Cylindrospermopsis raciborskii (Cyanobacteria) invasion at mid-latitudes:: Selection, wide physiological tolerance, or global warming?

The tropical bloom-forming cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) Seenaya et Subba Raju is causing increasing concern because of its potential toxicity and invasive behavior at mid-latitudes. This species has recently been identified in several temperate areas and was first reported in France in 1994, but the mechanisms underlying this acclimation process remain to be elucidated. We performed a range of laboratory experiments in an attempt to identify the physiological characteristics that could account for this behavior. We investigated the three following hypotheses: 1) clones of C. raciborskii adapted to temperate climate have been selected as it advanced north, 2) C. raciborskii has high physiological tolerance that allows it to prosper in a wide range of conditions, and 3) changes inclimate (global warming) have favored the development of C. raciborskii in mid-latitudes. Ten strains of C. raciborskii from Australia n=1), Brazil (2), France (2), Germany (1), Hungary (1), Mexico (1), and Senegal (2) were cultured at different temperatures and light intensities. The in vitro growth parameters (mu and T-opt) were the same for the tropical and temperate strains. All 10 strains displayed positive net growth in a wide range of temperatures (from 20 to 35degreesC) and light intensities (from 30 to 400 mumol photons.m(-2).s(-1)), with maximum growth rates at around 30degreesC and 80 mumol photons.m(-2).s(-1). This suggests that the colonization of mid-latitudes by C. raciborskii may result from a combination of its ability to tolerate a rather wide range of climatic conditions and the global warming phenomenon, which provides this species with better environmental conditions for its growth.