Epiphytic cyanobacterial strains in the roots of Salvinia auriculata and the effect of light and nutrients on the production of heterocyst, akinete and hormogonia

Abstract

In epiphytic associations, cyanobacteria form the periphyton with phytoplanktonic algae and with aquatic macrophytes. In this study, we found homocytous and heterocytous filamentous strains of epiphytic cyanobacteria associated with submerged leaves of the aquatic fern Salvinia auriculata. Filamentous morphotypes can produce adaptive structures such as heterocysts, akinetes, and hormogonia. Based on the premise that light limitation and nutrient limitation affect the adaptive strategies of cyanobacteria epiphytic we hypothesized that the heterocysts production would be greater under nutrient scarcity and full sunlight conditions, akinetes would be predominantly produced under growth-limiting conditions, such as nutrient scarcity and shade, and hormogonia would be abundantly produced under shade. In addition, for purposes of recording, identification and assembling a collection, we carry out the isolation of cultivable cyanobacteria. We conducted an experiment in a greenhouse applying a shade cloth and Hoagland’s solution to manipulate, respectively, the light intensity and the concentration of nutrients. Both factors, light and nutrients, affected the production of adaptive structures. Heterocysts were produced in greater numbers when no nutrient was added and under full light. Akinetes were produced mainly in the treatments under shade and no nutrient addition. In its turn, hormogonia were produced in the treatments with nutrients and mainly in the shade. Accordingly, akinete differentiation was negatively correlated with that of hormogonia. We conclude that the non-addition of nutrients stimulates heterocysts production, as well as akinete production. Therefore, due to the BFN BNF performed in heterocysts, we suggest that in oligotrophic aquatic environments, cyanobacteria epiphytic on the roots of aquatic macrophytes can supply fixed nitrogen to the aquatic ecosystem. Besides, with the shading produced by macrophytes, and available nutrients, the production of hormogonia is stimulated due to positive phototaxis. At the end of the experiment, nine morphologically distinct strains were isolated and taxonomically classified, up to the level of family and genus, and will allow us to assemble a collection for future research.