The role of resting cysts in the survival and succession of freshwater dinoflagellates

Abstract: Phytoplankton have adapted to survive in the variable and heterogeneous environment that is characteristic of lakes in temperate regions. One strategy for survival during periods unfavorable for growth is to form resting propagules. Dinoflagellates are bloom-forming phytoplankton which have the ability to form resistant and long-lived resting cysts. The role of cysts in the ecology and life history of dinoflagellates was investigated in this thesis. First, it was tested whether cysts have the ability to assimilate phosphorus (P) during dormancy, which could improve the survival of newly germinated cells when returning to the water column. Secondly, the role of resting cysts in regulating the seasonal succession of dinoflagellates in Lake Erken was examined. Also. biotic and abiotic factorscontrolling the timing of germination were tested.Cysts of the marine dinoflagellate Scrippsiella trochoidea which were incubated in P enriched medium attained a P content twice that of cysts incubated in P deplete medium. Cysts of the freshwater dinoflagellate Ceratium hirundinella had only a slightly higher P content when incubated in P rich medium. Some uptake may be taking place (although adsorption to the cyst wall cannot be excluded), but not enough to provide newly excysted cells with a competitive advantage over other phytoplankton. Phosphorus may, however, play a role in cyst survival and subsequent germination processes.Nine species of dinoflagellates were found in Lake Erken, of which one new thecate phototrophic species, Peridinium euryceps Rengefors et B. Meyer was encountered. This is a cryophilic-and a cyst-forming species, with a distinct and extremely flattened shape. All phototrophic dinoflagellates in Lake Erken rested as cysts during the major part of the year. Timing of germination was mainly regulated by an internal maturation period, a biological clock, and temperature. Furthermore, C. hirundinella cysts did not germinate when incubated in anoxic conditions. Cysts of Peridinium aciculiferum, a winter species, did not germinate in the presence of exudate from zooplankton (predators). These results suggest that cyst dormancy may also be a predator avoidance strategy for some species.To summarize, the results of the studies presented in this thesis show that resting cysts play a significant role in the seasonal succession of dinoflagellates. Also, the cyst stage appears to be a survival strategy for escaping herbivory. Furthermore the results suggest that nutrient uptake may occur during the cyst stage, which in turn can affect the survival of the cyst and the newly germinated cell.