Effect of nutrient concentration on growth and saxitoxin production of Raphidiopsis raciborskii (Cyanophyta) interacting with Monoraphidium contortum (Chlorophyceae)

Raphidiopsis raciborskii is the dominant species in several subtropical aquatic ecosystems. This study addressed the influence of nutrient concentration on cyanobacteria growth and saxitoxin synthesis. Therefore, we performed bioassays of interaction between the Chlorophyceae Monoraphidium contortum and R. raciborskii simulating oligotrophic and supereutrophic environments. Experiments were carried out in a climatized room for 15 days in pure cultures of each species (control) and mixed (interaction). The biomass growth (biovolume) and specific growth rates were measured. Saxitoxin was analyzed using the ELISA biochemical method. In the oligotrophic environment, the cell volume of R. raciborskii decreased. This species also showed senescence in interaction with M. contortum. However, there were no statistical differences in the saxitoxin synthesis in both conditions. In the supereutrophic environment, the growth of R. raciborskii and saxitoxin production was similar in both the control and interaction conditions. However, M. contortum growth decreased in interaction with the cyanobacteria. The increase in trophic status from oligo- to supereutrophic contributes to the growth of M. contortum in subtropical aquatic ecosystems, but the decrease in the area/volume ratio of cyanobacteria and the saxitoxin synthesis is an evident survival strategy in oligotrophic environments.