Apparent grazing losses of Labyrinthulomycetes protists in oceanic and coastal waters: an experimental elucidation

Labyrinthulomycetes are unicellular eukaryotes known for their ability to synthesize polyunsaturated fatty acids that are essential for growth and development of zooplankton. But relationship of these microorganisms with microzooplankton and their trophic status in food chain remains unexplored. Hence grazing losses of these protists were studied in coastal and oceanic waters. Traditional dilution plot experiments to study consumption of natural phytoplankton (herbivory) and bacteria (bacterivory) by microzooplankton were carried out, incorporating a modification to unravel their preying on Labyrinthulomycetes. It was incorporation of fluorescently-labeled Labyrinthulomycetes cells. Experiments in oceanic waters displayed grazing on phytoplankton and bacteria at all the three times of study while that on Labyrinthulomycetes at only one time out of the three. There was no significant grazing on phytoplankton in coastal waters and microzooplankton grazed on Labyrinthulomycetes and bacteria. When grazing was studied with respect to time, significant grazing on bacteria occurred in the first 6 h and on Labyrinthulomycetes occurred towards the end of 24 h of incubation. Grazing on natural Labyrinthulomycetes population instead of fluorescently-labeled cells also revealed that they were grazed. Overall, the grazing experiments demonstrated that microzooplankton can exert positive grazing pressure on Labyrinthulomycetes, though during certain times only, signalizing the probable ‘top-down control’ of Labyrinthulomycetes by microzooplankton during those times. Labyrinthulomycetes may therefore constitute an important part of microbial loop in the marine ecosystem by utilizing dissolved and particulate organic matter due to osmoheterotrophic mode of nutrition and in turn supplying it to the next trophic level viz., the microzooplankton.