INVOLVEMENT OF MICROCYSTINS AND COLONY SIZE IN THE BENTHIC RECRUITMENT OF THE CYANOBACTERIUM MICROCYSTIS (CYANOPHYCEAE)(1).

The benthic recruitment of Microcystis was simulated in vitro in order to characterize the colonies of Microcystis recruited and to study the impact of intracellular and extracellular microcystins (MCs), and the influence of colony size on the recruitment process. We observed recruitment dynamics consisting of a lag phase followed by a peak and then a return to low recruitment rates, mainly controlled by passive resuspension throughout the experiment, and by physiological processes during the recruitment peak. Ninety-seven percent of the Microcystis colonies recruited were <160 μm in maximum length, and their cells contained much greater amounts of MCs (0.26 ± 0.14 pg eq microcystin leucine-arginine variant [MC-LR] · cell(-1) ) than those in benthic colonies (0.021 ± 0.004 pg eq MC-LR · cell(-1) ). The MC content of recruited Microcystis varied significantly over time and was not related to changes in the proportion of potentially toxic genotypes, determined using real-time PCR. On the other hand, the changes in MC content in the potentially toxic Microcystis recruited were closely and negatively correlated with recruitment dynamics; the lowest MC contents corresponded to high recruitment rates, and the highest MC contents corresponded to low recruitment rates. Thus, depending on temperature and light conditions, these variations are thought to result from the selection of various subpopulations from among the smallest and the most toxic of the initial benthic population. Adding purified MC-LR to experimental treatments led to a decreased recruitment of Microcystis and more specifically of mcyB genotypes.