Effect of Phosphorus on the Synechococcus Cell Cycle in Surface Mediterranean Waters during Summer

Unveiling the responses of Alexandrium pacificum to phosphorus utilization by physiological and transcriptomic analysis

Phosphorus (P) is an essential macronutrient impacting bloom formation of marine dinoflagellates. The dinoflagellate Alexandrium pacificum is a cosmopolitan species known to frequently cause dense blooms in estuarine and coastal waters worldwide, while the physiological and molecular responses of A. pacificum to P utilization are still not well understood. Herein, the growth, P utilization, toxin production and transcriptomes of A. pacificum grown under P-deficient, inorganic P-replete, and organic P-replete conditions were compared. The results indicated that P-deficient adversely affected the growth of A. pacificum and significantly down-regulated the expression of genes related to P transport and material metabolism, but enhanced the production of toxin. On the other hand, no significant differences were observed in growth and toxin production between the organic and inorganic P-replete treatments. However, genes involved in P transport, utilization and TCA cycle were significantly changed in the organic P-replete compared with the inorganic P-replete group, and the mechanisms underlying the use of various organic P compounds were different. These findings suggested that A. pacificum evolved diverse organic P utilization strategies to adapt to low P conditions, which might be a crucial factor driving bloom formation in a low inorganic P environment.

Exploring the dynamics of marine picophytoplankton among the Yellow Sea, Indian Ocean and Pacific Ocean: The importance of temperature and nitrogen

Marine picophytoplankton (<2 μm) are the most abundant photosynthetic group and also important contributors to global primary production. However, it is still constrained to incorporate picophytoplankton into dynamic ecosystem models, as a result of our limited understanding of their global distribution and abundance. Here, we applied a large dataset consisted of 1817 in situ observations from the Yellow Sea, Indian Ocean, and Pacific Ocean to suggest that picophytoplankton abundance and distribution had a large variability among the three distinct regions. Based on the correlation analysis, aggregated boosted tree analysis, and generalized additive model, we proposed that water temperature and dissolved inorganic nitrogen (N) were key determinants in driving the large-scale variability of marine picophytoplankton. For example, we revealed that high temperature and low N would stimulate the growth of Prochlorococcus. Therefore, these results could provide some insights into the various environmental factors which affect the dynamics of picophytoplankton, as well as the dynamic ecosystem models.

Transcriptomic response of Daphnia magna to nitrogen- or phosphorus-limited diet

Effects of nutrient-imbalanced diet on the growth and fitness of zooplankton were widely reported as key issues to aquatic ecology. However, little is known about the molecular mechanisms driving the physiological changes of zooplankton under nutrient stress.In this study, we investigated the physiological fitness and transcriptomic response of Daphnia magna when exposed to nitrogen (N)-limited or phosphorus (P)-limited algal diet (Chlamydomonas reinhardtii) compared to regular algae (N and P saturated).D. magna showed higher ingestion rates and overexpression of genes encoding digestive enzymes when fed with either N-limited or P-limited algae, reflecting the compensatory feeding. Under P-limitation, both growth rate and reproduction rate of D. magna were greatly reduced, which could be attributed to the downregulated genes within the pathways of cell cycle and DNA replication. Growth rate of D. magna under N-limitation was similar to normal group, which could be explained by the high methylation level (by degradation of methionine) supporting the body development.Phenotypic changes of D. magna under nutrient stress were explained by gene and pathway regulations from transcriptome data. Generally, D. magna invested more on growth under N-limitation but kept maintenance (e.g., cell structure and defense to external stress) in priority under P-limitation. Post-translational modifications (e.g., methylation and protein folding) were important for D. magna to deal with nutrient constrains.This study reveals the fundamental mechanisms of zooplankton in dealing with elemental imbalanced diet and sheds light on the transfer of energy and nutrient in aquatic ecosystems.

Picocyanobacteria Community and Cyanophage Infection Responses to Nutrient Enrichment in a Mesocosms Experiment in Oligotrophic Waters

Prochlorococcus and Synechococcus are pico-sized cyanobacteria that play a fundamental role in oceanic primary production, being particularly important in warm, nutrient-poor waters. Their potential response to nutrient enrichment is expected to be contrasting and to differ from larger phytoplankton species. Here, we used a metagenomic approach to characterize the responses to nutrient enrichment in the community of picocyanobacteria and to analyze the cyanophage response during a mesocosms experiment in the oligotrophic Red Sea. Natural picoplankton community was dominated by Synechococcus clade II, with marginal presence of Prochlorococcus (0.3% bacterial reads). Increased nutrient input triggered a fast Synechococcus bloom, with clade II being the dominant, with no response of Prochlorococcus growth. The largest bloom developed in the mesocosms receiving a single initial input of nutrients, instead of daily additions. The relative abundances of cyanophage sequences in cellular metagenomes increased during the experiment from 12.6% of total virus reads up to 40% in the treatment with the largest Synechococcus bloom. The subsequent collapse of the bloom pointed to a cyanophage infection on Synechococcus that reduced its competitive capacity, and was then followed by a diatom bloom. The cyanophage attack appears to have preferentially affected the most abundant Synechococcus clade II, increasing the evenness within the host population. Our results highlight the relevance of host-phage interactions on determining population dynamics and diversity of Synechococcus populations.

Phosphorus deficiency induced by aluminum in a marine nitrogen-fixing cyanobacterium Crocosphaera watsonii WH0003

Large amounts of aluminum (Al) enter the ocean through atmospheric dust deposition and river runoffs. However, few studies have reported the effects of Al on marine phytoplankton, especially nitrogen-fixing cyanobacteria. By using the isotope tracer method and quantitative reverse transcription PCR (RT-qPCR), we examined the physiological effect of Al (0.2, 2 and 20 μM) on the unicellular marine nitrogen-fixing cyanobacterium Crocosphaera watsonii in Aquil* medium. We show that Al has an inhibitory physiological effect on C. watsonii, including changes in growth rate, nitrogen fixation rate, carbon fixation rate, cell size, fast rise chlorophyll fluorescence kinetics, cellular photosynthetic pigment and C/N/P content, the same as that of the phosphorus deficient treatment. The ratio of cellular elements C:N:P showed that phosphorus was deficient in the cell of C. watsonii after Al treatment (2 and 20 μM). In addition, Al stimulated the expression of phosphorus-related genes pstS, phoH, phoU, ppK and ppX in C. watsonii. All these results suggest that Al-treated C. watsonii is phosphorus-limited, and that the phosphorus deficiency induced by Al may be one mechanism behind aluminum's toxicity.

Effects of Phosphorus on Interspecific Competition between two cell-size Cyanobacteria: Synechococcus sp. and Microcystis aeruginosa

Pico-cyanobacteria and micro-cyanobacteria coexist ubiquitously in many lakes. Differences in cell size and abilities to utilize nutrients may influence their distribution patterns. In this study, Synechococcus sp. and Microcystis aeruginosa were chosen as pico- and micro-cyanobacteria, respectively. Gradient phosphorus treatments (0.002, 0.01, 0.05, and 0.25 mg P L^-1) were designed in mono- and co-cultures. Growth curves were recorded and fitted by the Monod equation. Moreover, the interspecific competition was analyzed by the Lotka-Volterra model. When mono-cultured in lower P conditions (≤ 0.01 mg P L^-1), Synechococcus sp. obtained much higher biomass than M. aeruginosa. But, M. aeruginosa grew faster than Synechococcus sp. in higher P groups (≥ 0.05 mg P L^-1) (p < 0.05). Synechococcus sp. has abilities to thrive in low-phosphorus environments, whereas M. aeruginosa favored high-phosphorus conditions. In co-cultures, Synechococcus sp. strongly inhibited M. aeruginosa at each P treatment.

Role of temperature and nutrients on the growth and toxin production of Prorocentrum hoffmannianum (Dinophyceae) from the Florida Keys

The benthic dinoflagellate Prorocentrum hoffmannianum M.A. Faust is typical of tropical warm waters and produces biotoxins responsible for diarrhetic shellfish poisoning (DSP). In this study, the effect of temperature and nutrient limitation on growth and toxin production of P. hoffmannianum isolated from field samples collected in the Florida Keys was investigated. Batch culture experiments were ran at two temperatures (i.e. 21 ± 0.1 and 27 ± 0.1 °C) and under nitrogen-limited (14.7 μmol L^-1 N-NO₃- and 18.1 μmol L^-1 P-PO₄^3-) and phosphorus-limited (441 μmol L^-1 N-NO₃- and 0.6 μmol L^-1 P-PO₄^3-) levels with respect to control nutrient conditions (441 μmol L^-1 N-NO₃-and 18.1 μmol L^-1 P-PO₄^3-). Both temperature and nutrient conditions significantly affected growth rates and maximum yield of P. hoffmannianum with the maximum values being recorded at the higher temperature and in the replete medium. Production of okadaic acid was induced under all conditions (from 13.5 to 859.8 pg cell^-1), with values up to one order of magnitude higher than those observed in other DSP toxin producing species. Toxin production was enhanced under P limitation at 27 °C, corroborating the theory that toxin production is modulated by cell physiological conditions, which are in turn affected by a wide spectrum of factors, including environmental stressors such as nutrient availability. Toxin fraction released in the growth medium was negligible. No okadaic acid esters were detected in this strain of P. hoffmannianum.

Variability of ultraplankton composition and distribution in an oligotrophic coastal ecosystem of the NW Mediterranean Sea derived from a two-year survey at the single cell level

Ultraplankton [heterotrophic prokaryotes and ultraphytoplankton (<10 μm)] were monitored weekly over two years (2009 & 2010) in a coastal area of the NW Mediterranean Sea. Six clusters were differentiated by flow cytometry on the basis of their optical properties, two heterotrophic prokaryote (HP) subgroups labelled LNA and HNA (low and high nucleic acid content respectively), Prochlorococcus, Synechococcus, autotrophic picoeukaryotes and nanoeukaryotes. HP represented an important component of the microbial assemblage over the survey with relatively small abundance variation through seasons. The carbon biomass ratio HP/ultraphytoplankton averaged 0.45, however this ratio exceeded 1 during spring. Ultraphytoplankton biomass made about 50% of the total autotrophic carbon estimates but this contribution increased up to 97% and 67% during the 2009 and 2010 spring periods respectively. Within ultraphytoplankton, nanoeukaryote represent the most important ultraphytoplankton group in terms of autotrophic carbon biomass (up to 70%). Picoeukaryote maximum abundance occurred in winter. Synechococcus was the most abundant population (maximum 1.2 x 10 5 cells cm-3) particularly in spring where it represented up to 54% of ultraphytoplankton carbon biomass. The warmer winter-spring temperatures and the lengthening of the stratification period created a favorable situation for the earlier appearance of Synechococcus and its persistence throughout summer, paralleling Prochlorococcus development. Prochlorococcus was dominant over summer and autumn with concentrations up to 1.0 × 10 5 cells cm-3. While the abundance of Synechococcus throughout survey was of the same order as that reported in western Mediterranean Sea, Prochlorococcus was more abundant and similar to the more typical oligotrophic and warm waters. The abundance variation of the ultraplankton components through the survey was relatable to variations in the hydrological and nutrient conditions.

Anthropogenic chemical cues can alter the swimming behaviour of juvenile stages of a temperate fish

Human pressure on coastal areas is affecting essential ecosystems including fish nursery habitats. Among these anthropogenic uses, the seasonal increment in the pressure due to leisure activities such as coastal tourism and yachting is an important environmental stressor in many coastal zones. These pressures may elicit understudied impacts due to, for example, sunscreens or other seasonal pollutants. The island of Majorca, northwest Mediterranean Sea, experiences one of the highest number of tourist visits per capita in the world, thus the surrounding coastal habitat is subject to high anthropogenic seasonal stress. Studies on early stages of fishes have observed responses to coastal chemical cues for the selection or avoidance of habitats. However, the potential interferences of human impacts on these signals are largely unknown. A choice chamber was used to determine water type preference and behaviour in naïve settled juvenile gilt-head sea bream (Sparus aurata), a temperate species of commercial interest. Fish were tested individually for behavioural changes with respect to water types from potential beneficial habitats, such as seawater with extract of the endemic seagrass Posidonia oceanica, anthropogenically influenced habitats such as water extracted from a commercial and recreational harbour and seawater mixed with sunscreen at concentrations observed in coastal waters. Using a Bayesian approach, we investigated a) water type preference; b) mean speed; and c) variance in the movement (as an indicator of burst swimming activity, or "sprint" behaviour) as behavioural descriptors with respect to water type. Fish spent similar percentage of time in treatment and control water types. However, movement descriptors showed that fish in sunscreen water moved slower (98.43% probability of being slower) and performed fewer sprints (90.1% probability of having less burst in speed) compared to control water. Less evident increases in sprints were observed in harbour water (73.56% more sprints), and seagrass (79.03% more) in comparison to control water. When seagrass water was tested against harbour water, the latter elicited a higher number of sprints (91.66% increase). We show that juvenile gilt-head seabream are able to react to a selection of naturally occurring chemically different odourscapes, including the increasingly important presence of sunscreen products, and provide a plausible interpretation of the observed behavioural patterns.

Genetic and ecophysiological traits of Synechococcus strains isolated from coastal and open ocean waters of the Arabian Sea

The picocyanobacterium Synechococcus is a prominent primary producer in the marine environment. The marine Synechococcus strains are clustered into different clades representing ecologically distinct genotypes. In this study, we compared phylogeny, photophysiology and cell cycles of four novel phycoerythrin-containing Synechococcus strains (clade II of subcluster 5.1) isolated from different depths of the water column (surface and subsurface waters) in coastal and offshore regions of the eastern Arabian Sea. The surface water strains possessed a lesser number of thylakoid layers and had a higher zeaxanthin to chlorophyll a ratio than subsurface strains indicating possible influence of light intensity available at their niche. The DNA distribution pattern of the four strains was bimodal in optimal cellular physiology conditions with cell division restricted to the light period and synchronized with the light-dark cycle. The presence of phycourobilin or phycoerythrobilin and the ratio between these two chromophores in all four strains varied according to available spectral wavelength in situ This study indicates that the timing of cell division is conserved within these genotypically identical Synechococcus strains, despite their having different chromophore ratios. We conclude that the timing of cell division of the Synechococcus strains has a genetic basis rather than being determined by phenotypic characters, such as chromophore content and ratio.

Phosphorus Deficiency Inhibits Cell Division But Not Growth in the Dinoflagellate Amphidinium carterae

Phosphorus (P) is an essential nutrient element for the growth of phytoplankton. How P deficiency affects population growth and the cell division cycle in dinoflagellates has only been studied in some species, and how it affects photosynthesis and cell growth remains poorly understood. In the present study, we investigated the impact of P deficiency on the cell division cycle, the abundance of the carbon-fixing enzyme Rubisco, and other cellular characteristics in the Gymnodiniales peridinin-plastid species Amphidinium carterae. We found that under P-replete condition, the cell cycle actively progressed in the culture in a 24-h diel cycle with daily growth rates markedly higher than the P-deficient cultures, in which cells were arrested in the G1 phase and cell size significantly enlarged. The results suggest that, as in previously studied dinoflagellates, P deficiency likely disenables A. carterae to complete DNA duplication or check-point protein phosphorylation. We further found that under P-deficient condition, overall photosystem II quantum efficiency (Fv/Fm ratio) and Rubisco abundance decreased but not significantly, while cellular contents of carbon, nitrogen, and proteins increased significantly. These observations indicated that under P-deficiency, this dinoflagellate was able to continue photosynthesis and carbon fixation, such that proteins and photosynthetically fixed carbon could accumulate resulting in continued cell growth in the absence of division. This is likely an adaptive strategy thereby P-limited cells can be ready to resume the cell division cycle upon resupply of phosphorus.

Nutrient Limitation in Surface Waters of the Oligotrophic Eastern Mediterranean Sea: an Enrichment Microcosm Experiment

The growth rates of planktonic microbes in the pelagic zone of the Eastern Mediterranean Sea are nutrient limited, but the type of limitation is still uncertain. During this study, we investigated the occurrence of N and P limitation among different groups of the prokaryotic and eukaryotic (pico-, nano-, and micro-) plankton using a microcosm experiment during stratified water column conditions in the Cretan Sea (Eastern Mediterranean). Microcosms were enriched with N and P (either solely or simultaneously), and the PO4 turnover time, prokaryotic heterotrophic activity, primary production, and the abundance of the different microbial components were measured. Flow cytometric and molecular fingerprint analyses showed that different heterotrophic prokaryotic groups were limited by different nutrients; total heterotrophic prokaryotic growth was limited by P, but only when both N and P were added, changes in community structure and cell size were detected. Phytoplankton were N and P co-limited, with autotrophic pico-eukaryotes being the exception as they increased even when only P was added after a 2-day time lag. The populations of Synechococcus and Prochlorococcus were highly competitive with each other; Prochlorococcus abundance increased during the first 2 days of P addition but kept increasing only when both N and P were added, whereas Synechococcus exhibited higher pigment content and increased in abundance 3 days after simultaneous N and P additions. Dinoflagellates also showed opportunistic behavior at simultaneous N and P additions, in contrast to diatoms and coccolithophores, which diminished in all incubations. High DNA content viruses, selective grazing, and the exhaustion of N sources probably controlled the populations of diatoms and coccolithophores.

Summer monsoon onset-induced changes of autotrophic pico- and nanoplankton in the largest monsoonal estuary along the west coast of India

This study presents the response of autotrophic pico- and nanoplankton to southwest monsoon-associated hydrographical transformations in the Cochin backwaters (CBW), the largest monsoonal estuary along the west coast of India. By the onset of the southwest monsoon, the euhaline/mesohaline conditions in the downstream/upstream of CBW usually transform into oligohaline/limnohaline. The flow cytometer analysis revealed the dominance of picoeukaryotes > Synechococcus > nanoautotrophs, with Prochlorococcus either very low or entirely absent. Synechococcus abundance was high during the pre-southwest monsoon (10(6) L(-1)), which dwindled with heavy fresh water influx during the southwest monsoon (10(5) L(-1)). The drastic drop in salinity and faster flushing of the CBW during the southwest monsoon replaced the euhaline/mesohaline strain of Synechococcus with an oligohaline/limnohaline strain. Epifluorescence microscopy analyses showed that, among the two strains of Synechococcus, the phycoerythrin-rich (PE-rich) one was dominant in the mesohaline/euhaline conditions, whereas the phycocyanin-rich (PC-rich) strain dominated in oligohaline/limnohaline conditions. Although Synechococcus abundance diminished during the southwest monsoon, the total abundance of picoplankton community remained virtually unchanged in the upstream due to an increase in the abundance of picoeukaryotes. On the other hand, the autotrophic nanoplankton abundance increased from pre-monsoon levels of av. 3.8 × 10(6)-av. 9.5 × 10(6) L(-1) at the onset of the southwest monsoon. Utilizing suitable multivariate analyses, the study illustrated the differential response and niche preference of various smaller communities of autotrophs to the southwest monsoon-associated hydrographical ramifications in a large monsoonal estuary, which may be applicable to similar such estuaries situated along the Indian coastline.

Phosphorus physiological ecology and molecular mechanisms in marine phytoplankton

Phosphorus (P) is an essential nutrient for marine phytoplankton and indeed all life forms. Current data show that P availability is growth-limiting in certain marine systems and can impact algal species composition. Available P occurs in marine waters as dissolved inorganic phosphate (primarily orthophosphate [Pi]) or as a myriad of dissolved organic phosphorus (DOP) compounds. Despite numerous studies on P physiology and ecology and increasing research on genomics in marine phytoplankton, there have been few attempts to synthesize information from these different disciplines. This paper is aimed to integrate the physiological and molecular information on the acquisition, utilization, and storage of P in marine phytoplankton and the strategies used by these organisms to acclimate and adapt to variations in P availability. Where applicable, we attempt to identify gaps in our current knowledge that warrant further research and examine possible metabolic pathways that might occur in phytoplankton from well-studied bacterial models. Physical and chemical limitations governing cellular P uptake are explored along with physiological and molecular mechanisms to adapt and acclimate to temporally and spatially varying P nutrient regimes. Topics covered include cellular Pi uptake and feedback regulation of uptake systems, enzymatic utilization of DOP, P acquisition by phagotrophy, P-limitation of phytoplankton growth in oceanic and coastal waters, and the role of P-limitation in regulating cell size and toxin levels in phytoplankton. Finally, we examine the role of P and other nutrients in the transition of phytoplankton communities from early succession species (diatoms) to late succession ones (e.g., dinoflagellates and haptophytes).

Physiology and evolution of nitrate acquisition in Prochlorococcus

Prochlorococcus is the numerically dominant phototroph in the oligotrophic subtropical ocean and carries out a significant fraction of marine primary productivity. Although field studies have provided evidence for nitrate uptake by Prochlorococcus, little is known about this trait because axenic cultures capable of growth on nitrate have not been available. Additionally, all previously sequenced genomes lacked the genes necessary for nitrate assimilation. Here we introduce three Prochlorococcus strains capable of growth on nitrate and analyze their physiology and genome architecture. We show that the growth of high-light (HL) adapted strains on nitrate is ∼17% slower than their growth on ammonium. By analyzing 41 Prochlorococcus genomes, we find that genes for nitrate assimilation have been gained multiple times during the evolution of this group, and can be found in at least three lineages. In low-light adapted strains, nitrate assimilation genes are located in the same genomic context as in marine Synechococcus. These genes are located elsewhere in HL adapted strains and may often exist as a stable genetic acquisition as suggested by the striking degree of similarity in the order, phylogeny and location of these genes in one HL adapted strain and a consensus assembly of environmental Prochlorococcus metagenome sequences. In another HL adapted strain, nitrate utilization genes may have been independently acquired as indicated by adjacent phage mobility elements; these genes are also duplicated with each copy detected in separate genomic islands. These results provide direct evidence for nitrate utilization by Prochlorococcus and illuminate the complex evolutionary history of this trait.

Diel variation of the cellular carbon to nitrogen ratio of Chlorella autotrophica (Chlorophyta) growing in phosphorus- and nitrogen-limited continuous cultures

We investigated the relationship between daily growth rates and diel variation of carbon (C) metabolism and C to nitrogen (N) ratio under P- and N-limitation in the green algae Chlorella autotrophica. To do this, continuous cultures of C. autotrophica were maintained in a cyclostat culture system under 14:10 light:dark cycle over a series of P- and N-limited growth rates. Cell abundance, together with cell size, as reflected by side scatter signal from flow cytometric analysis demonstrated a synchronized diel pattern with cell division occurring at night. Under either type of nutrient limitation, the cellular C:N ratio increased through the light period and decreased through the dark period over all growth rates, indicating a higher diel variation of C metabolism than that of N. Daily average cellular C:N ratios were higher at lower dilution rates under both types of nutrient limitation but cell enlargement was only observed at lower dilution rates under P-limitation. Carbon specific growth rates during the dark period positively correlated with cellular daily growth rates (dilution rates), with net loss of C during night at the lowest growth rates under N-limitation. Under P-limitation, dark C specific growth rates were close to zero at low dilution rates but also exhibited an increasing trend at high dilution rates. In general, diel variations of cellular C:N were low when dark C specific growth rates were high. This result indicated that the fast growing cells performed dark C assimilation at high rates, hence diminished the uncoupling of C and N metabolism at night.

PAHs reduce DNA synthesis and delay cell division in the widespread primary producer Prochlorococcus

The cyanobacteria Prochlorococcus is the most abundant primary producer in the ocean. In a global study across the Atlantic, Indian and Pacific Oceans, we tested the effect of organic pollutants on the growth and cell division of natural Prochlorococcus populations. Sub-lethal concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) altered Prochlorococcus cell division by reducing DNA synthesis and decreasing the percentage of cells entering mitosis. Cell division time increased with PAHs dosage by 1.2 h per μg L(-1) of mixture added. At PAHs dosages >1 μg L(-1), Prochlorococcus cell division tended to arrest at S-phase (DNA synthesis). As a consequence, population growth was significantly reduced in the presence of PAHs. The presence of PAHs resulted in a predictable alteration of the cell cycle of the widespread cyanobacteria.The dosages tested are above concentrations in the open ocean, but found in the coastal ocean, where Prochlorococcus growth must be inhibited.

Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Nearly half of the Earth's surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet--Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentation barely detectable by flow cytometry, suggesting low efficiency of CO2 fixation compared with other phytoplankton living in the same waters. To test the latter assumption, CO2 fixation rates of flow cytometrically sorted (14)C-labelled phytoplankton cells were directly compared in surface waters of the open Atlantic Ocean (30°S to 30°N). CO2 fixation rates of Prochlorococcus are at least 1.5-2.0 times higher than CO2 fixation rates of the smallest plastidic protists and Synechococcus cyanobacteria when normalised to photosynthetic pigmentation assessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcus cells to harvest plentiful sunlight more effectively than other phytoplankton.

Sunscreen products as emerging pollutants to coastal waters

A growing awareness of the risks associated with skin exposure to ultraviolet (UV) radiation over the past decades has led to increased use of sunscreen cosmetic products leading the introduction of new chemical compounds in the marine environment. Although coastal tourism and recreation are the largest and most rapidly growing activities in the world, the evaluation of sunscreen as source of chemicals to the coastal marine system has not been addressed. Concentrations of chemical UV filters included in the formulation of sunscreens, such as benzophehone 3 (BZ-3), 4-methylbenzylidene camphor (4-MBC), TiO₂ and ZnO, are detected in nearshore waters with variable concentrations along the day and mainly concentrated in the surface microlayer (i.e. 53.6-577.5 ng L⁻¹ BZ-3; 51.4-113.4 ng L⁻¹ 4-MBC; 6.9-37.6 µg L⁻¹ Ti; 1.0-3.3 µg L⁻¹ Zn). The presence of these compounds in seawater suggests relevant effects on phytoplankton. Indeed, we provide evidences of the negative effect of sunblocks on the growth of the commonly found marine diatom Chaetoceros gracilis (mean EC₅₀ = 125±71 mg L⁻¹). Dissolution of sunscreens in seawater also releases inorganic nutrients (N, P and Si forms) that can fuel algal growth. In particular, PO₄³⁻ is released by these products in notable amounts (up to 17 µmol PO₄³⁻g⁻¹). We conservatively estimate an increase of up to 100% background PO₄³⁻ concentrations (0.12 µmol L⁻¹ over a background level of 0.06 µmol L⁻¹) in nearshore waters during low water renewal conditions in a populated beach in Majorca island. Our results show that sunscreen products are a significant source of organic and inorganic chemicals that reach the sea with potential ecological consequences on the coastal marine ecosystem.

Molecular ecology of the marine cyanobacterial genera Prochlorococcus and Synechococcus

Oxygenic photoautotrophs of the genera Synechococcus and Prochlorococcus contribute significantly to primary production and are now widely accepted as the most abundant members of the picophytoplankton in the world's oceans. Since they represent one of the few cultured and representative groups of marine microorganisms, study of their physiology and biochemistry has progressed rapidly since their discovery. The recent and on-going sequencing of the complete genomes of representative strains will further hasten our understanding, and allow a complete interrogation, of the metabolism of these organisms. Moreover, since they inhabit a relatively simple environment they provide an excellent model system to begin to identify the underlying molecular mechanisms which allow their success in water columns with large vertical gradients of light and nutrients. Such work should provide novel insights into the genetic adaptations of these important marine microbes to their environment. We review here molecular ecological methods that are already available or which are currently being developed for these organisms. Such methods allow community structure, growth rate and nutrient status analysis, potentially at the single cell level, and can be used to define the niches, or identify the biotic or abiotic factors, which might control the productivity of specific genotypes. These techniques will undoubtedly provide the tools for answering more discerning questions concerning their ecology. How the complete genome sequence information is providing insights, and can further facilitate our understanding, of the ecology of these organisms is also discussed.

Effects of different nitrate and phosphate concentrations on the growth and toxin production of an Alexandrium tamarense strain collected from Drake Passage

Nitrate (N) and phosphate (P) are believed to be two of the most important nutrients for the growth and toxin production for Alexandrium species. The study of the growth and toxicity characteristics of the Alexandrium spp. under the change of N and P can help us to understand the dynamics of algal bloom and toxification events in natural environments. A strain of Alexandrium tamarense (designated as Kci) was successfully isolated from the Drake Passage in 2001 and the clonal culture has been kept in our laboratory (Ho et al., 2003, 2012). In order to extend our understanding on the growth physiology and toxicity of this A. tamarense strain, growth and cellular toxin content were examined in unialgal batch cultures under different concentrations of N and P. The effects of variable N, P concentrations on growth, cellular toxicity (fg STXeq. cell(-1)), and toxin composition (% molar) were determined in both exponential and stationary growth phases. The toxin profile, determined by high-performance liquid chromatography with fluorescence detection (HPLC-FD), was found to be remained relatively stable and was consistently dominated by the N-sulfocarbamoyl C-toxins (>90%) under different conditions and growth phases. There were also trace amounts of other carbamate gonyautoxins consistently expressed. The cellular toxicity varied under different N and P concentrations, as well as different growth stages. A positive correlation was observed between cellular toxicity and N concentrations, but the toxicity was enhanced when P was depleted. Both cell densities and growth rate of the cells were severely suppressed under N- or P-depletion. However, the biovolume of the cells tended to be larger at N- or P-depleted cultures. Results from the present study provide valuable insight for the ecophysiology of Alexandrium species in the coastal ecosystem of Drake Passage.

Diversity and Distribution of Marine Synechococcus: Multiple Gene Phylogenies for Consensus Classification and Development of qPCR Assays for Sensitive Measurement of Clades in the Ocean

Marine Synechococcus is a globally significant genus of cyanobacteria that is comprised of multiple genetic lineages or clades. These clades are thought to represent ecologically distinct units, or ecotypes. Because multiple clades often co-occur together in the oceans, Synechococcus are ideal microbes to explore how closely related bacterial taxa within the same functional guild of organisms co-exist and partition marine habitats. Here we sequenced multiple gene loci from cultured strains to confirm the congruency of clade classifications between the 16S-23S rDNA internally transcribed spacer (ITS), 16S rDNA, narB, ntcA, and rpoC1 loci commonly used in Synechococcus diversity studies. We designed quantitative PCR (qPCR) assays that target the ITS for 10 Synechococcus clades, including four clades, XV, XVI, CRD1, and CRD2, not covered by previous assays employing other loci. Our new qPCR assays are very sensitive and specific, detecting down to tens of cells per ml. Application of these qPCR assays to field samples from the northwest Atlantic showed clear shifts in Synechococcus community composition across a coastal to open-ocean transect. Consistent with previous studies, clades I and IV dominated cold, coastal Synechococcus communities. Clades II and X were abundant at the two warmer, off-shore stations, and at all stations multiple Synechococcus clades co-occurred. qPCR assays developed here provide valuable tools to further explore the dynamics of microbial community structure and the mechanisms of co-existence.

Bacterioplankton groups involved in the uptake of phosphate and dissolved organic phosphorus in a mesocosm experiment with P-starved Mediterranean waters

The use of inorganic phosphate (Pi) and dissolved organic phosphorus (DOP) by different bacterial groups was studied in experimental mesocosms of P-starved eastern Mediterranean waters in the absence (control mesocosms) and presence of additional Pi (P-amended mesocosms). The low Pi turnover times in the control mesocosms and the increase in heterotrophic prokaryotic abundance and production upon Pi addition confirmed that the bacterial community was originally P-limited. The bacterioplankton groups taking up Pi and DOP were identified by means of microautoradiography combined with catalysed reporter deposition fluorescence in situ hybridization. Incubations with leucine were also performed for comparative purposes. All the probe-identified groups showed a high percentage of cells taking up Pi and DOP in the control, P-limited, mesocosms throughout the experiment. However, in response to Pi addition two contrasting scenarios in Pi use were observed: (i) on day 1 of the experiment Pi addition caused a clear reduction in the percentage of SAR11 cells taking up Pi, whereas Gammaproteobacteria, Roseobacter and Bacteroidetes showed similar percentages to the ones in the control mesocosms and (ii) on day 4 of the experiment, probably when the bacterial community had fully responded to the P input, all the probe-identified groups showed low percentages of cells taking up the substrate as compared with the control mesocosms. These differences are likely related to different P requirements among the bacterial groups and point out to the existence of two contrasting strategies in P use.

Temporal orchestration of glycogen synthase (GlgA) gene expression and glycogen accumulation in the oceanic picoplanktonic cyanobacterium Synechococcus sp. strain WH8103

Glycogen is accumulated during the latter half of the diel cycle in Synechococcus sp. strain WH8103 following a midday maximum in glgA (encoding glycogen synthase) mRNA abundance. This temporal pattern is quite distinct from that of Prochlorococcus and may highlight divergent regulatory control of carbon/nitrogen metabolism in these closely related picocyanobacteria.

DIEL IN SITU PICOPHYTOPLANKTON CELL DEATH CYCLES COUPLED WITH CELL DIVISION(1)

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.

Enumeration and Cell Cycle Analysis of Natural Populations of Marine Picoplankton by Flow Cytometry Using the Nucleic Acid Stain SYBR Green I

The novel dye SYBR Green I binds specifically to nucleic acids and can be excited by blue light (488-nm wavelength). Cell concentrations of prokaryotes measured in marine samples with this dye on a low-cost compact flow cytometer are comparable to those obtained with the UV-excited stain Hoechst 33342 (bis-benzimide) on an expensive flow cytometer with a water-cooled laser. In contrast to TOTO-1 and TO-PRO-1, SYBR Green I has the advantage of clearly discriminating both heterotrophic bacteria and autotrophic Prochlorococcus cells, even in oligotrophic waters. As with TOTO-1 and TO-PRO-1, two groups of heterotrophic bacteria (B-I and B-II-like types) can be distinguished. Moreover, the resolution of DNA distribution obtained with SYBR Green I is similar to that obtained with Hoechst 33342 and permits the analysis of the cell cycle of photosynthetic prokaryotes over the whole water column.

Individual-based modeling of phytoplankton: Evaluating approaches for applying the cell quota model

Present phytoplankton models typically use a population-level (lumped) modeling (PLM) approach that assumes average properties of a population within a control volume. For modern biogeochemical models that formulate growth as a nonlinear function of the internal nutrient (e.g. Droop kinetics), this averaging assumption can introduce a significant error. Individual-based (agent-based) modeling (IBM) does not make the assumption of average properties and therefore constitutes a promising alternative for biogeochemical modeling. This paper explores the hypothesis that the cell quota (Droop) model, which predicts the population-average specific growth or cell division rate, based on the population-average nutrient cell quota, can be applied to individual algal cells and produce the same population-level results. Three models that translate the growth rate calculated using the cell quota model into discrete cell division events are evaluated, including a stochastic model based on the probability of cell division, a deterministic model based on the maturation velocity and fraction of the cell cycle completed (maturity fraction), and a deterministic model based on biomass (carbon) growth and cell size. The division models are integrated into an IBM framework (iAlgae), which combines a lumped system representation of a nutrient with an individual representation of algae. The IBM models are evaluated against a conventional PLM (because that is the traditional approach) and data from a number of steady and unsteady continuous (chemostat) and batch culture laboratory experiments. The stochastic IBM model fails the steady chemostat culture test, because it produces excessive numerical randomness. The deterministic cell cycle IBM model fails the batch culture test, because it has an abrupt drop in cell quota at division, which allows the cell quota to fall below the subsistence quota. The deterministic cell size IBM model reproduces the data and PLM results for all experiments and the model parameters (e.g. maximum specific growth rate, subsistence quota) are the same as those for the PLM. In addition, the model-predicted cell age, size (carbon) and volume distributions are consistent with those derived analytically and compare well to observations. The paper discusses and illustrates scenarios where intra-population variability in natural systems leads to differences between the IBM and PLM models.

Marine bacterioplankton production of polysaccharidic and proteinaceous particles under different nutrient regimes

The influence of inorganic nutrient concentrations on the ability of bacterioplankton to produce and degrade polysaccharidic transparent exopolymer particles (TEPs) and proteinaceous Coomassie-stained particles (CSPs) was investigated in an 11-day experiment. The dynamics of these particles were followed in prefiltered (1 microm) northern Adriatic seawater enclosures enriched either with 1 microM orthophosphate (main limiting nutrient in this area), 10 microM ammonium or both orthophosphate and ammonium. These enclosures were referenced to a nonenriched control. A high potential for bacterial TEP and CSP production was observed (10(4) - 10(5) L(-1) for particles larger than 4 microm). In conditions of high orthophosphate concentration (either orthophosphate enriched or both orthophosphate and ammonium enriched), lower abundances and surface areas of CSPs were obtained, whereas TEP dynamics were more affected by unbalanced enrichments where only orthophosphate or ammonium was added. The impact of unbalanced nutrient ratios on TEPs was indicated by their higher abundance but low capacity for Alcian blue absorption, implying a change in their structure. Inorganic nutrient availability was thus proven to affect the bacterial potential for producing and degrading bacterially derived TEPs and CSPs.

Accounting for intrapopulation variability in biogeochemical models using agent-based methods

Present biogeochemical models typically use a lumped-system (population-level) modeling (LSM) approach that assumes average properties of a population within a control volume. For modern models that formulate phytoplankton growth as a nonlinear function of the internal nutrient (e.g., Droop kinetics), this averaging assumption can introduce a significant error. Agent-based (individual-based) modeling (ABM) is an alternative approach that does not make the assumption of average properties. This paper presents a new agent-based phytoplankton model called iAlgae. The model is contrasted to a conventional lumped-system model, constructed based on identical underlying sub-models of nutrient uptake (including luxury uptake) and growth (cell quota, Droop model). The two models are validated against laboratory data and applied to a realistic scenario, consisting of a point source nutrient discharge into a river. For the realistic scenario, the ABM-predicted phytoplankton bloom is significantly lower than the LSM-predicted one, which is due to the intrapopulation distribution in cell quotas (due to different life histories of individuals) and nonlinearity of the growth rate model. In the ABM, a fraction of the population accumulates nutrients in excess of their immediate growth requirement (luxury uptake), leaving less for the remainder. Because the model is nonlinear, this results in a suboptimal (from a population perspective) utilization of nutrient and a lower population-level growth rate, compared to the case of no intrapopulation variability assumed by the LSM model. In general, the ABM and LSM approaches can produce significantly different results when incompletely mixed conditions lead to intrapopulation variability in cell properties (i.e., cell quota) and the model equations are nonlinear.

Phosphate acquisition genes in Prochlorococcus ecotypes: evidence for genome-wide adaptation

The cyanobacterium Prochlorococcus is the numerically dominant phototroph in the oligotrophic oceans. This group consists of multiple ecotypes that are physiologically and phylogenetically distinct and occur in different abundances along environmental gradients. Here we examine adaptations to phosphate (P) limitation among ecotypes. First, we used DNA microarrays to identify genes involved in the P-starvation response in two strains belonging to different ecotypes, MED4 (high-light-adapted) and MIT9313 (low-light-adapted). Most of the up-regulated genes under P starvation were unique to one strain. In MIT9313, many ribosomal genes were down-regulated, suggesting a general stress response in this strain. We also observed major differences in regulation. The P-starvation-induced genes comprise two clusters on the chromosome, the first containing the P master regulator phoB and most known P-acquisition genes and the second, absent in MIT9313, containing genes of unknown function. We examined the organization of the phoB gene cluster in 11 Prochlorococcus strains belonging to diverse ecotypes and found high variability in gene content that was not congruent with rRNA phylogeny. We hypothesize that this genome variability is related to differences in P availability in the oceans from which the strains were isolated. Analysis of a metagenomic library from the Sargasso Sea supports this hypothesis; most Prochlorococcus cells in this low-P environment contain the P-acquisition genes seen in MED4, although a number of previously undescribed gene combinations were observed.

Changes in Synechococcus population size and cellular ribosomal RNA content in response to predation and nutrient limitation

A mathematical model of predator-prey interactions was used to predict the relationship between population size and cellular growth rate in a two-tiered trophic system consisting of Synechococcus PCC 6301 and Tetrahymena pyriformis. As predicted, axenic chemostat cultures of Synechococcus responded to increased nutrient availability by expanding the equilibrium population size without a concurrent change in growth rate. Likewise, the addition of the predator Tetrahymena pyriformis decreased the Synechococcus population size by 85% and increased the Synechococcus growth rate. Synechococcus populations in the surface waters of the Gulf of Mexico were sampled to ascertain whether the relationship between population size and cellular 16S rRNA concentration conformed to that predicted by the model. Direct counts of autofluorescent cells in size-fractionated seawater samples provided an estimate of Synechococcus population size. The growth rate of in situ populations was estimated by measuring the extent of hybridization of an oligonucleotide probes complementary to Synechococcus 16S rRNA, based on evidence that ribosomal RNA content increases concurrently with growth rate. The comparison of in situ population sizes and specific growth rates revealed that relatively large Synechococcus populations were growing slowly, indicative of nutrient limitation, and that quickly growing populations were relatively small, as predicted for predator-limited populations.

Physiological diversity and niche adaptation in marine Synechococcus

During the twenty years or so since the discovery of tiny photosynthetic cells of the genus Synechococcus in marine oceanic systems, a tremendous expansion of interest has been seen in the literature pertaining to these organisms. The fact that they are ubiquitous and abundant in major oceanic regimes underlies their ecological importance as significant contributors to marine C fixation. Recent advances in the physiology and biochemistry of these organisms are presented here, focusing on strains of the MC-A and MC-B clusters; it is stressed that the data contained herein should be put into the context of the ecological niche occupied by particular genotypes in situ. This system is ripe for joining the often separate disciplines of molecular ecology and microbial physiology and provides a great opportunity to tease out the underlying processes that both mediate organism evolution and also the environmental factors that dictate this.

In vivo regulation of glutamine synthetase activity in the marine chlorophyll b-containing cyanobacterium Prochlorococcus sp. strain PCC 9511 (oxyphotobacteria)

The physiological regulation of glutamine synthetase (GS; EC 6.3.1.2) in the axenic Prochlorococcus sp. strain PCC 9511 was studied. GS activity and antigen concentration were measured using the transferase and biosynthetic assays and the electroimmunoassay, respectively. GS activity decreased when cells were subjected to nitrogen starvation or cultured with oxidized nitrogen sources, which proved to be nonusable for Prochlorococcus growth. The GS activity in cultures subjected to long-term phosphorus starvation was lower than that in equivalent nitrogen-starved cultures. Azaserine, an inhibitor of glutamate synthase, provoked an increase in enzymatic activity, suggesting that glutamine is not involved in GS regulation. Darkness did not affect GS activity significantly, while the addition of diuron provoked GS inactivation. GS protein determination showed that azaserine induces an increase in the concentration of the enzyme. The unusual responses to darkness and nitrogen starvation could reflect adaptation mechanisms of Prochlorococcus for coping with a light- and nutrient-limited environment.

Diel rhythms in ribulose-1,5-bisphosphate carboxylase/oxygenase and glutamine synthetase gene expression in a natural population of marine picoplanktonic cyanobacteria (Synechococcus spp.)

Diel periodicity in the expression of key genes involved in carbon and nitrogen assimilation in marine Synechococcus spp. was investigated in a natural population growing in the surface waters of a cyclonic eddy in the northeast Atlantic Ocean. Synechococcus sp. cell concentrations within the upper mixed layer showed a net increase of three- to fourfold during the course of the experiment (13 to 22 July 1991), the population undergoing approximately one synchronous division per day. Consistent with the observed temporal pattern of phycoerythrin (CpeBA) biosynthesis, comparatively little variation was found in cpeBA mRNA abundance during either of the diel cycles investigated. In marked contrast, the relative abundance of transcripts originating from the genes encoding the large subunit of ribulose bisphosphate carboxylase/oxygenase (rbcL) and glutamine synthetase (glnA) showed considerable systematic temporal variation and oscillated during the course of each diel cycle in a reciprocal rhythm. Whereas activation of rbcL transcription was clearly not light dependent, expression of glnA appeared sensitive to endogenous changes in the physiological demands for nitrogen that arise as a natural consequence of temporal periodicity in photosynthetic carbon assimilation. The data presented support the hypothesis that a degree of temporal separation may exist between the most active periods of carbon and nitrogen assimilation in natural populations of marine Synecoccoccus spp.

An immunological approach to detect phosphate stress in populations and single cells of photosynthetic picoplankton

In the marine cyanobacterium Synechococcus sp. strain WH7803, PstS is a 32-kDa cell wall-associated phosphate-binding protein specifically synthesized under conditions of restricted inorganic phosphate (P1) availability (D. J. Scanlan, N. H. Mann, and N. G. Carr, Mol. Microbiol. 10:181-191, 1993). We have assessed its use as a potential diagnostic marker for the P status of photosynthetic picoplankton. Expression of PstS in Synechococcus sp. strain WH7803 was observed when the P1 concentration fell below 50 nM, demonstrating that the protein is induced at concentrations of P1 typical of oligotrophic conditions. PstS expression could be specifically detected by use of standard Western blotting (immunoblotting) techniques in natural mesocosm samples under conditions in which the N/P ratio was artificially manipulated to force P depletion. In addition, we have developed an immunofluorescence assay that can detect PstS expression in single Synechococcus cells both in laboratory cultures and natural samples. We show that antibodies raised against PstS cross-react with P-depleted Prochlorococcus cells, extending the use of these antibodies to both major groups of prokaryotic photosynthetic picoplankton. Furthermore, DNA sequencing of a Prochlorococcus pstS homolog demonstrated high amino acid sequence identity (77%) with the marine Synechococcus sp. strain WH7803 protein, including those residues in Escherichia coli PstS known to be directly involved in phosphate binding.