Sexual reproduction and bloom dynamics of toxic dinoflagellates from Australian estuarine waters

Of the dinoflagellates known to form toxic blooms in Australian estuaries three, Alexandrium minurum, Alexandrium catenella and Gymnodinium catenatum produce paralytic shellfish toxins that can lead to the potentially fatal paralytic shellfish poisoning in humans. These three species and a fourth dinoflagellate Protoceratium reticulatum all have the capacity to reproduce both by vegetative cell division and by sexual reproduction. The product of sexual reproduction in all cases is a resting cyst, which can sink to the sediments and remain dormant for a time. Resting cyst production results in a coupling between the benthic and pelagic components of estuarine systems and forms an important part of the ecological strategy of dinoflagellates. A variety of aspects of sexual reproduction of these four species were investigated. These included: life cycle description (A. minutum); mating system studies (all four species); assessing the use of sexually compatible strains in high biomass culture systems (A. minutum); experimental studies of encystment and excystment cues (A, minutum and G. catenatum); and a three-year field study of vegetative and sexual reproduction of G. catenatum in an estuary in south-east Tasmania. The complete life cycle of A, minutum was described and was found to be similar to other Alexandrium species. Alexandrium minutum was also found to produce temporary resting cysts in response to cold shock and to have a short (approximately 4 weeks) minimum requisite dormancy period. The mating systems of all four species were investigated. Alexandrium catenella strains from three Australian populations were found to have simple heterothallic mating systems with essentially two mating types. The other three species all had mating systems of greater complexity with gradients of affinity between strains rather than discrete mating types. These mating type affinities were also found to vary over years in A. minutum and G. catenatum. Cluster analysis was demonstrated to be a useful method for describing these gradients. Encystment and excystment of A. minutum and G. catenatum involves both biological and environmental factors. Both these dinoflagellates had defined temperature range.s. for encystment and excystment and evidence was found for the action of pheromone-like substances in encystment. Low irradiance and low nutrients influenced encystment, and nutrient depletion had a greater effect on encystment of A, minutum than of G. catenatum. The influence of algicidal bacteria on both encystment and excystment was tested for the first time and found to have no effect while low oxygen and low light greatly reduced excystment. The capacity for sexual reproduction by A, rninutum did not significantly effect biomass production in high biomass culture systems while growth rate increased with aeration. In the three year field study, four G. catenatum blooms were observed, two in summer and two in autumn. Sexual reproduction occurred throughout summer blooms but was below detection levels in autumn blooms and probably controlled by temperature. The importance of river flow for dinoflagellate blooms in this system was identified. Dinoflagellate blooms, including G. catenatum, were absent in the first year of the study which may be attributed to increased flushing of the estuary due to high and persistent flow of the Huon River in that year which may also have reduced diatom biomass and subsequent nutrient remineralisation. A dual strategy for the contribution of resting cysts in this system is suggested whereby frequent germination of resting cysts from shallow sediments contributes to bloom populations and overwintering biomass while deeply buried and rarely resuspended resting cysts contribute to long-term survival of the population in the estuary. The results of all investigations for A. minutum and G. catenatum were synthesised into conceptual frameworks of sexual reproduction and bloom dynamics within natural systems.