Biochemical prey recognition by planktonic protozoa

Planktonic flagellates and ciliates are the major consumers of phytoplankton and bacterioplankton in aquatic environments, playing a pivotal role in carbon cycling and nutrient regeneration. Despite certain unicellular predators using chemosensory responses to locate and select their prey, the biochemical mechanisms behind prey reception and selection have not been elucidated. Here we identify a Ca2+-dependent, mannose-binding lectin on the marine dinoflagellate Oxyrrhis marina, which is used as a feeding receptor for recognizing prey. Blocking the receptor using 20 ?M mannose-BSA inhibited ingestion of phytoplankton prey, Isochrysis galbana, by 60%. In prey selection studies, O. marina ingested twice as many 6 ?m diameter beads coated with mannose-BSA as those coated with galNac-BSA. When pre-incubated with mannose-BSA, O. marina was no longer able to discriminate between different sugar-coated beads. Thus, these findings reveal molecular mechanisms of protozoan prey recognition. Our results also indicate the functional similarity between cellular recognition used by planktonic protozoa to discriminate between different prey items, and those used by metazoan phagocytic blood cells to recognize invading microorganisms.<br/

Picophytoplankton in freshwater ecosystems: the importance of small-sized phototrophs

About 40 years have passed since the discovery of picophytoplankton; the present knowledge of the taxonomy, physiology and ecology of these tiny photoautotrophic cells offers new perspectives on the importance of the microbial contribution to global biogeochemical cycles and food webs. This review focuses on the relationships among the components of picophytoplankton (picocyanobacteria and the picoplanktic eukaryotes) and biotic and abiotic environmental factors. The dynamics of picophytoplankton in aquatic ecosystems are strictly dependent upon basin size and trophy, temperature, and nutrient and light limitation, but they are also regulated by grazing and viral-induced lysis. The review considers: the pros and cons of the molecular approach to the study of the taxonomy of freshwater Synechococcus spp.; the importance of ecological aspects in understanding the puzzle of picophytoplankton phylogeny (genotype vs ecotype); and the role of biotic vs abiotic interactions in controlling picophytoplankton dynamics. Biotic, top-down control mechanisms are reviewed as well as knowledge of other biological interactions