Alterações ultraestruturais de cianobactérias (Cylindrospermopsis raciborskii e Microcystis aeruginosa) em interação interespecífica com ênfase em corpúsculos lipídicos

Abstract

Cyanobacteria are photosynthetic unicellular microorganisms responsible for most of the primary production in aquatic ecosystems. In many environments, several species of cyanobacteria cause blooms and release toxins that are harmful to other communities. In many places, it is common that there is dominance of a particular species of cyanobacteria, which may be switched over seasonal periods. Thus, there is a natural interaction between different species of cyanobacteria in which we observe changes in the behavior of the same so that they can suppress their competitors and thus dominate the environment. Several studies have evaluated interactions between cyanobacteria at the population level, but few ones investigate physiological, metabolic and ultrastructural aspects of the cells during this process. Lipid bodies (LBs), also known as lipid droplets or inclusions, are organelles present in all cell types, including cyanobacteria. In eukaryotic cells, these organelles are involved in varied cellular functions such as lipid metabolism, cellular signaling, intracellular transport and inflammation. The Cell Biology at UFJF has been studying the formation and the role of LBs in mammalian cells and, more recently, in unicellular protozoan parasites. LBs are functionally active organelles, which can be rapidly formed in response to stimuli independent of the cell type involved. The occurrence and the functional role of LBs in cyanobacteria are poorly known. Previous work of our group has identified by TEM that the number of LBs within cyanobacteria changed in response to ultraviolet radiation. The presente work aimed to study the ultrastructure of cyanobacteria strains Cylindrospermopsis raciborskii (CYRF-01) and Microcystis aeruginosa (MIRF-01) in interaction for 24n and investigate whether this interaction affects the formation of CLs. Samples of the control groups (MIRF-01 and CYR-01) and interaction group (MIRF-01/CYRF-01) were collected at 0h and 24h. For analysis and quantification of LCs we used: (1) Oil Red-O staining, which enables visualization of CLs by both bright-field and fluorescence microscopy and (2) TEM,.which provides identification of morphological aspects of LBs at high resolution. In parallel, we analyzed the density and cell viability through routine quantification with a Neubauer chamber and a membrane permeability marker, respectively. The results in both light microscopy and TEM showed a significant reduction in LB numbers (p <0.001) for the two species after 24h of interaction compared with the control groups. However, it was observed a greater reduction in the number of LBs in parallel to the increase in size in CYRF-01 compared to MIRF-01. Thus, our data suggest that LCs in cyanobacteria are dynamic organelles, able to respond to environmental stresses, as in interspecific competition and with features that can go beyond just energy storage. IN addition to CLs, other organelles and structures were analyzed and quantified such as external membrane vesicles (EMVs), polyphosphate granules, carboxysomes and thylakoid membranes. These analyses demonstrated that CYRF-01 seems to be much more responsive to environmental stimuli, since just this cyanobacterium released EMVs. Furthermore, the proportion of EMVs significantly increased in C. raciborskii cells after 24h of interaction (p <0.004). In parallel, the numbers of phosphate granules reduced in response to interaction compared to the control group (p = 0.014) as well as the area occupied by thylakoids (p <0.0001). Values found in MIRF-01 were not statistically significant (p = 0.589 and p = 0.921). The interaction did not affect cell density (p = 0.811), however, cell viability analysis showed that the interaction there is a higher quantity of non-viable cells in both strains (p <0.05 in both). However, there is a greater amount of nonviable cells, in proportion in MIRF-01 (10.81 ± 3.11%) than in CYRF-01 (7.74% ± 1.47) after the interaction. Collectively, the results of this study reinforce the importance of individual analysis of factors such as interspecific interactions that interfere directly in cyanobacteria community. This kind of interaction induces changes at the cellular level in the species involved. Therefore, a better knowledge of these responses is important to understand the community dynamics of these organisms and their relationships in aquatic ecosystems.