. A distinct shift of the picocyanobacterial community was observed from the Bering Sea to the Chukchi Sea, which reflected the change of water temperature. Our study demonstrates that oceanic systems contain a large pool of diverse picocyanobacteria, and further suggest that new genotypes or ecotypes of picocyanobacteria will continue to emerge, as microbial consortia are explored with advanced sequencing technology.     

Different physiological responses of cyanobacteria to ultraviolet‐B radiation under iron‐replete and iron‐deficient conditions: Implications for underestimating the negative effects of UV‐B radiation

Iron deficiency has been considered one of the main limiting factors of phytoplankton productivity in some aquatic systems including oceans and lakes. Concomitantly, solar ultraviolet‐B radiation has been shown to have both deleterious and positive impacts on phytoplankton productivity. However, how iron‐deficient cyanobacteria respond to UV‐B radiation has been largely overlooked in aquatic systems. In this study, physiological responses of four cyanobacterial strains (Microcystis and Synechococcus), which are widely distributed in freshwater or marine systems, were investigated under different UV‐B irradiances and iron conditions. The growth, photosynthetic pigment composition, photosynthetic activity, and nonphotochemical quenching of the different cyanobacterial strains were drastically altered by enhanced UV‐B radiation under iron‐deficient conditions, but were less affected under iron‐replete conditions. Intracellular reactive oxygen species (ROS) and iron content increased and decreased, respectively, with increased UV‐B radiation under iron‐deficient conditions for both Microcystis aeruginosa FACHB 912 and Synechococcus sp. WH8102. On the contrary, intracellular ROS and iron content of these two strains remained constant and increased, respectively, with increased UV‐B radiation under iron‐replete conditions. These results indicate that iron‐deficient cyanobacteria are more susceptible to enhanced UV‐B radiation. Therefore, UV‐B radiation probably plays an important role in influencing primary productivity in iron‐deficient aquatic systems, suggesting that its effects on the phytoplankton productivity may be underestimated in iron‐deficient regions around the world.     

Differential Sunlight Sensitivity of Picophytoplankton from Surface Mediterranean Coastal Waters

We tested the sensitivity of coastal picophytoplankton exposed to natural sunlight in short-term experiments. Cell abundance and cell-specific chlorophyll fluorescence were significantly reduced in Prochlorococcus spp. but not in Synechococcus, whereas picoeukaryotes had an intermediate response. These results are the first direct evidence of a differential sensitivity to sunlight of these ubiquitous marine members of unicellular phytoplankton.     

Urea degradation by picophytoplankton in the euphotic zone of Lake Biwa

The ability of photoautotrophic picoplankton Synechococcus to degrade urea was examined in the euphotic zone of Lake Biwa. Samples were divided into pico (0.2–2.0 μm) and larger (>2.0 μm) size fractions by filtration. The rates of urea degradation (the sum of the rates of incorporation of carbon into phytoplankton cells and of liberation of CO₂ into water) measured by radiocarbon urea were 8 and 17 μmol urea m⁻³ day⁻¹ in June and July, respectively, for the picophytoplankton in the surface water, and 196 and 96 μmol urea m⁻³ day⁻¹, respectively for the larger phytoplankton. The rates decreased with depth, somewhat similar to the vertical profiles of the photosynthetic rate. The urea degradation rates were obviously high under light conditions. In daylight, urea was degraded into two phases, carbon incorporation and CO₂ liberation, whereas in the dark it was degraded only into the CO₂ liberation phase. The contribution of picophytoplankton to total phytoplankton in urea degradation was high in the subsurface to lower euphotic layer. Urea degradation activity was higher in the picophytoplankton fraction than in the larger phytoplankton fraction. Shorter residence times of urea were obtained in the upper euphotic zone. The contribution of picophytoplankton to urea cycling was 4% to 35%. The present results suggest that the picophytoplankton Synechococcus is able to degrade urea and effectively makes use of regenerated urea as a nitrogen source in the euphotic layer, and that picophytoplankton play an important role in the biogeochemical nitrogen cycle in Lake Biwa. Received: June 25, 1998 / Accepted: February 10, 1999     

Codon usage patterns and adaptive evolution of marine unicellular cyanobacteria Synechococcus and Prochlorococcus

Marine unicellular cyanobacteria, represented by Synechococcus and Prochlorococcus, dominate the total phytoplankton biomass and production in oligotrophic ocean. In this study, we employed comparative genomics approaches to extensively investigate synonymous codon usage bias and evolutionary rates in a large number of closely related species of marine unicellular cyanobacteria. Although these two groups of marine cyanobacteria have a close phylogenetic relationship, we find that they are highly divergent not only in codon usage patterns but also in the driving forces behind the diversification. It is revealed that in Prochlorococcus, mutation and genome compositional constraints are the main forces contributing to codon usage bias, whereas in Synechococcus, translational selection. In addition, nucleotide substitution rate analysis indicates that they are not evolving at a constant rate after the divergence and that the average d_N/d_S values of core genes in Synechococcus are significantly higher than those in Prochlorococcus. Our evolutionary genomic analysis provides the first insight into codon usage, evolutionary genetic mechanisms and environmental adaptation of Synechococcus and Prochlorococcus after divergence.     

Co-culture with Synechococcus facilitates growth of Prochlorococcus under ocean acidification conditions

Anthropogenic CO₂ emissions are projected to lower the pH of the ocean 0.3 units by 2100. Previous studies suggested that Prochlorococcus and Synechococcus, the numerically dominant phytoplankton in the oceans, have different responses to elevated CO₂ that may result in a dramatic shift in their relative abundances in future oceans. Here we showed that the exponential growth rates of these two genera respond to future CO₂ conditions in a manner similar to other cyanobacteria, but Prochlorococcus strains had significantly lower realized growth rates under elevated CO₂ regimes due to poor survival after exposure to fresh culture media. Despite this, a Synechococcus strain was unable to outcompete a Prochlorococcus strain in co-culture at elevated CO₂. Under these conditions, Prochlorococcus' poor response to elevated CO₂ disappeared, and Prochlorococcus' relative fitness showed negative frequency dependence, with both competitors having significant fitness advantages when initially rare. These experiments suggested that the two strains should be able to coexist indefinitely in co-culture despite sharing nearly identical nutritional requirements. We speculate that negative frequency dependence exists due to reductive Black Queen evolution that has resulted in a passively mutualistic relationship analogous to that connecting Prochlorococcus with the ‘helper’ heterotrophic microbes in its environment.     

RELATIONSHIP BETWEEN CELL CYCLE AND LIGHT‐LIMITED GROWTH RATE IN OCEANIC PROCHLOROCOCCUS (MIT9312) AND SYNECHOCOCCUS (WH8103) (CYANOBACTERIA)1

The relationships between growth rate, cell‐cycle parameters, and cell size were examined in two unicellular cyanobacteria representative of open‐ocean environments: Prochlorococcus (strain MIT9312) and Synechococcus (strain WH8103). Chromosome replication time, C, was constrained to a fairly narrow range of values (～4–6 h) in both species and did not appear to vary with growth rate. In contrast, the pre‐ and post‐DNA replication periods, B and D, respectively, decreased with increasing growth rate from maxima of ～30 and 10–20 h to minima of ～4–6 and 2–3 h, respectively. The combined duration of the chromosome replication and postreplication periods (C+D), a quantity often used in the estimation of Prochlorococcus in situ growth rates, varied ～2.4‐fold over the range of growth rates examined. This finding suggests that assumptions of invariant C+D may adversely influence Prochlorococcus growth rate estimates. In both strains, cell mass was the greatest in slowly growing cells and decreased 2‐ to 3‐fold over the range of growth rates examined here. Estimated cell mass at the start of replication appeared to decrease with increasing growth rate, indicating that the initiation of chromosome replication in Prochlorococcus and Synechococcus is not a simple function of cell biomass, as suggested previously. Taken together, our results reflect a notable degree of similarity between oceanic Synechococcus and Prochlorococcus strains with respect to their growth‐rate‐specific cell‐cycle characteristics.     

Aerobic Anoxygenic Phototrophic Bacteria in the Mid-Atlantic Bight and the North Pacific Gyre

The abundance of aerobic anoxygenic phototrophic (AAP) bacteria, cyanobacteria, and heterotrophs was examined in the Mid-Atlantic Bight and the central North Pacific Gyre using infrared fluorescence microscopy coupled with image analysis and flow cytometry. AAP bacteria comprised 5% to 16% of total prokaryotes in the Atlantic Ocean but only 5% or less in the Pacific Ocean. In the Atlantic, AAP bacterial abundance was as much as 2-fold higher than that of Prochlorococcus spp. and 10-fold higher than that of Synechococcus spp. In contrast, Prochlorococcus spp. outnumbered AAP bacteria 5- to 50-fold in the Pacific. In both oceans, subsurface abundance maxima occurred within the photic zone, and AAP bacteria were least abundant below the 1% light depth. The abundance of AAP bacteria rivaled some groups of strictly heterotrophic bacteria and was often higher than the abundance of known AAP bacterial genera (Erythrobacter and Roseobacter spp.). Concentrations of bacteriochlorophyll a (BChl a) were low (~1%) compared to those of chlorophyll a in the North Atlantic. Although the BChl a content of AAP bacteria per cell was typically 20- to 250-fold lower than the divinyl-chlorophyll a content of Prochlorococcus, the pigment content of AAP bacteria approached that of Prochlorococcus in shelf break water. Our results suggest that AAP bacteria can be quite abundant in some oceanic regimes and that their distribution in the water column is consistent with phototrophy.