Lake Big Momela, one of the East African soda lakes in Northern Tanzania characterised by highly saline-alkaline conditions, making them inhospitable to a range of organisms, although supporting massive growths of some adapted planktonic microorganisms that serve as food for birds, such as Lesser Flamingo. The temporal dynamics of plankton, with an emphasis on cyanobacteria, were examined in 2007 using morphological traits and ribosomal genetic markers (16S and 18S rRNA). Cyanobacterial genes encoding for hepatotoxins (mcyE and ndaF) were also screened. Rotifers and copepods dominated the zooplankton, whereas cyanobacteria, such as Anabaenopsis elenkinii and Arthrospira fusiformis dominated the phytoplankton community, and these being related to representatives in other East African soda lakes. The cyanobacteria community also showed distinct seasonal patterns influenced by environmental parameters, mainly salinity, pH and nitrate. Significant positive correlations were found between phytoplankton abundance and nitrate concentrations (r = 0.617, p = 0.033). No signals of the hepatotoxin synthetase genes mcyE and ndaF were retrieved from cyanobacteria during the whole year. In general, our data illustrate the presence of rich planktonic communities, including some unique and potentially endemic cyanobacteria.

Unicellular cyanobacteria are ubiquitous photoautotrophic microbes that contribute substantially to global primary production. Picocyanobacteria such as Synechococcus and Prochlorococcus depend on chlorophyll a-binding protein complexes to capture light energy. In addition, Synechococcus has accessory pigments organized into phycobilisomes, and Prochlorococcus contains chlorophyll b. Across a surface water transect spanning the sparsely studied tropical Indian Ocean, we examined Synechococcus and Prochlorococcus occurrence, taxonomy and habitat preference in an evolutionary context. Shotgun sequencing of size fractionated microbial communities from 0.1 mu m to 20 mu m and subsequent phylogenetic analysis indicated that cyanobacteria account for up to 15% of annotated reads, with the genera Prochlorococcus and Synechococcus comprising 90% of the cyanobacterial reads, even in the largest size fraction (3.0-20 mm). Phylogenetic analyses of cyanobacterial lightharvesting genes (chl-binding pcb/isiA, allophycocyanin (apcAB), phycocyanin (cpcAB) and phycoerythin (cpeAB)) mostly identified picocyanobacteria clades comprised of overlapping sequences obtained from Indian Ocean, Atlantic and/or Pacific Oceans samples. Habitat reconstructions coupled with phylogenetic analysis of the Indian Ocean samples suggested that large Synechococcus-like ancestors in coastal waters expanded their ecological niche towards open oligotrophic waters in the Indian Ocean through lineage diversification and associated streamlining of genomes (e.g. loss of phycobilisomes and acquisition of Chl b); resulting in contemporary small celled Prochlorococcus. Comparative metagenomic analysis with picocyanobacteria populations in other oceans suggests that this evolutionary scenario may be globally important.

Seagrasses are photoautotrophic, ecologically important components of many globally widespread coastal ecosystems, in which combined nitrogen may limit their production. We examined the biodiversity and diazotrophic capacity of microbial epiphytes associated with the phyllosphere of the seagrass Cymodocea rotundata of the Western Indian Ocean. Light microscopy, 16S rRNA and nifH gene analysis revealed the dominance of cyanobacteria in the epiphytic microbial community. Most phylotypes were related to free-living uncultured benthic cyanobacteria, while some to cyanobacterial endosymbionts of marine diatoms. Novel and potentially diazotrophic species, some of known pantropical distribution, were also discovered. Significant diel nitrogenase activities (acetylene reduction assay) were recorded (up to 358 +/- 232nmolC2H4g1 of seagrass FWh1). The nifH gene expression patterns showed that heterocystous phylotypes may be the dominant diazotrophs during the day and non-heterocystous at night. These data show that C.rotundata is colonized by diverse diazotrophic cyanobacteria species and suggest that these may be beneficial partners of seagrasses in nitrogen-depleted waters.

Here, we investigated how changes in water nutrient stoichiometry may change the nutritional quality of phytoplankton for mesozooplankton. For 6 d, we added nutrients to nine 1300 l mesocosms with natural summer phytoplankton from the Baltic Sea to create communities that were N-limited (P-treatment), Si-limited (PN-treatment) and not limited by nutients (PNSi-treatment). With the addition of P, no major changes occurred in phytoplankton biomass or species composition with time. With the addition of P and N, with or without excess of Si, C: N ratios approached the Redfield ratio, biomass increased and diatoms and dinoflagellates became dominant. Analysis of the 18S rRNA gene showed that the availability and diversity of phytoplankton other than diatoms and dinoflagellates, like prasinophytes or heterotrophic flagellates and ciliates, were not affected by the nutrient treatments. We then tested how the modified phytoplankton communities affected somatic growth in a natural mesozooplankton community dominated by the copepod genus Acartia. After 6 d of grazing, the zooplankton more than doubled their C content per individual when grazing on the Si-limited phytoplankton (PN-treatment) while total community biomass was maintained. In the other 2 treatments, the C content per individual remained the same and total community biomass decreased by ca. 60%, which suggests that the zooplankton was not optimally fed. Thus, a phytoplankton community with stoichiometry close to the Redfield ratio provided the best nutritional quality for the zooplankton, but not when the diatoms were Si saturated. This study shows that nutrient stoichiometry in the seawater can affect zooplankton growth by modification of the phytoplankton community. It also shows that the food quality of phytoplankton does not depend only on the taxonomic composition, but also on the nutrient stoichiometry.

Although cyanobacterial diazotrophs are common in Arctic terrestrial and freshwater habitats, they have been assumed to be absent from Arctic marine habitats. We report here a high diversity of cyanobacterial nifH genes in Fram Strait and the Greenland Sea. The nifH gene encodes the iron protein of the nitrogenase enzyme complex, which is essential for biological N2 fixation. Using primers specific for nifH genes we uncovered communities of autotrophic and heterotrophic bacteria in sea ice brine and seawater between latitudes 65 and 81 degrees N. Cyanobacteria (Oscillatoriales and Chroococcales) with known marine planktonic and benthic distributions were distinguished, alongside a mix of metabolically versatile eubacteria (nifH Clusters I and III). Using primers selective for cyanobacterial nifH genes we identified filamentous non-heterocystous Trichodesmium-like and LPP (Leptolyngbya, Phormidium and Plectonema)-like Oscillatoriales, as well as Cyanothece-like Chroococcales in a brine sample from 81 degrees N. The occurrence of Trichodesmium-like cyanobacteria was further confirmed by sequences of the hetR gene of Trichodesmium. Microscopic examinations confirmed the presence of viable filamentous and unicellular cyanobacteria. Our results reveal the potential for microbial N2 fixation in the Arctic seas. However, it is still left to determine if these genes are also metabolically active before any biogeochemical importance of diazotrophy in the polar oceans can be assessed.

Prokaryotic epiphytes on leaves of three seagrass species, Thalassodendron ciliatum, Thalassia hemprichii, and Cymodocea rotundata, from two Kenyan coastal sites, Nyali (a high-nutrient site) and Vipingo (a low-nutrient site), were characterized genetically and morphol. Denaturing gradient gel electrophoresis (DGGE) and clone libraries of PCR-amplified 16S rRNA gene fragments were used to study prokaryotes assocd. with these seagrasses. In general, the epiphytic coverage was greater in the high-nutrient site, while the microbial diversity was linked to seagrass species rather than the study sites. Cytophaga-Flavobacteria-Bacteroides (CFB) were assocd. with T. ciliatum and T. hemprichii mainly in the nutrient-poor site, while a-, b-, and g-proteobacteria were assocd. with all three species at the two study sites. Some bacteria phylotypes were closely related to sequences of microorganisms previously recovered from wastewaters or other contaminated sources, indicating the influx of land-based wastes into these coastal lagoon ecosystems. The abundance of potential nitrogen (N2)-fixing cyanobacteria on C. rotundata, particularly in the low-nutrient site, suggested that this assocn. may have been acquired to meet N demands. Unicellular cyanobacteria were dominant and assocd. with C. rotundata and T. hemprichii (with those on T. hemprichii being closely related to cyanobacterial symbiotic species), while T. ciliatum was almost devoid of cyanobacterial assocns. at the same site (Nyali), which suggests specificity in the cyanobacteria-seagrass assocns. The abundance of prokaryotic epiphytes was considered to be linked to water depth and tidal exposure.