Neither plant nor animal - The role of mixotrophic protists in aquatic environments

Planktonic organisms have traditionally been categorized into ‘plant-like’ phytoplankton and ‘animal-like’ zooplankton. However, many protists combine the ability to photosynthesize with the capacity to feed on other microbes in a mixotrophic lifestyle. Mixotrophs are increasingly recognized as important components of microbial food webs, but due to their nutritional versatility the net contribution of mixotrophs to primary production and ecosystem respiration can be variable. In laboratory experiments I found mixotrophs with contrasting physiological strategies to show opposite responses in their nutrition when exposed to changing environmental conditions. Disentangling how mixotrophs impact biogeochemical cycles in nature is currently still hampered by methodological difficulties. While the cell biological and molecular signatures of photosynthesis are readily detectable, detecting the ability to feed via phagocytosis is more difficult. We identified transcriptional patterns associated with different nutritional conditions in a mixotrophic marine chrysophyte. Controlled manipulation of prey availability, and synchronization of cell division over the diurnal cycle was used to separate direct effects of feeding from factors reflecting progression through the cell cycle. The observed transcriptional patterns provide the first indicators for detection of nutritional conditions in nature. Collectively, these results will help dissect the diverse trophic roles of microbial eukaryotes that shape the marine carbon cycle.