Unique Characteristics of Microbial Cells in Aquatic Ecosystems

‭Another characteristic that can be noted are these elements that are morphologically‬ ‭different from the other cells that make up the colony:‬ ‭These are heterocysts, cells used for the fixation of atmospheric nitrogen and its conversion‬ ‭into ammonia. These cells are unable to carry out photosynthesis because they lack the‬ ‭second photosystem. The nitrogenase enzyme, which catalyzes the reduction of atmospheric‬ ‭nitrogen to ammonia, works only in an anoxic environment, so photosynthesis and nitrogen‬ ‭fixation are incompatible and cannot occur within the same cell. The role of‬ ‭Cyanobacteria as nitrogen fixers can be very important in nutrient-poor waters by allowing some‬ ‭of the atmospheric nitrogen to be recovered. Another cyanobacterium abundant in the oceans‬ ‭is‬‭Prochlorococcus‬‭, particularly widespread in tropical‬‭oceanic areas, but is also present in the‬ ‭Mediterranean. In size it does not even reach a micron and has a pigmentary set that is‬ ‭completely different from that of other phytoplankton and emits a characteristic red fluorescence.‬ ‭The study of phytoplankton of such small dimensions can only be evaluated from the point of‬ ‭view of abundance through flow cytometric techniques.‬ ‭ n important phytoplankton component is represented by the Haptophyta‬ A ‭phylum, which can be colonial or unicellular. They are forms equipped with‬ ‭two flagella and a third elongated thread-like structure called aptoneme‬ ‭which allows the cell to move but also to anchor. In the figure, you can see‬ ‭the presence of a coating made of organic flakes which can have different‬ ‭shapes or sizes.also in the Haptophyta phylum there is a particularly‬ ‭widespread group, that of coccolithophorids: they are rounded in shape,‬ ‭have a short and rigid haptoneme; their covering is made up of calcium‬ ‭carbonate plaques called coccoliths and they can have variegated‬ ‭ornamentations. Coccoliths are important in the study of sediments from a‬ ‭paleontological point of view, in fact there are many fossil coccoliths in‬ ‭sedimentary rocks which can give rise to large formations, typically white‬ ‭due to the presence of calcium carbonate.‬ ‭Two groups belong to the Ochrophyta phylum, Chrysophyceae and‬ ‭Silicoflagellate, both with siliceous skeletons. In the former the skeleton is‬ ‭external and made up of siliceous scales intertwined with each other, as in‬ ‭the case of‬‭Mallomonas‬‭. Chrysophyceae owe their name‬‭to the presence of‬ ‭pigments such as carotenes and fucoxanthin which give their characteristic‬ ‭yellow color. In the Silicoflagellates we have an example of‬‭Dictyocha‬‭: they‬ ‭have a star-shaped internal skeleton. They then have two flagella of different‬ ‭lengths and one of them may have flagella hairs. The siliceous skeletons of‬ ‭the Silicoflagellates are also widespread in marine sediments‬ ‭Another very widespread group of algae are the Prasinophyceae, which are part of the‬ ‭cyanophyceae, they have a number of flagella that varies from 1 to 4 or are devoid of them, they are‬ ‭often covered in layers of scales of different types, up to 7 different types in the same cell.‬ ‭The‬ ‭Cryptophyceae‬ ‭belong‬ ‭to‬ ‭the‬ ‭Cryptophyte‬ ‭phylum,‬ ‭which‬ ‭are‬‭particularly‬ ‭ bundant‬ ‭in‬ ‭coastal‬ ‭waters,‬ ‭with‬ ‭a‬ ‭drop‬ ‭shape,‬ ‭two‬ ‭flagella‬ ‭and‬ ‭an‬ ‭organic‬ a ‭plate coating. They are predominantly nano planktonic forms.‬ ‭One of the most abundant and rich taxa from a specific point of view is represented by diatoms,‬ ‭members of the Bacillariophyceae class. There can be solitary forms or colonial forms. In every‬ ‭diatom the cytoplasm is enclosed in a peptic wall always impregnated with silica: this structure is‬ ‭called a frustule. Depending on the morphology of the frustule, diatoms can be divided into‬ ‭pennate, with an elongated shape and bilateral symmetry, or centric, with a circular shape and‬ ‭radial symmetry. They are particularly abundant at high latitudes and in temperate climates‬ ‭during the cold season. Diatom blooms occur when there is high availability of silicon‬ ‭(particularly after the full circulation phase at the end of winter). When the quantity of silicon‬ ‭decreases, diatoms disappear and are replaced by other algae. Since different species of‬ ‭diatoms have different needs, the amount of silicon is crucial in defining the specific composition‬ ‭in the diatom community.‬ ‭In the image we see a diatom schematized in its components:‬ ‭In the frustule or theca we see an upper part, called epitheca, and a lower‬ ‭part or hypotheca. These two cases fit together like the lid of a box and the‬ ‭bottom. Each theca has a valve face and a connective face. The surface of the‬ ‭frustule, in particular the valve face, presents an ornamentation, more or less‬ ‭evident depending on the species, with the presence of pores or alveoli‬ ‭variously arranged: in some species it is radial, as in the figure.‬