Rivulariaceae

Abstract

Summary: The Rivulariaceae are treated here as all the cyanobacteria which have trichomes with a marked taper and a basal heterocyst for much of their growth cycle. Molecular sequencing of Calothrix and Rivularia shows that these are heterogeneous and this probably also applies to Dichothrix and Gloeotrichia. The unispecific Isactis has received little study, but is morphologically close to some Calothrix. These should all be treated as form-genera for ecological descriptive purposes until more sequencing studies have been made. Sacconema and Gardnerula are not considered distinct enough to be treated as distinct genera. Colony formation occurs by aggregation of hormogonia and this may lead to the inclusion of more than one genotype. The group as a whole occurs in environments with highly variable P concentrations, usually short periods of relatively high ambient P followed by much longer periods of low P; a few possible exceptions are discussed. Typically organic P exceeds inorganic P and the Rivulariaceae as a whole are especially efficient at using organic phosphates. N2 fixation in Rivulariaceae is highest during the period of high P. In the case of Gloeotrichia echinulata, which sometimes forms blooms in lakes, P acquisition takes place mainly while the organism is growing near the sediments. Its competitive success is favoured by a combination of P-rich sediments and relatively low P concentrations in the water. Long colourless multicellular hairs are formed by many species, including all those with distinct hemispherical and spherical colonies. The hairs not only enhance the surface area for phosphatase activities, but also aid P acquisition from environments where the ambient P concentration is mostly low, but with occasional much higher pulses. Although most phosphatase activities take place at the cell surface, some activity often occurs in sheaths and probably also inside the cytoplasmic membrane of older hair cells, suggesting that organic phosphates can sometimes pass through the membrane. Sufficient is known about the relationship between morphology and the environment in the Rivulariaceae to make them excellent environmental indicators. Many of the morphological characters used for classical taxonomic descriptions are expressed only when the organisms are P-limited, but some culture collections and many experimental studies have used media with very high P concentrations, making past interpretation of results doubtful, if not wrong.