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Saxitoxin and Related Paralytic Shellfish Toxins
Published in Dongyou Liu, Handbook of Foodborne Diseases, 2018
Leanne Andrea Pearson, Brett Anthony Neilan
The vegetative cells of filamentous cyanobacteria are photosynthetically active and reproduce through binary fission. In Nostocalean PST-producing species, vegetative cells are capable of differentiating into heterocysts; specialized cells that perform nitrogen fixation. Heterocysts are somewhat larger and rounder than vegetative cells, have a thicker cell envelope, and do not usually divide.9 Heterocyst differentiation is triggered by the absence of a combined nitrogen source.9
The Azolia-Anabaena Symbiosis
Published in Peter M. Gresshoff, Molecular Biology of Symbiotic Nitrogen Fixation, 2018
Besides their characteristic shape and size, heterocysts can readily be distinguished from the vegetative cells by three structural properties: (1) they are weakly pigmented; (2) they have a thick outer envelope; and (3) near each junction with a vegetative cell they contain a polar granule.35 The ability to form heterocysts is associated with a physiological property, the capacity of nitrogen fixation under aerobic conditions.43 The heterocyst envelope is composed of three layers and a laminated inner layer.44 As the two inner layers are formed, the initially rounded cell pole in contact with the adjacent vegetative cell changes into a short, narrow tubular neck around which the two inner envelope layers are thickened. This modification reduces the contact with the adjacent vegetative cell to a small flattened area at the end of the neck, in which microplasmodesmata develop.44
Diversity and Utilization of Marine Cyanobacteria
Published in Gokare A. Ravishankar, Ranga Rao Ambati, Handbook of Algal Technologies and Phytochemicals, 2019
Cyanobacteria are traditionally classified on the basis of their morphology into five orders representing Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales and Stigonematales as given in Bergey’s Manual (Holt et al., 1994). There are two major morphological types of cyanobacteria: coccoid and filamentous forms. Coccoid species range from single cells (Synechococcus; Figure 14.1, b) to colonies (Gloeocapsa and Chroococcus; Figure 14.1, c and e) or masses of various shapes (Microcystis; Figure 14.1, a). In some, cells are arranged in rows resulting in a flat plate (Merismopedia; Figure 14.1, f), or they may be radially arranged in spherical colonies (Gomphospharia; Figure 14.1, d). Filamentous forms produce a row of cells referred to as a trichome, a result of cell division in one plane and failure of the cells to secrete sheath material between the cells or in the plane of division. Trichomes may be simple straight (Oscillatoria; Figure 14.1, h) or in the form of aggregated bundles (Trichodesmium; Figure 14.1, k) and/or permanently spirally coiled (Spirulina; Figure 14.1, g). The trichome with the enclosing sheath is referred to as a filament (Lyngbya and Phormidium; Figure 14.1, i and j). Some filamentous species are characterized by true cell differentiation and form special cells called ‘heterocysts’ which, unlike normal vegetative cells, lack an oxygenic photosystem, biliprotein pigments and carboxysomes but possess an extraordinarily thick cell wall that is impervious to oxygen (Anabaena; Figure 14.1, m). The number and distribution of these cells is often of taxonomic importance. These are considered sites of nitrogen fixation, providing vegetative cells with combined nitrogen but are not viable when disc onnected from the trichome.
An overview on cyanobacterial blooms and toxins production: their occurrence and influencing factors
Published in Toxin Reviews, 2022
Isaac Yaw Massey, Muwaffak Al osman, Fei Yang
It is well established that nitrogen fixation is an important feature of some cyanobacteria species and in terms of nutrition nitrogen-fixing, cyanobacteria are considered the most self-sufficient among other organisms. They are photoautotrophs that require only light energy, CO2, dinitrogen (N2), water and some minerals (Paerl and Huisman 2009, Paerl and Otten 2013, Paerl et al.2016, 2001). Heterocysts are specialized nitrogen-fixing cells. Heterocysts have thick cell wall, do not pose photosynthetic membrane and are larger, clearer and highly refractive under light microscope appearance. They may occur within the filament of photosynthetic cells or terminally on a filament (Paerl and Huisman 2009, Paerl and Otten 2013, Paerl et al.2016, 2001). Due to the differences in size, shape and location of heterocysts, they form a significant component in species identification. Within the heterocysts, the enzyme nitrogenase reduces molecular nitrogen to ammonia, which is incorporated into the amido group of glutamine. The thickened cell wall enables molecular oxygen to enter the cell, to be reduced (Bryant 1994, Paerl et al.2016, 2001), thus helping to maintain a highly reducing environment within the cell, necessary for nitrogen reduction.