Picocyanobacteria Synechococcus Research Articles

Proteomics analysis reveals differential acclimation of coastal and oceanic Synechococcus to climate warming and iron limitation.

In many oceanic regions, anthropogenic warming will coincide with iron (Fe) limitation. Interactive effects between warming and Fe limitation on phytoplankton physiology and biochemical function are likely, as temperature and Fe availability affect many of the same essential cellular pathways. However, we lack a clear understanding of how globally significant phytoplankton such as the picocyanobacteria Synechococcus will respond to these co-occurring stressors, and what underlying molecular mechanisms will drive this response. Moreover, ecotype-specific adaptations can lead to nuanced differences in responses between strains. In this study, Synechococcus isolates YX04-1 (oceanic) and XM-24 (coastal) from the South China Sea were acclimated to Fe limitation at two temperatures, and their physiological and proteomic responses were compared. Both strains exhibited reduced growth due to warming and Fe limitation. However, coastal XM-24 maintained relatively higher growth rates in response to warming under replete Fe, while its growth was notably more compromised under Fe limitation at both temperatures compared with YX04-1. In response to concurrent heat and Fe stress, oceanic YX04-1 was better able to adjust its photosynthetic proteins and minimize the generation of reactive oxygen species while reducing proteome Fe demand. Its intricate proteomic response likely enabled oceanic YX04-1 to mitigate some of the negative impact of warming on its growth during Fe limitation. Our study highlights how ecologically-shaped adaptations in Synechococcus strains even from proximate oceanic regions can lead to differing physiological and proteomic responses to these climate stressors.

Dual thermal ecotypes coexist within a nearly genetically identical population of the unicellular marine cyanobacterium Synechococcus.

The extent and ecological significance of intraspecific functional diversity within marine microbial populations is still poorly understood, and it remains unclear if such strain-level microdiversity will affect fitness and persistence in a rapidly changing ocean environment. In this study, we cultured 11 sympatric strains of the ubiquitous marine picocyanobacterium Synechococcus isolated from a Narragansett Bay (RI) phytoplankton community thermal selection experiment. Thermal performance curves revealed selection at cool and warm temperatures had subdivided the initial population into thermotypes with pronounced differences in maximum growth temperatures. Curiously, the genomes of all 11 isolates were almost identical (average nucleotide identities of >99.99%, with >99% of the genome aligning) and no differences in gene content or single nucleotide variants were associated with either cool or warm temperature phenotypes. Despite a very high level of genomic similarity, sequenced epigenomes for two strains showed differences in methylation on genes associated with photosynthesis. These corresponded to measured differences in photophysiology, suggesting a potential pathway for future mechanistic research into thermal microdiversity. Our study demonstrates that present-day marine microbial populations can harbor cryptic but environmentally relevant thermotypes which may increase their resilience to future rising temperatures.

Biogenic silica accumulation in picoeukaryotes: Novel players in the marine silica cycle.

It is well known that the biological control of oceanic silica cycling is dominated by diatoms, with sponges and radiolarians playing additional roles. Recent studies have revealed that some smaller marine organisms (e.g. the picocyanobacterium Synechococcus) also take up silicic acid (dissolved silica, dSi) and accumulate silica, despite not exhibiting silicon dependent cellular structures. Here, we show biogenic silica (bSi) accumulation in five strains of picoeukaryotes (<2-3μm), including three novel isolates from the Baltic Sea, and two marine species (Ostreococcus tauri and Micromonas commoda), in cultures grown with added dSi (100 μM). Average bSi accumulation in these novel biosilicifiers was between 30 and 92 amol Si cell-1 . Growth rate and cell size of the picoeukaryotes were not affected by dSi addition. Still, the purpose of bSi accumulation in these smaller eukaryotic organisms lacking silicon dependent structures remains unclear. In line with the increasing recognition of picoeukaryotes in biogeochemical cycling, our findings suggest that they can also play a significant role in silica cycling.

Photoinhibition of the Picophytoplankter Synechococcus Is Exacerbated by Ocean Acidification

The marine picocyanobacterium Synechococcus accounts for a major fraction of the primary production across the global oceans. However, knowledge of the responses of Synechococcus to changing pCO2 and light levels has been scarcely documented. Hence, we grew Synechococcus sp. CB0101 at two CO2 concentrations (ambient CO2 AC:410 μatm; high CO2 HC:1000 μatm) under various light levels between 25 and 800 μmol photons m−2 s−1 for 10–20 generations and found that the growth of Synechococcus strain CB0101 is strongly influenced by light intensity, peaking at 250 μmol m−2 s−1 and thereafter declined at higher light levels. Synechococcus cells showed a range of acclimation in their photophysiological characteristics, including changes in pigment content, optical absorption cross section, and light harvesting efficiency. Elevated pCO2 inhibited the growth of cells at light intensities close to or greater than saturation, with inhibition being greater under high light. Elevated pCO2 also reduced photosynthetic carbon fixation rates under high light but had smaller effects on the decrease in quantum yield and maximum relative electron transport rates observed under increasing light intensity. At the same time, the elevated pCO2 significantly decreased particulate organic carbon (POC) and particulate organic nitrogen (PON), particularly under low light. Ocean acidification, by increasing the inhibitory effects of high light, may affect the growth and competitiveness of Synechococcus in surface waters in the future scenario.

Coexistence of Synechococcus and Microcystis Blooms in a Tropical Urban Reservoir and Their Links with Microbiomes.

Bacteria play a crucial role in driving ecological processes in aquatic ecosystems. Studies have shown that bacteria-cyanobacteria interactions contributed significantly to phytoplankton dynamics. However, information on the contribution of bacterial communities to blooms remains scarce. Here, we tracked changes in the bacterial community during the development of a cyanobacterial bloom in an equatorial estuarine reservoir. Two forms of blooms were observed simultaneously corresponding to the lotic and lentic characteristics of the sampling sites where significant spatial variabilities in physicochemical water quality, cyanobacterial biomass, secondary metabolites, and cyanobacterial/bacterial compositions were detected. Microcystis dominated the upstream sites during peak periods and were succeeded by Synechococcus when the bloom subsided. For the main body of the reservoir, a mixed bloom featuring coccoid and filamentous cyanobacteria (Microcystis, Synechococcus, Planktothricoides, Nodosilinea, Raphidiopsis, and Prochlorothrix) was observed. Concentrations of the picocyanobacteria Synechococcus remained high throughout the study, and their positive correlations with cylindrospermopsin and anatoxin-a suggested that they could produce cyanotoxins, which pose more damaging impacts than previously supposed. Succession of different cyanobacteria (Synechococcus and Microcystis) following changes in nutrient composition and ionic strength was demonstrated. The microbiomes associated with blooms were unique to the dominant cyanobacteria. Generic and specialized bloom biomarkers for the Microcystis and downstream mixed blooms were also identified. Microscillaceae, Chthoniobacteraceae, and Roseomonas were the major heterotrophic bacteria associated with Microcystis bloom, whereas Phycisphaeraceae and Methylacidiphilaceae were the most prominent groups for the Synechococcus bloom. Collectively, bacterial community can be greatly deviated by the geological condition, monsoon season, cyanobacterial density, and dominant cyanobacteria.

Contrasting sensitivity among oligotrophic marine microbial communities to priority PAHs

Oligotrophic areas represent a large proportion of the oceans, wherein microbial food webs largely determine carbon flux dynamics and biogeochemical cycles. However, little is known regarding the sensitivity of microbial planktonic communities to pollutants in such areas. Organic pollutants such as polycyclic aromatic hydrocarbons (PAH/s) are toxic oil derivatives that occur as complex mixtures and reach marine environments through different sources. Therefore, our study analyzed the PAH tolerance of natural photosynthetic and heterotrophic bacteria and eukaryotes from the oligotrophic Red Sea, which is uniquely susceptible to high oil contamination. Natural communities sampled from the surface layer were exposed to a concentration gradient of a mixture of 16 priority PAHs at in situ conditions for 48 h. The populations of the dominant picocyanobacteria Synechococcus sp., picophytoeukaryotes, and low nucleic acid (LNA) bacteria decreased upon exposure to PAHs in a strong dose-dependent manner. Chlorophyll-a, which was measured as an indicator of the total autotrophic community response, also decreased substantially. High nucleic acid (HNA) bacteria, however, exhibited lower growth inhibition (<50%). The lethal concentration (LC10) thresholds to the 16-PAH mixture demonstrated contrasting sensitivities among the microbial communities studied increasing from picoeukaryotes (5.98 ± 2.08 μg L−1) < chlorophyll-a (19.51 ± 8.11 μg L−1) < LNA bacteria (23.63 ± 10.64 μg L−1) < Synechococcus sp. (26.77 ± 13.34 μg L−1) < HNA bacteria (97.13 ± 17.28 μg L−1). The sensitivity of Red Sea Synechococcus and picophytoeukaryotes to the 16-PAH mixture was between 2 and 6.5 times higher compared to single PAH compounds tested previously. However, some populations of HNA bacteria and Synechococcus sp., were highly tolerant, suggesting an adaptation to chronic pollution. Concerningly, the LC10 toxicity thresholds approached the ambient PAH concentrations in the Red Sea, suggesting that environmental oil pollution actively shapes the microbial community structures in the region.

An update on dissolved methane distribution in the subtropical North Atlantic Ocean

Abstract. Methane (CH4) is a potent greenhouse gas and plays a significant role in recent increasing global temperatures. The oceans are a natural source of methane contributing to atmospheric methane concentrations, yet our understanding of the oceanic methane cycle is poorly constrained. Accumulating evidence indicates that a significant part of oceanic CH4 is produced in oxygenated surface waters as a by-product of phytoplanktonic activity. This study focused on the subtropical North Atlantic Ocean (26∘ N, 80′ W and 26∘ N, 18′ W) where the distribution of dissolved CH4 concentrations and associated air–sea fluxes during winter 2020 were investigated. Water samples from 64 stations were collected from the upper water column up to depths of 400 m. The upper oxic mixed layer was oversaturated in dissolved CH4 with concentrations ranging 3–7 nmol L−1, with the highest concentrations of 7–10 nmol L−1 found to the east of the transect, consistent with other subtropical regions of the world's oceans. The high anomalies of dissolved CH4 were found to be associated with phosphate-depleted waters and regions where the abundance of the ubiquitous picocyanobacteria Synechococcus and Prochlorococcus were elevated. Although other phytoplanktonic phyla cannot be excluded, this suggests that cyanobacteria contribute to the release of CH4 in this region. The calculation of air–sea fluxes further confirmed the subtropical North Atlantic Ocean as a source of CH4. This study provides evidence to corroborate the key role that picocyanobacteria play in helping to explain the oversaturation of CH4 found in surface mixed layer of the open ocean, otherwise known as the “ocean methane paradox”.

Highly Diverse Synechococcus Pigment Types in the Eastern Indian Ocean.

Marine picocyanobacteria Synechococcus exhibit highly diverse pigment types (PTs) and hence possess great advantage to utilize different spectrum of light effectively and to occupy a wide range of light niches. In this study, we explored the diversity of Synechococcus PTs in the eastern Indian Ocean (EIO), surface water of Strait of Malacca (SSM), and coastal waters of Sri Lanka (SSL). All the detected PTs were phycourobilin (PUB) containing PT 3 and showed distinct distribution patterns. Low PUB PT 3a and partial chromatic acclimater PT 3eA dominated in coastal and shallow waters (SSM and SSL). In contrast, high PUB and chromatic acclimaters PT 3dA and PT 3c/3dB were mainly distributed in open ocean (EIO). PT 3dA and PT 3c/3dB occurred at similar depths of the lower euphotic layers but showed distinct distribution pattern that are partially exclusive, indicating that they compete with each other for the same light niche. Interestingly, the newly described PT 3f was detected with high relative abundances at all stations and particularly dominated in the upper euphotic layer in EIO, which was confirmed with PT-specific quantitative polymerase chain reaction (qPCR). The relative abundance of PT 3f was negatively correlated with nutrient level, implying that PT 3f is adapted to oligotrophic waters. Pronounced niche partition of different PTs was observed in the upper and lower layers of euphotic zone in EIO and SSM/SSL. Light, nutrients, and strong stratification may play important roles in the niche partition of different PTs. Further analysis about ecologically significant taxonomic units revealed high diversity within each PT at different locations, which provided insights for understanding specific PT with wide range of niches.

Photosynthetic Pigments Changes of Three Phenotypes of Picocyanobacteria Synechococcus sp. under Different Light and Temperature Conditions

It is estimated that the genus Synechococcus is responsible for about 17% of net primary production in the Global Ocean. Blooms of these organisms are observed in tropical, subtropical and even temperate zones, and they have been recorded recently even beyond the polar circle. The long-term scenarios forecast a growing expansion of Synechococcus sp. and its area of dominance. This is, among others, due to their high physiological plasticity in relation to changing environmental conditions. Three phenotypes of the genus Synechococcus sp. (Type 1, Type 2, and Type 3a) were tested in controlled laboratory conditions in order to identify their response to various irradiance (10, 55, 100 and 145 µmol photons m−2 s−1) and temperature (15, 22.5 and 30 °C) conditions. The highest total pigment content per cell was recorded at 10 μmol photons m−2 s−1 at all temperature variants with the clear dominance of phycobilins among all the pigments. In almost every variant the highest growth rate was recorded for the Type 1. The lowest growth rates were observed, in general, for the Type 3a. However, it was recognized to be less temperature sensitive in comparison to the other two types and rather light-driven with the highest plasticity and adaptation potential. The highest amounts of carotenoids were produced by Type 2 which also showed signs of the cell stress even around 55 μmol photons m−2 s−1 at 15 °C and 22.5 °C. This may imply that the Type 2 is the most susceptible to higher irradiances. Picocyanobacteria Synechococcus sp. require less light intensity to achieve the maximum rate of photosynthesis than larger algae. They also tolerate a wide range of temperatures which combined together make them gain a powerful competitive advantage. Our results will provide key information for the ecohydrodynamical model development. Thus, this work would be an important link in forecasting future changes in the occurrence of these organisms in the context of global warming.

Dynamics and functional diversity of the smallest phytoplankton on the Northeast US Shelf

Picophytoplankton are the most abundant primary producers in the ocean. Knowledge of their community dynamics is key to understanding their role in marine food webs and global biogeochemical cycles. To this end, we analyzed a 16-y time series of observations of a phytoplankton community at a nearshore site on the Northeast US Shelf. We used a size-structured population model to estimate in situ division rates for the picoeukaryote assemblage and compared the dynamics with those of the picocyanobacteria Synechococcus at the same location. We found that the picoeukaryotes divide at roughly twice the rate of the more abundant Synechococcus and are subject to greater loss rates (likely from viral lysis and zooplankton grazing). We describe the dynamics of these groups across short and long timescales and conclude that, despite their taxonomic differences, their populations respond similarly to changes in the biotic and abiotic environment. Both groups appear to be temperature limited in the spring and light limited in the fall and to experience greater mortality during the day than at night. Compared with Synechococcus, the picoeukaryotes are subject to greater top-down control and contribute more to the region's primary productivity than their standing stocks suggest.

The Importance of Allelopathic Picocyanobacterium Synechococcus sp. on the Abundance, Biomass Formation, and Structure of Phytoplankton Assemblages in Three Freshwater Lakes.

The contribution of picocyanobacteria to summer phytoplankton blooms, accompanied by an ecological crisis, is a new phenomenon in Europe. This issue requires careful investigation. We studied allelopathic activity of freshwater picocyanobacterium Synechococcus sp. on phytoplankton assemblages from three freshwater lakes. In this study, the allelopathic activity of the Synechococcus sp. on the total abundance, biomass, as well as structure of the phytoplankton assemblages were investigated. Our results indicated that addition of exudates obtained from Synechococcus sp. affected the number of cells and biomass of the phytoplankton communities; the degree of inhibition or stimulation was different for each species, causing a change in the phytoplankton abundance and dominance during the experiment. We observed that some group of organisms (especially cyanobacteria from the genus Aphanothece, Limnothrix, Microcystis, and Synechococcus) showed tolerance for allelopathic compounds produced and released by Synechococcus sp. It is also worth noting that in some samples, Bacillariophyceae (e.g., Amphora pediculus, Navicula pygmaea, and Nitzschia paleacea) were completely eliminated in the experimental treatments, while present in the controls. This work demonstrated that the allelopathic activity exhibited by the Synechococcus sp. is probably one of the major competitive strategies affecting some of the coexisting phytoplankton species in freshwater ecosystems. To our best knowledge this is the first report of the allelopathic activity of Synechococcus sp. in the freshwater reservoirs, and one of the few published works showing allelopathic properties of freshwater picocyanobacteria on coexisting phytoplankton species.

Ultraphytoplankton community composition in Southwestern and Eastern Mediterranean Basin: Relationships to water mass properties and nutrients

The composition, biomass, and spatial and vertical distribution of ultraphytoplankton (< 10 μm) were investigated at the single-cell level by flow cytometry during the INCOMMET oceanographic cruise carried out during the fall of November 2013, covering eleven stations spread out along coastal Tunisian waters at less than 100 m depth. Flow cytometry analysis of samples taken at different stations and depths allowed the determination of five major groups of ultraphytoplankton; among them two populations of pico-cyanobacteria Synechococcus and Prochlorococcus, picoeukaryotes, nanoeukaryotes and cryptophytes. In addition to these autotrophic groups, three unknown groups were resolved and were distinguished by their fluorescence and size signals. The unknown groups' integrated biomass was larger than that of the other ultraphytoplankton components. The abundance of picoeukaryotes, nanoeukaryotes and cryptophytes increased gradually southwards towards the Gulf of Gabès, whereas Prochlorococcus were more abundant in the Gulf of Hammamet at more than 60 m depth. Structural equation modeling analysis showed weak links between most of the cell groups and nutrients concentrations. Clustering analysis based on the abundance of the phytoplankton groups resulted in aggregation into three clusters and two major zones, discriminating the Gulf of Gabès from the more northern Bays. The results suggest that the physical features, essentially the water mass properties, were the main drivers shaping the ultraphytoplankton community composition and that the autotrophic biomasses were not determined by nutrient control.

First data on cyanobacterial biodiversity in benthic biofilms during mass mortality of endemic sponges in Lake Baikal

This study investigates in detail species diversity of cyanobacteria which proliferated on the surface of diseased sponges and bedrock in Lake Baikal (Siberia, Russia) during the ecological crisis that has been progressing in the littoral zone since 2011 and caused numerous negative changes in biota. In 2014–2015, we examined samples from diseased and healthy sponges as well as rock substrates using Sanger sequencing and high-throughput 16S rRNA gene analysis. Based on morphology, we identified 15 cyanobacterial species in biofoulings. Tychonema sp., Symplocastrum sp., Pseudanabaena galeata and Tolypothrix distorta dominated on diseased sponges. In communities of healthy sponges, we detected mainly picocyanobacteria Synechococcus. Fascicles of Symplocastrum sp. and T. distorta dominated on bedrock substrates. Phylogenetically, most 16S rDNAs clustered among Tychonema, Pseudanabaena, Geitlerinema, Kamptonema, Leptolyngbya, and Tolypothrix spp. Cyanobacterial proliferation in the littoral zone of Lake Baikal should be considered as one of the most important negative ecological processes in recent times which requires an urgent study in order to understand the causes of its emergence and development. Mat-forming cyanobacteria Tychonema characterized by the highest tropism to sponges plays a special role in the disease and death of sponges.

TEP production under oxidative stress of the picocyanobacterium Synechococcus

Transparent exopolymer particles (TEP) are mainly acidic polysaccharides directly or indirectly formed by phytoplankton and bacteria. These particles are often colonized by picoplankton and considered a hot spot for microbial activity. Recent studies suggested an important role of Synechococcus in TEP production found in lakes and prompted us to further investigate this issue using monoclonal xenic cultures of Synechococcus. We tested TEP production under oxidative stress in two treatments, one with hydrogen peroxide and another treated with ultraviolet radiation (UVR) and high photosynthetic active radiation (PAR), compared with an unstressed control. Our results showed a cell-normalized TEP production, ranging from 12 to 238 ng C cell-1 among strains, not only under stress but also in the control with non-limiting nutrients. Our data prove that freshwater communities of Synechococcus and their associated heterotrophic microflora, are capable of producing TEP even during growth phase. The oxidative stress induced extra production of TEP up to 400 ng C cell-1 in one of our phycocyanin-type (PC) strain. The phycoerythrin-type (PE) strains increased TEP production, particularly under UV-PAR stress, whereas the PC strains did it under H2O2 stress. This study provides new perspectives on the potential role of freshwater Synechococcus in TEP production.

The microbial mercury link in oligotrophic lakes: Bioaccumulation by picocyanobacteria in natural gradients of dissolved organic matter

Andean Patagonian lakes are oligotrophic systems characterized by low dissolved organic carbon (DOC) levels and moderate to high Hg concentration that determine naturally high Hg/DOC ratios and bioavailability. In these lakes, microbial food webs are extremely important in Hg trophodynamics, being that the picophytoplankton fraction is a major entrance path of Hg2+ into pelagic food webs. This study analyzed the bioaccumulation of Hg2+ by the picocyanobacteria Synechococcus sp. using the radiotracer 197Hg2+ and water from four Andean Patagonian lakes presenting a natural gradient of DOM concentration and quality. Hg2+ bioaccumulation by Synechococcus was calculated as the uptake of Hg2+ per biovolume unit (volume concentration factor VCF; pL μm−3). Hg uptake showed a wide variation (13 < VCF< 300 pL μm−3) in the natural DOC gradient tested (0.7–4 mg L−1; Hg2+/DOC ratio: 1.8–14 ng mg−1). The bioaccumulation of Hg2+ in Synechococcus decreased exponentially with DOC concentration. Differences in the quality of dissolved organic matter (DOM) among lake water influenced also Hg2+ bioaccumulation. Naturally degraded DOM, with low molecular weight/size, promoted higher Hg uptakes in Synechococcus compared to humic DOM, rich in high molecular weight/size aromatic compounds, that retained Hg in the dissolved phase. In Andean Patagonian lakes picocyanobacteria are pivotal organisms in the Hg cycling, taking dissolved Hg2+ and transferring it to pelagic food webs, as well as fueling the benthic Hg pathway through sedimentation.

Allelopathic effects of Chara species (C. aspera, C. baltica, and C. canescens) on the bloom-forming picocyanobacterium Synechococcus sp.

The role of macroalgal allelopathy in aquatic systems has received increasing attention as a potential means of controlling cyanobacterial blooms. However, the allelopathic activity of Chara sp. on coexisting and bloom-forming picocyanobacteria is still largely unknown. Therefore, the laboratory experiments were conducted to investigate the allelopathic activity of extracts of Chara aspera, C. baltica, and C. canescens on the growth, the fluorescence parameters: maximum and effective quantum yield of photosystem II (PSII) photochemistry (Fv/Fm and ΦPSII, respectively) and photosynthesis parameters such as the initial slope of photosynthesis-irradiance (P-E) curves (alpha) and photosynthetic capacity (Pm) of the picocyanobacterium Synechococcus sp. Batch cultures of picocyanobacterium were exposed to three concentrations of extracts originating from three charophyte cultures and the effect was followed at three sampling times. Dried specimens of C. aspera, C. baltica, and C. canescens were extracted in the water-based matrix and the initial Synechococcus sp. inoculum, derived from unialgal culture media, was used. We found both negative and positive allelopathic effects of all tested Chara extracts on Synechococcus sp. The strongest adverse impact of picocyanobacterium growth was caused by C. baltica. This study clearly demonstrated that the allelopathic effect depends on the Chara species identity. Our results also suggested that some allelopathic Chara sp. have the potential to mitigate harmful cyanobacterial blooms in systems dominated by Synechococcus sp.

The chemical form of silicon in marine Synechococcus

The widely distributed marine picocyanobacterium Synechococcus is found throughout the upper oceans. Recent observations that Synechococcus accumulates silicon indicate that it may influence the global cycle of this element. While diatoms, the organisms that dominate the marine Si cycle, have absolute Si requirements due to their precipitation of exoskeleton frustules composed of hydrated opal-A (i.e., biogenic silica), no requirements or biological roles are known for the Si associated with Synechococcus. Even less is known about the biochemical form(s) of Si in picocyanobacteria or whether it differs from diatomaceous silica. Using bulk X-ray absorption near-edge spectroscopy (XANES) across the Si K-edge, we investigated the chemical speciation of Si in dried Synechococcus laboratory isolates from several clades, representing both coastal and oligotrophic isolates grown at a range of Si(OH)4 concentrations. Silicon associated with Synechococcus is bound to oxygen and is spectroscopically distinct from opal-A precipitated by diatoms. The closest XANES spectral analogues found in the literature are dried Mg-Si gels precipitated under basic conditions, suggesting that some Synechococcus Si may be present in vivo as a hydrated siliceous network with Mg and/or Ca cations. Slight spectral variations across isolates and growth conditions suggest strain-specificity in Si chemistry and the potential for siliceous phases to bind organic matter.

Allelopathic activity of the bloom‐forming picocyanobacterium Synechococcus sp. on the coexisting microalgae: The role of eutrophication

Allelopathic picocyanobacteria have been responsible for harmful incidents with severe ecological impacts in many parts of the world. The allelopathic interactions that have been shown to be implied in the dominance of various species of phytoplankton, forming massive blooms, are influenced by environmental factors such as nutrient concentration. This study aims to determine in what extent the allelopathic activity of the bloom‐forming picocyanobacterium Synechococcus sp. is impacted by water stoichiometry. We measured the allelopathic activity of Synechococcus sp. on growth, chlorophyll fluorescence, and photosynthetic performance of green algae Chlorella vulgaris and Oocystis submarina and diatoms Bacillaria paxillifer and Skeletonema marinoi by addition of cell‐free filtrate obtained from cultures growing in nutrient‐sufficient (NP), nitrogen‐deficient (−N), or phosphorus‐deficient (−P) culture medium. These studies indicated that sufficient amounts of nutrients affected the picocyanobacterium increasing its production of allelochemicals. Conversely, the weakest allelopathic activity was recorded after the addition of a filtrate obtained from Synechococcus sp. grown at −N medium. The highest decline in growth, the maximum quantum yield of photosystem II (PSII) photochemistry (Fv/Fm) and the photosynthetic capacity (Pm) were observed for diatom S. marinoi. On the other hand, Synechococcus sp. filtrate had no allelopathic effects on O. submarina. While confirming the allelopathic activity of Synechococcus sp. these findings show that production of allelopathic substances is influenced by the availability of nutrients.

Distinct molecular signatures in dissolved organic matter produced by viral lysis of marine cyanobacteria.

Dissolved organic matter (DOM) plays a central role in the microbial ecology and biogeochemistry of aquatic environments, yet little is known about how the mechanism of DOM release from its ultimate source, primary producer biomass, affects the molecular composition of the inputs to the dissolved pool. Here we used a model marine phytoplankton, the picocyanobacterium Synechococcus WH7803, to compare the composition of DOM released by three mechanisms: exudation, mechanical cell lysis and infection by the lytic phage S-SM1. A broad, untargeted analytical approach reveals the complexity of this freshly sourced DOM, and comparative analysis between DOM produced by the different mechanisms suggests that exudation and viral lysis are sources of unsaturated, oxygen-rich and possibly novel biomolecules. Furthermore, viral lysis of WH7803 by S-SM1 releases abundant peptides derived from specific proteolysis of the major light-harvesting protein phycoerythrin, raising the possibility that phage infection of these abundant cyanobacteria could be a significant source of high molecular weight dissolved organic nitrogen compounds.

Allelopathic activity of the three strains of Baltic picocyanobacterium Synechococcus sp. on selected algae and cyanobacteria

Allelopathic activity of the three strains of Baltic picocyanobacterium Synechococcus sp. on selected algae and cyanobacteria