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

Parker, N 2002 , 'Sexual reproduction and bloom dynamics of toxic dinoflagellates from Australian estuarine waters', PhD thesis, University of Tasmania.

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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
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.

Item Type: Thesis - PhD
Authors/Creators:Parker, N
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