Coordinated transcriptional response to environmental stress by a Synechococcus virus.

Viruses are a major control on populations of microbes. Often, their virulence is examined in controlled laboratory conditions. Yet, in nature, environmental conditions lead to changes in host physiology and fitness that may impart both costs and benefits on viral success. Phosphorus (P) is a major abiotic control on the marine cyanobacterium Synechococcus. Some viruses infecting Synechococcus have acquired, from their host, a gene encoding a P substrate binding protein (PstS), thought to improve virus replication under phosphate starvation. Yet, pstS is uncommon among cyanobacterial viruses. Thus, we asked how infections with viruses lacking PstS are affected by P scarcity. We show that the production of infectious virus particles of such viruses is reduced in low P conditions. However, this reduction in progeny is not caused by impaired phage genome replication, thought to be a major sink for cellular phosphate. Instead, transcriptomic analysis showed that under low P conditions, a PstS-lacking cyanophage increased the expression of a specific gene set that included mazG, hli2, and gp43 encoding a pyrophosphatase, a high-light inducible protein and DNA polymerase, respectively. Moreover, several of the upregulated genes were controlled by the host's phoBR two-component system. We hypothesize that recycling and polymerization of nucleotides liberates free phosphate and thus allows viral morphogenesis, albeit at lower rates than when phosphate is replete or when phages encode pstS. Altogether, our data show how phage genomes, lacking obvious P-stress-related genes, have evolved to exploit their host's environmental sensing mechanisms to coordinate their own gene expression in response to resource limitation.

Differential global distribution of marine picocyanobacteria gene clusters reveals distinct niche-related adaptive strategies.

The ever-increasing number of available microbial genomes and metagenomes provides new opportunities to investigate the links between niche partitioning and genome evolution in the ocean, especially for the abundant and ubiquitous marine picocyanobacteria Prochlorococcus and Synechococcus. Here, by combining metagenome analyses of the Tara Oceans dataset with comparative genomics, including phyletic patterns and genomic context of individual genes from 256 reference genomes, we show that picocyanobacterial communities thriving in different niches possess distinct gene repertoires. We also identify clusters of adjacent genes that display specific distribution patterns in the field (eCAGs) and are thus potentially involved in the same metabolic pathway and may have a key role in niche adaptation. Several eCAGs are likely involved in the uptake or incorporation of complex organic forms of nutrients, such as guanidine, cyanate, cyanide, pyrimidine, or phosphonates, which might be either directly used by cells, for example for the biosynthesis of proteins or DNA, or degraded to inorganic nitrogen and/or phosphorus forms. We also highlight the enrichment of eCAGs involved in polysaccharide capsule biosynthesis in Synechococcus populations thriving in both nitrogen- and phosphorus-depleted areas vs. low-iron (Fe) regions, suggesting that the complexes they encode may be too energy-consuming for picocyanobacteria thriving in the latter areas. In contrast, Prochlorococcus populations thriving in Fe-depleted areas specifically possess an alternative respiratory terminal oxidase, potentially involved in the reduction of Fe(III) to Fe(II). Altogether, this study provides insights into how phytoplankton communities populate oceanic ecosystems, which is relevant to understanding their capacity to respond to ongoing climate change.

Trade-offs of lipid remodeling in a marine predator-prey interaction in response to phosphorus limitation.

Phosphorus (P) is a key nutrient limiting bacterial growth and primary production in the oceans. Unsurprisingly, marine microbes have evolved sophisticated strategies to adapt to P limitation, one of which involves the remodeling of membrane lipids by replacing phospholipids with non-P-containing surrogate lipids. This strategy is adopted by both cosmopolitan marine phytoplankton and heterotrophic bacteria and serves to reduce the cellular P quota. However, little, if anything, is known of the biological consequences of lipid remodeling. Here, using the marine bacterium Phaeobacter sp. MED193 and the ciliate Uronema marinum as a model, we sought to assess the effect of remodeling on bacteria-protist interactions. We discovered an important trade-off between either escape from ingestion or resistance to digestion. Thus, Phaeobacter grown under P-replete conditions was readily ingested by Uronema, but not easily digested, supporting only limited predator growth. In contrast, following membrane lipid remodeling in response to P depletion, Phaeobacter was less likely to be captured by Uronema, thanks to the reduced expression of mannosylated glycoconjugates. However, once ingested, membrane-remodeled cells were unable to prevent phagosome acidification, became more susceptible to digestion, and, as such, allowed rapid growth of the ciliate predator. This trade-off between adapting to a P-limited environment and susceptibility to protist grazing suggests the more efficient removal of low-P prey that potentially has important implications for the functioning of the marine microbial food web in terms of trophic energy transfer and nutrient export efficiency.

Membrane lipid renovation in Pseudomonas aeruginosa - implications for phage therapy?

Pseudomonas aeruginosa is an important Gram-negative pathogen with intrinsic resistance to many clinically used antibiotics. It is particularly troublesome in nosocomial infections, immunocompromised patients, and individuals with cystic fibrosis. Antimicrobial resistance (AMR) is a huge threat to global health, with a predicted 10 million people dying from resistant infections by 2050. A promising therapy for combatting AMR infections is phage therapy. However, more research is required to investigate mechanisms that may influence the efficacy of phage therapy. An important overlooked aspect is the impact of membrane lipid remodelling on phage binding ability. P. aeruginosa undergoes changes in membrane lipids when it encounters phosphorus stress, an environmental perturbation that is likely to occur during infection. Lipid changes include the substitution of glycerophospholipids with surrogate glycolipids and the over-production of ornithine-containing aminolipids. Given that membrane lipids are known to influence the structure and function of membrane proteins, we propose that changes in the composition of membrane lipids during infection may alter phage binding and subsequent phage infection dynamics. Consideration of such effects needs to be urgently prioritised in order to develop the most effective phage therapy strategies for P. aeruginosa infections.

2-Aminoethylphosphonate utilization in Pseudomonas putida BIRD-1 is controlled by multiple master regulators.

Bacteria possess various regulatory mechanisms to detect and coordinate a response to elemental nutrient limitation. In pseudomonads, the two-component system regulators CbrAB, NtrBC and PhoBR, are responsible for regulating cellular response to carbon (C), nitrogen (N) and phosphorus (P) respectively. Phosphonates are reduced organophosphorus compounds produced by a broad range of biota and typified by a direct C-P bond. Numerous pseudomonads can use the environmentally abundant phosphonate species 2-aminoethylphosphonate (2AEP) as a source of C, N, or P, but only PhoBR has been shown to play a role in 2AEP utilization. On the other hand, utilization of 2AEP as a C and N source is considered substrate inducible. Here, using the plant-growth-promoting rhizobacterium Pseudomonas putida BIRD-1 we present evidence that 2AEP utilization is under dual regulation and only occurs upon depletion of C, N, or P, controlled by CbrAB, NtrBC, or PhoBR respectively. However, the presence of 2AEP was necessary for full gene expression, i.e. expression was substrate inducible. Mutation of a LysR-type regulator, termed AepR, upstream of the 2AEP transaminase-phosphonatase system (PhnWX), confirmed this dual regulatory mechanism. To our knowledge, this is the first study identifying coordination between global stress response and substrate-specific regulators in phosphonate metabolism.

A widely distributed phosphate-insensitive phosphatase presents a route for rapid organophosphorus remineralization in the biosphere.

The regeneration of bioavailable phosphate from immobilized organophosphorus represents a key process in the global phosphorus cycle and is facilitated by enzymes known as phosphatases. Most bacteria possess at least one of three phosphatases with broad substrate specificity, known as PhoA, PhoX, and PhoD, whose activity is optimal under alkaline conditions. The production and activity of these phosphatases is repressed by phosphate availability. Therefore, they are only fully functional when bacteria experience phosphorus-limiting growth conditions. Here, we reveal a previously overlooked phosphate-insensitive phosphatase, PafA, prevalent in Bacteroidetes, which is highly abundant in nature and represents a major route for the regeneration of environmental phosphate. Using the enzyme from Flavobacterium johnsoniae, we show that PafA is highly active toward phosphomonoesters, is fully functional in the presence of excess phosphate, and is essential for growth on phosphorylated carbohydrates as a sole carbon source. These distinct properties of PafA may expand the metabolic niche of Bacteroidetes by enabling the utilization of abundant organophosphorus substrates as C and P sources, providing a competitive advantage when inhabiting zones of high microbial activity and nutrient demand. PafA, which is constitutively synthesized by soil and marine flavobacteria, rapidly remineralizes phosphomonoesters releasing bioavailable phosphate that can be acquired by neighboring cells. The pafA gene is highly diverse in plant rhizospheres and is abundant in the global ocean, where it is expressed independently of phosphate availability. PafA therefore represents an important enzyme in the context of global biogeochemical cycling and has potential applications in sustainable agriculture.

Transporter characterisation reveals aminoethylphosphonate mineralisation as a key step in the marine phosphorus redox cycle.

The planktonic synthesis of reduced organophosphorus molecules, such as alkylphosphonates and aminophosphonates, represents one half of a vast global oceanic phosphorus redox cycle. Whilst alkylphosphonates tend to accumulate in recalcitrant dissolved organic matter, aminophosphonates do not. Here, we identify three bacterial 2-aminoethylphosphonate (2AEP) transporters, named AepXVW, AepP and AepSTU, whose synthesis is independent of phosphate concentrations (phosphate-insensitive). AepXVW is found in diverse marine heterotrophs and is ubiquitously distributed in mesopelagic and epipelagic waters. Unlike the archetypal phosphonate binding protein, PhnD, AepX has high affinity and high specificity for 2AEP (Stappia stellulata AepX Kd 23 ± 4 nM; methylphosphonate Kd 3.4 ± 0.3 mM). In the global ocean, aepX is heavily transcribed (~100-fold>phnD) independently of phosphate and nitrogen concentrations. Collectively, our data identifies a mechanism responsible for a major oxidation process in the marine phosphorus redox cycle and suggests 2AEP may be an important source of regenerated phosphate and ammonium, which are required for oceanic primary production.

Phosphorus stress induces the synthesis of novel glycolipids in Pseudomonas aeruginosa that confer protection against a last-resort antibiotic.

Pseudomonas aeruginosa is a nosocomial pathogen with a prevalence in immunocompromised individuals and is particularly abundant in the lung microbiome of cystic fibrosis patients. A clinically important adaptation for bacterial pathogens during infection is their ability to survive and proliferate under phosphorus-limited growth conditions. Here, we demonstrate that P. aeruginosa adapts to P-limitation by substituting membrane glycerophospholipids with sugar-containing glycolipids through a lipid renovation pathway involving a phospholipase and two glycosyltransferases. Combining bacterial genetics and multi-omics (proteomics, lipidomics and metatranscriptomic analyses), we show that the surrogate glycolipids monoglucosyldiacylglycerol and glucuronic acid-diacylglycerol are synthesised through the action of a new phospholipase (PA3219) and two glycosyltransferases (PA3218 and PA0842). Comparative genomic analyses revealed that this pathway is strictly conserved in all P. aeruginosa strains isolated from a range of clinical and environmental settings and actively expressed in the metatranscriptome of cystic fibrosis patients. Importantly, this phospholipid-to-glycolipid transition comes with significant ecophysiological consequence in terms of antibiotic sensitivity. Mutants defective in glycolipid synthesis survive poorly when challenged with polymyxin B, a last-resort antibiotic for treating multi-drug resistant P. aeruginosa. Thus, we demonstrate an intriguing link between adaptation to environmental stress (nutrient availability) and antibiotic resistance, mediated through membrane lipid renovation that is an important new facet in our understanding of the ecophysiology of this bacterium in the lung microbiome of cystic fibrosis patients.

Proteomics insights into the Burkholderia cenocepacia phosphorus stress response.

The Burkholderia cepacia complex is a group of Burkholderia species that are opportunistic pathogens causing high mortality rates in patients with cystic fibrosis. An environmental stress often encountered by these soil-dwelling and pathogenic bacteria is phosphorus limitation, an essential element for cellular processes. Here, we describe cellular and extracellular proteins differentially regulated between phosphate-deplete (0 mM, no added phosphate) and phosphate-replete (1 mM) growth conditions using a comparative proteomics (LC-MS/MS) approach. We observed a total of 128 and 65 unique proteins were downregulated and upregulated respectively, in the B. cenocepacia proteome. Of those downregulated proteins, many have functions in amino acid transport/metabolism. We have identified 24 upregulated proteins that are directly/indirectly involved in inorganic phosphate or organic phosphorus acquisition. Also, proteins involved in virulence and antimicrobial resistance were differentially regulated, suggesting B. cenocepacia experiences a dramatic shift in metabolism under these stress conditions. Overall, this study provides a baseline for further research into the biology of Burkholderia in response to phosphorus stress.

Niche-adaptation in plant-associated Bacteroidetes favours specialisation in organic phosphorus mineralisation.

Bacteroidetes are abundant pathogen-suppressing members of the plant microbiome that contribute prominently to rhizosphere phosphorus mobilisation, a frequent growth-limiting nutrient in this niche. However, the genetic traits underpinning their success in this niche remain largely unknown, particularly regarding their phosphorus acquisition strategies. By combining cultivation, multi-layered omics and biochemical analyses we first discovered that all plant-associated Bacteroidetes express constitutive phosphatase activity, linked to the ubiquitous possession of a unique phosphatase, PafA. For the first time, we also reveal a subset of Bacteroidetes outer membrane SusCD-like complexes, typically associated with carbon acquisition, and several TonB-dependent transporters, are induced during Pi-depletion. Furthermore, in response to phosphate depletion, the plant-associated Flavobacterium used in this study expressed many previously characterised and novel proteins targeting organic phosphorus. Collectively, these enzymes exhibited superior phosphatase activity compared to plant-associated Pseudomonas spp. Importantly, several of the novel low-Pi-inducible phosphatases and transporters, belong to the Bacteroidetes auxiliary genome and are an adaptive genomic signature of plant-associated strains. In conclusion, niche adaptation to the plant microbiome thus appears to have resulted in the acquisition of unique phosphorus scavenging loci in Bacteroidetes, enhancing their phosphorus acquisition capabilities. These traits may enable their success in the rhizosphere and also present exciting avenues to develop sustainable agriculture.

Elucidation of glutamine lipid biosynthesis in marine bacteria reveals its importance under phosphorus deplete growth in Rhodobacteraceae.

Marine microorganisms employ multiple strategies to cope with transient and persistent nutrient limitation, one of which, for alleviating phosphorus (P) stress, is to substitute membrane glycerophospholipids with non-P containing surrogate lipids. Such a membrane lipid remodelling strategy enables the most abundant marine phytoplankton and heterotrophic bacteria to adapt successfully to nutrient scarcity in marine surface waters. An important group of non-P lipids, the aminolipids which lack a diacylglycerol backbone, are poorly studied in marine microbes. Here, using a combination of genetic, lipidomics and metagenomics approaches, we reveal for the first time the genes (glsB, olsA) required for the formation of the glutamine-containing aminolipid. Construction of a knockout mutant in either glsB or olsA in the model marine bacterium Ruegeria pomeroyi DSS-3 completely abolished glutamine lipid production. Moreover, both mutants showed a considerable growth cost under P-deplete conditions and the olsA mutant, that is unable to produce the glutamine and ornithine aminolipids, ceased to grow under P-deplete conditions. Analysis of sequenced microbial genomes show that glsB is primarily confined to the Rhodobacteraceae family, which includes the ecologically important marine Roseobacter clade that are key players in the marine sulphur and nitrogen cycles. Analysis of the genes involved in glutamine lipid biosynthesis in the Tara ocean metagenome dataset revealed the global occurrence of glsB in marine surface waters and a positive correlation between glsB abundance and N* (a measure of the deviation from the canonical Redfield ratio), suggesting glutamine lipid plays an important role in the adaptation of marine Rhodobacteraceae to P limitation.

Manganese Is Essential for PlcP Metallophosphoesterase Activity Involved in Lipid Remodeling in Abundant Marine Heterotrophic Bacteria.

In vast areas of the ocean, microbes must adapt to the availability of scarce nutrients, and a key strategy for reducing the cellular phosphorus (P) quota is to remodel membranes by replacing phospholipids with non-P surrogate lipids. A metallophosphoesterase, PlcP, is essential for lipid remodeling in cosmopolitan marine bacteria of the Roseobacter (e.g., Phaeobacter sp. strain MED193) and SAR11 (e.g., Pelagibacter sp. strain HTCC7211) clades, and transcription of plcP is known to be induced by P limitation. In order to better understand PlcP-mediated lipid remodeling, we sought to characterize PlcP for its metal ion requirement and to determine its selectivity for native bacterial phospholipids. Here, we report the occurrence of a highly conserved binuclear ion center in PlcPs from MED193 and HTCC7211 and show that manganese is the preferred metal for metallophosphoesterase activity. PlcP displayed high activity towards the major bacterial phospholipids, e.g., phosphatidylglycerol but also phosphatidic acid, a key intermediate in phospholipid biosynthesis. In contrast, phosphatidylserine and phosphatidylinositol, both of which are rare lipids in bacteria, are not preferred substrates. These data suggest that PlcP undertakes a generic lipid remodeling role during the cellular response of marine bacteria to P deficiency and that manganese availability may play a key role in regulating the lipid remodeling process.IMPORTANCE Membrane lipids form the structural basis of all cells. In the marine environment, it is well established that phosphorus availability significantly affects lipid composition in cosmopolitan marine bacteria, whereby non-phosphorus-containing lipids are used to replace phospholipids in response to phosphorus stress. Central to this lipid remodeling pathway is a newly identified phospholipase C-type metallophosphoesterase (PlcP). However, little is known about how PlcP activity is regulated. Here, we determined the role of metal ions in regulating PlcP activity and compared PlcP substrate specificities in PlcP enzymes from two model marine bacteria from the marine Roseobacter clade and the SAR11 clade. Our data provide new insights into the regulation of lipid remodeling in these marine bacteria.

Identification of extracellular glycerophosphodiesterases in Pseudomonas and their role in soil organic phosphorus remineralisation.

In soils, phosphorus (P) exists in numerous organic and inorganic forms. However, plants can only acquire inorganic orthophosphate (Pi), meaning global crop production is frequently limited by P availability. To overcome this problem, rock phosphate fertilisers are heavily applied, often with negative environmental and socio-economic consequences. The organic P fraction of soil contains phospholipids that are rapidly degraded resulting in the release of bioavailable Pi. However, the mechanisms behind this process remain unknown. We identified and experimentally confirmed the function of two secreted glycerolphosphodiesterases, GlpQI and GlpQII, found in Pseudomonas stutzeri DSM4166 and Pseudomonas fluorescens SBW25, respectively. A series of co-cultivation experiments revealed that in these Pseudomonas strains, cleavage of glycerolphosphorylcholine and its breakdown product G3P occurs extracellularly allowing other bacteria to benefit from this metabolism. Analyses of metagenomic and metatranscriptomic datasets revealed that this trait is widespread among soil bacteria with Actinobacteria and Proteobacteria, specifically Betaproteobacteria and Gammaproteobacteria, the likely major players.

The 'known' genetic potential for microbial communities to degrade organic phosphorus is reduced in low-pH soils.

In soil, bioavailable inorganic orthophosphate is found at low concentrations and thus limits biological growth. To overcome this phosphorus scarcity, plants and bacteria secrete numerous enzymes, namely acid and alkaline phosphatases, which cleave orthophosphate from various organic phosphorus substrates. Using profile hidden Markov modeling approaches, we investigated the abundance of various non specific phosphatases, both acid and alkaline, in metagenomes retrieved from soils with contrasting pH regimes. This analysis uncovered a marked reduction in the abundance and diversity of various alkaline phosphatases in low-pH soils that was not counterbalanced by an increase in acid phosphatases. Furthermore, it was also discovered that only half of the bacterial strains from different phyla deposited in the Integrated Microbial Genomes database harbor alkaline phosphatases. Taken together, our data suggests that these 'phosphatase lacking' isolates likely increase in low-pH soils and future research should ascertain how these bacteria overcome phosphorus scarcity.

Viruses Inhibit CO2 Fixation in the Most Abundant Phototrophs on Earth.

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus are the most numerous photosynthetic organisms on our planet [1, 2]. With a global population size of 3.6 × 10(27) [3], they are responsible for approximately 10% of global primary production [3, 4]. Viruses that infect Prochlorococcus and Synechococcus (cyanophages) can be readily isolated from ocean waters [5-7] and frequently outnumber their cyanobacterial hosts [8]. Ultimately, cyanophage-induced lysis of infected cells results in the release of fixed carbon into the dissolved organic matter pool [9]. What is less well known is the functioning of photosynthesis during the relatively long latent periods of many cyanophages [10, 11]. Remarkably, the genomes of many cyanophage isolates contain genes involved in photosynthetic electron transport (PET) [12-18] as well as central carbon metabolism [14, 15, 19, 20], suggesting that cyanophages may play an active role in photosynthesis. However, cyanophage-encoded gene products are hypothesized to maintain or even supplement PET for energy generation while sacrificing wasteful CO2 fixation during infection [17, 18, 20]. Yet this paradigm has not been rigorously tested. Here, we measured the ability of viral-infected Synechococcus cells to fix CO2 as well as maintain PET. We compared two cyanophage isolates that share different complements of PET and central carbon metabolism genes. We demonstrate cyanophage-dependent inhibition of CO2 fixation early in the infection cycle. In contrast, PET is maintained throughout infection. Our data suggest a generalized strategy among marine cyanophages to redirect photosynthesis to support phage development, which has important implications for estimates of global primary production.

Comparative genomic, proteomic and exoproteomic analyses of three Pseudomonas strains reveals novel insights into the phosphorus scavenging capabilities of soil bacteria.

Bacteria that inhabit the rhizosphere of agricultural crops can have a beneficial effect on crop growth. One such mechanism is the microbial-driven solubilization and remineralization of complex forms of phosphorus (P). It is known that bacteria secrete various phosphatases in response to low P conditions. However, our understanding of their global proteomic response to P stress is limited. Here, exoproteomic analysis of Pseudomonas putida BIRD-1 (BIRD-1), Pseudomonas fluorescens SBW25 and Pseudomonas stutzeri DSM4166 was performed in unison with whole-cell proteomic analysis of BIRD-1 grown under phosphate (Pi) replete and Pi deplete conditions. Comparative exoproteomics revealed marked heterogeneity in the exoproteomes of each Pseudomonas strain in response to Pi depletion. In addition to well-characterized members of the PHO regulon such as alkaline phosphatases, several proteins, previously not associated with the response to Pi depletion, were also identified. These included putative nucleases, phosphotriesterases, putative phosphonate transporters and outer membrane proteins. Moreover, in BIRD-1, mutagenesis of the master regulator, phoBR, led us to confirm the addition of several novel PHO-dependent proteins. Our data expands knowledge of the Pseudomonas PHO regulon, including species that are frequently used as bioinoculants, opening up the potential for more efficient and complete use of soil complexed P.

Lipid remodelling is a widespread strategy in marine heterotrophic bacteria upon phosphorus deficiency.

Upon phosphorus (P) deficiency, marine phytoplankton reduce their requirements for P by replacing membrane phospholipids with alternative non-phosphorus lipids. It was very recently demonstrated that a SAR11 isolate also shares this capability when phosphate starved in culture. Yet, the extent to which this process occurs in other marine heterotrophic bacteria and in the natural environment is unknown. Here, we demonstrate that the substitution of membrane phospholipids for a variety of non-phosphorus lipids is a conserved response to P deficiency among phylogenetically diverse marine heterotrophic bacteria, including members of the Alphaproteobacteria and Flavobacteria. By deletion mutagenesis and complementation in the model marine bacterium Phaeobacter sp. MED193 and heterologous expression in recombinant Escherichia coli, we confirm the roles of a phospholipase C (PlcP) and a glycosyltransferase in lipid remodelling. Analyses of the Global Ocean Sampling and Tara Oceans metagenome data sets demonstrate that PlcP is particularly abundant in areas characterized by low phosphate concentrations. Furthermore, we show that lipid remodelling occurs seasonally and responds to changing nutrient conditions in natural microbial communities from the Mediterranean Sea. Together, our results point to the key role of lipid substitution as an adaptive strategy enabling heterotrophic bacteria to thrive in the vast P-depleted areas of the ocean.

Contribution of cyanobacterial alkane production to the ocean hydrocarbon cycle.

Hydrocarbons are ubiquitous in the ocean, where alkanes such as pentadecane and heptadecane can be found even in waters minimally polluted with crude oil. Populations of hydrocarbon-degrading bacteria, which are responsible for the turnover of these compounds, are also found throughout marine systems, including in unpolluted waters. These observations suggest the existence of an unknown and widespread source of hydrocarbons in the oceans. Here, we report that strains of the two most abundant marine cyanobacteria, Prochlorococcus and Synechococcus, produce and accumulate hydrocarbons, predominantly C15 and C17 alkanes, between 0.022 and 0.368% of dry cell weight. Based on global population sizes and turnover rates, we estimate that these species have the capacity to produce 2-540 pg alkanes per mL per day, which translates into a global ocean yield of ∼ 308-771 million tons of hydrocarbons annually. We also demonstrate that both obligate and facultative marine hydrocarbon-degrading bacteria can consume cyanobacterial alkanes, which likely prevents these hydrocarbons from accumulating in the environment. Our findings implicate cyanobacteria and hydrocarbon degraders as key players in a notable internal hydrocarbon cycle within the upper ocean, where alkanes are continually produced and subsequently consumed within days. Furthermore we show that cyanobacterial alkane production is likely sufficient to sustain populations of hydrocarbon-degrading bacteria, whose abundances can rapidly expand upon localized release of crude oil from natural seepage and human activities.

Elemental composition of natural populations of key microbial groups in Atlantic waters.

Intracellular carbon (C), nitrogen (N) and phosphorus (P) content of marine phytoplankton and bacterioplankton can vary according to cell requirements or physiological acclimation to growth under nutrient limited conditions. Although such variation in macronutrient content is well known for cultured organisms, there is a dearth of data from natural populations that reside under a range of environmental conditions. Here, we compare C, N and P content of Synechococcus, Prochlorococcus, low nucleic acid (LNA) content bacterioplankton and small plastidic protists inhabiting surface waters of the North and South subtropical gyres and the Equatorial Region of the Atlantic Ocean. While intracellular C:N ratios ranged between 3.5 and 6, i.e. below the Redfield ratio of 6.6, all the C:P and N:P ratios were up to 10 times higher than the corresponding Redfield ratio of 106 and 16, respectively, reaching and in some cases exceeding maximum values reported in the literature. Similar C:P or N:P ratios in areas with different concentrations of inorganic phosphorus suggests that this is not just a response to the prevailing environmental conditions but an indication of the extremely low P content of these oceanic microbes.

Comparable light stimulation of organic nutrient uptake by SAR11 and Prochlorococcus in the North Atlantic subtropical gyre.

Subtropical oceanic gyres are the most extensive biomes on Earth where SAR11 and Prochlorococcus bacterioplankton numerically dominate the surface waters depleted in inorganic macronutrients as well as in dissolved organic matter. In such nutrient poor conditions bacterioplankton could become photoheterotrophic, that is, potentially enhance uptake of scarce organic molecules using the available solar radiation to energise appropriate transport systems. Here, we assessed the photoheterotrophy of the key microbial taxa in the North Atlantic oligotrophic gyre and adjacent regions using (33)P-ATP, (3)H-ATP and (35)S-methionine tracers. Light-stimulated uptake of these substrates was assessed in two dominant bacterioplankton groups discriminated by flow cytometric sorting of tracer-labelled cells and identified using catalysed reporter deposition fluorescence in situ hybridisation. One group of cells, encompassing 48% of all bacterioplankton, were identified as members of the SAR11 clade, whereas the other group (24% of all bacterioplankton) was Prochlorococcus. When exposed to light, SAR11 cells took 31% more ATP and 32% more methionine, whereas the Prochlorococcus cells took 33% more ATP and 34% more methionine. Other bacterioplankton did not demonstrate light stimulation. Thus, the SAR11 and Prochlorococcus groups, with distinctly different light-harvesting mechanisms, used light equally to enhance, by approximately one-third, the uptake of different types of organic molecules. Our findings indicate the significance of light-driven uptake of essential organic nutrients by the dominant bacterioplankton groups in the surface waters of one of the less productive, vast regions of the world's oceans-the oligotrophic North Atlantic subtropical gyre.