Flagellates and the microbial loop
ABSTRACT Flagellate Protozoa, particularly nanoflagellates, dominate the protozoan communities of the planktonic habit. They play a cardinal role in transferring carbon and in nutrient regeneration within the microbial plankton. They feed mainly on heterotrophic bacteria, but are capable of exploiting dissolved organic carbon and phototrophic bacteria, as well as some components of the phytoplankton. Many of the coloured photosynthetic nanoflagellates are capable of phagotrophy on bacteria, thereby supplementing their carbon budgets by mixotrophy. Heterotrophic flagellate abundances are controlled by bottom-up factors which influence the productivity of bacteria, but may also be subject to top-down control from grazers. Cladocera, ciliates and rotifers exploit them as food, but may also compete with them for bacterial food resources. Aggregates of particulate organic matter are a common feature of the planktonic environment. They form foci of high microbial activity, and many flagellates are adapted to living in the attached mode, achieving high densities on particles. In some stable aquatic water columns, for example meromictic lakes, flagellates form distinct strata in specialized communities which attract specific populations of predators.