Chemosensory behavioural response of freshwater phytoplanktonic flagellates

Unlike behavioural responses to physical gradients, active responses to chemical gradients, and their physiological and ecological implications, have rarely been studied in freshwater phytoplanktonic flagellates. This study used microscale preference chambers to investigate the population and individual cell responses of five species exposed to four chemical gradients which commonly develop with depth in lakes: phosphate, oxygen, carbon dioxide and pH. Upon exposure of nutrient-replete and -depleted cells to a phosphate gradient, only nutrient-depleted cells of the autotrophic Chlamydomonas moewusii responded, accumulating at high concentrations. In contrast, all species responded to an oxygen gradient with Ceratium furcoides, Chlamydomonas moewusii, Dinobryon sertularia and Plagioselmis nannoplanctica preferring high concentrations whereas Euglena gracilis preferred low concentrations. In addition, all species displayed a strong affinity for carbon dioxide which was not mediated by detecting pH. Analysis of the swimming trajectories of individual cells showed that directed chemotaxes, rather than speed-dependent chemokineses, were responsible for the observed preferences. These complex and diverse species-dependent chemosensory responses may optimize photosynthesis, facilitate nutrient retrieval during migration, increase growth rate and may influence spatial and temporal distribution, contributing to the delineation of niche separation in phytoplanktonic flagellates.