Morphological characteristics of Cylindrospermopsis raciborskii (Wołoszyńska) Seenayya et Subba Raju in laboratory cultures

Interspecific competition reveals Raphidiopsis raciborskii as a more successful invader than Microcystis aeruginosa

As a successful invasive cyanobacterial species, Raphidiopsis raciborskii is distributed globally and shows a tendency to replace Microcystis aeruginosa in many subtropical and temperate waters, but the ecological traits that contribute to its invasiveness are still unclear. In this study, we found that R. raciborskii occurred in 149 sites in 42 lakes in eastern and central China and coexisted with M. aeruginosa at most sites. Based on field results, a combination of invasion and competition experiments using a biomass gradient to evaluate the invasiveness and competitiveness of R. raciborskii compared with M. aeruginosa was conducted. In invasive groups, both R. raciborskii and M. aeruginosa were shown to have positive specific growth rates, indicating that R. raciborskii could coexist with M. aeruginosa. Furthermore, R. raciborskii was shown to grow faster from invasion while M. aeruginosa reduced growth for invasion. In competitive groups, R. raciborskii reached a higher maximum biomass and grew longer than M. aeruginosa. The specific growth rate of R. raciborskii was not inhibited by M. aeruginosa biomass, whereas the growth of M. aeruginosa was inhibited by R. raciborskii biomass. It was shown during the whole experiment that R. raciborskii tended to replace M. aeruginosa to become dominant owing to its faster growth rate and the eventual decline in growth of M. aeruginosa. With an increase in biomass of M. aeruginosa, the vegetative cell size and filament length of R. raciborskii gradually increased. This study has demonstrated that the inherent invasive traits of R. raciborskii, size differences, niche differences, and relative fitness differences between R. raciborskii and M. aeruginosa are crucial reasons for the invasive success of R. raciborskii. Our results revealed the invasiveness and domination of R. raciborskii from a new perspective.

A Greek Cylindrospermopsis raciborskii strain: Missing link in tropic invader's phylogeography tale

The cyanobacterium Cylindrospermopsis raciborskii represents a challenge for researchers and it is extensively studied for its toxicity and invasive behaviour, which is presumably enhanced by global warming. Biogeography studies indicate a tropical origin for this species, with Greece considered as the expansion route of C. raciborskii in Europe. The widening of its geographic distribution and the isolation of strains showing high optimum growth temperature underline its ecological heterogeneity, suggesting the existence of different ecotypes. The dominance of species like C. raciborskii along with their ecotoxicology and potential human risk related problems, render the establishment of a clear phylogeography model essential. In the context of the present study, the characterization of Cylindrospermopsis raciborskii TAU-MAC 1414 strain, isolated from Lake Karla, with respect to its phylogeography and toxic potential, is attempted. Our research provides new insights on the origin of C. raciborskii in the Mediterranean region; C. raciborskii expanded in Mediterranean from North America, whilst the rest of the European strains may originate from Asia and Australia. Microcystin synthetase genes, phylogenetic closely related with Microcystis strains, were also present in C. raciborskii TAU-MAC 1414. We were unable to unambiguously confirm the presence of MC-LR, using LC-MS/MS. Our results are shedding light on the expansion and distribution of C. raciborskii, whilst they pose further questions on the toxic capacity of this species.