Although intensification of toxic cyanobacterial blooms over the last decade is a matter of growing concern due to bloom impact on water quality, the biological role of most of the toxins produced is not known. In this critical review we focus primarily on the biological role of two toxins, microcystins and cylindrospermopsin, in inter- and intra-species communication and in nutrient acquisition. We examine the experimental evidence supporting some of the dogmas in the field and raise several open questions to be dealt with in future research. We do not discuss the health and environmental implications of toxin presence in the water body.
Recent evidence suggests that there is a dynamic microbial biota living on the surface and in the mucus layer of many hermatypic coral species that plays an essential role in coral well-being. Most of the studies published to date emphasize the importance of prokaryotic communities associated with the coral mucus in coral health and disease. In this study, we report the presence of a protist (Fng1) in the mucus of the hermatypic coral Fungia granulosa from the Gulf of Eilat. This protist was identified morphologically and molecularly as belonging to the family Thraustochytridae (phylum Stramenopile, order Labyrinthulida), a group of heterotrophs widely distributed in the marine environment. Morphological examination of this strain revealed a nonmotile organism c. 35 mum in diameter, which is able to thrive on carbon-deprived media, and whose growth and morphology are inoculum dependent. Its fatty acid production profile revealed an array of polyunsaturated fatty acids. A similar protist was also isolated from the mucus of the coral Favia sp. In light of these findings, its possible contribution to the coral holobiont is discussed.
Recent investigations of coral-associated microbial communities have revealed that coral surfaces are replete with microorganisms that may play important roles in colony wellbeing. In this study we show that the surfaces of a number of large polyped coral species are covered by a layer of aggregate-like microorganisms. These microorganisms are embedded in the mucus and in the tissue of solitary coral Fungia granulosa and in a number of faviid species. They are found on the coral surface and in the coral tissue. They are dispersed in a patchy distribution, with the highest density occurring in the area of the polyp mouth. Microscopic investigation revealed that the microorganisms found on and in tissues of F. granulosa are approximately 5 to 30 µm in diameter and are made up of unique coccoid bodies of approximately 1 µm in diameter. Transmission electron microscopy (TEM) revealed that they contain a nucleus, mitochondria and golgi, indicating they are eukaryotic in nature. The morphological data lead us to identify these organisms as stramenopile protists. This premise was strengthened by molecular investigation of samples taken from the surface mucus of the coral F. granulosa. The possible role of these protists is discussed. KEY WORDS: Coral · Protist · Stramenopile aggregates · Nutrition · MucusResale or republication not permitted without written consent of the publisher
Summary Microcystis sp. are major players in the global intensification of toxic cyanobacterial blooms endangering the water quality of freshwater bodies. A novel green alga identified as Scenedesmus sp., designated strain huji (hereafter S. huji), was isolated from water samples containing toxic Microcystis sp. withdrawn from Lake Kinneret (Sea of Galilee), Israel, suggesting that it produces secondary metabolites that help it withstand the Microcystis toxins. Competition experiments suggested complex interaction between these two organisms and use of spent cell‐free media from S. huji caused severe cell lysis in various Microcystis strains. We have isolated active metabolites from the spent S. huji medium. Application of the concentrated allelochemicals interfered with the functionality and perhaps the integrity of the Microcystis cell membrane, as indicated by the rapid effect on the photosynthetic variable fluorescence and leakage of phycobilins and ions. Although some activity was observed towards various bacteria, it did not alter growth of eukaryotic organisms such as the green alga Chlamydomonas reinhardtii.
Summary The reasons for the apparent dominance of the toxic cyanobacterium Microcystis sp., reflected by its massive blooms in many fresh water bodies, are poorly understood. We show that in addition to a large array of secondary metabolites, some of which are toxic to eukaryotes, Microcystis sp. secretes large amounts of fibrous exopolysaccharides that form extremely long fibres several millimetres in length. This phenomenon was detected in field and laboratory cultures of various Microcystis strains. In addition, we have identified and characterized three of the proteins associated with the fibres and the genes encoding them in Microcystis sp. PCC 7806 but were unable to completely delete them from its genome. Phylogenetic analysis of the most abundant one, designated IPF‐469, showed its presence only in cyanobacteria. Its closest relatives were detected in Synechocystis sp. PCC 6803 and in Cyanothece sp. strains; in the latter the genomic organization of the IPF‐469 was highly conserved. IPF‐469 and the other two proteins identified here, a haloperoxidase and a haemolysin‐type calcium‐binding protein, may be part of the fibres secretion pathway. The biological role of the fibres in Microcystis sp. is discussed.
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