Freshwater ecosystems provide multiple services and functions to all life. Phytoplankton forms the base of freshwater ecosystems. Therefore, a diverse and stably structured phytoplankton community is the key to have a good and functional freshwater ecosystem. Cyanobacteria is an important member of phytoplankton community. Malfunction of phytoplankton community, such as the incidence of cyanobacteria harmful algae bloom, can severely reduce ecosystem contribution to all parties. Cyanobacteria growth is affected by many factors, which can be both biotic and abiotic. For biotic factors, such as species interactions, among which, parasitism can strongly change phytoplankton community structure. For abiotic factors, apart from temperature and light, for example key role nutrients significantly change cyanobacteria growth dynamic.

Chytrid infection is an important driver of freshwater phytoplankton ecology. Intense studies have been done recently to investigate chytrid and phytoplankton interactions, with both observational and experimental research work being carried out. In the field, many and continue increasing chytrid species genetic information are characterized by new gene sequencing techniques, whereas in laboratory, only a handful of hosts and chytrid models are used to gain knowledge on host and parasite relations, both with themselves and with their surrounding environments. In chapter 2, I described the discovery of a novel chytrid parasitic species that infects solely on heterocyst of nitrogen fixing cyanobacterium, Dolichospermum planktonicum. Life cycles were characterized and matched with the typical life history of zoosporic chytrid fungi. Genetic identification has placed its origin within the phylum Chytridiomycota. Since stable host and chytrid culture were established under laboratorial conditions, subsequently experimental research on this system can take place, which add another host and chytrid model to the scientific exploration of aquatic host and parasite interactions.

Environmental factors such as nutrients can directly and indirectly affect host and chytrid interaction. In chapter 3, we studied the effect of nitrogen availability on the performance of both two players involved. Strong effect of nutrient on chytrid infection and host performance were observed. Chytrid infection strongly decreased host growth only under nitrogen free conditions, and overall nitrogen fixation rate decreased upon infection as well, but heterocyst survived infection displayed strong increase in nitrogenase activity.