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Culture studies of two toxic dinoflagellate species, Alexandrium minutum and Gymnodinium catenatum

Buttery, Michelle June (2000) Culture studies of two toxic dinoflagellate species, Alexandrium minutum and Gymnodinium catenatum. PhD thesis, Murdoch University.

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Abstract

This study examined different aspects of culturing the toxic dinoflagellates Alexandrium minutum, and a toxic (GCDE08) and a non-toxic strain (GCDE06) of Gymnodinium catenatum. These included 1) growth in different semi-defined media, and the effects of nutrient composition and concentration on growth; 2) the effects of selenium on growth; 3) the effects of mixing on growth, and; 4) growth in a semi-continuous dialysis system.

Of the seven media tested in this study, GSe medium resulted in highest cell densities and specific growth rates for A. minutum (1.05x10^5 cells.mL^-1 and 0.345 respectively) and both strains of G. catenatum (GCDE06 - 1.25x10^4 cells.mL^-1 and 0.433; GCDE08 - 1.68x10^4 cells.mL^-1 and 0.162). Next best growth was achieved in F/2 medium, with maximum cell densities of 6.85 x10^4 cells.mL^-1, 6.70x10^2 cells.mL^-1 and 8.40x10^2cells.mL^-1 recorded for A. minutum, GCDE06 and GCDE08 respectively. The addition of selenium, soil extract, and selenium plus soil extract to F/2 medium resulted in an increase in cell densities and growth rates for both species.

The results obtained in the media experiments led to further investigation on the effects of selenium on the growth of these two dinoflagellates. Preliminary experiments with A. minutum in media with and without selenium revealed no significant differences in cell densities and growth rates for the two treatments. For G. catenatum, cell density and growth rate was slightly higher in selenium free medium; these results were in conflict with those obtained in the previous media experiments with selenium, so it was decided to test further the effects of this trace metal on the growth of this species.

Growing the non-toxic G. catenatum strain (GCDE06) over several transfers in selenium-free medium resulted in a decrease in both growth rate (0.077 vs 0.110) and maximum cell density (4.40x10^3 cells.mL^-1 vs 6.20x10^3 cells.mL^-1) compared to continuous growth in selenium-containing medium. Similar results were also obtained for the toxic G. catenation strain (GCDE08).

These results indicate that A. minutum does not require additional selenium for growth, while the growth of both G. catenatum strains was slightly improved with the addition of selenium. The seawater used in this study had a relatively high background concentration of selenium (0.335 ng.mL^-1), and both A minutum and G. catenatum cells were found to accumulate high levels of selenium (0.008 fg.µm^-3 and 0.036 fg.µm^-3), so this could also explain why additional selenium was not essential for growth in this present study.

Of the mixing methods tested in this study, it was found that stirring using magnetic stirrers had the most adverse effects on the growth of both A minutum and G. catenatum; cell densities were reduced after stirring was applied to exponential phase cultures. Both species were able to tolerate gentle aeration, although aeration decreased growth rates.

The results of the mixing trial in this study led to the development of a semicontinuous dialysis system so as to achieve steady-state cultures with high cell densities. Aleandrium minutum grew at a higher rate in the dialysis system than in a control batch culture (0.182 vs 0.161), though the maximum cell density was lower (9.37x104 cells.mL^-1 vs 1.23x103 cells.mL^-1). For the non-toxic strain of G. catenatum, both the growth rate and maximum cell density were lower (0.151 vs 0.148, and, 4.55x10^3 cells.mL^-1 vs 1.66x10^3 cells.mL^-1) in the dialysis system. Similar results were obtained for the toxic G. catenatum strain (0.182 vs 0.118, and, 1.40x10^2 cells.mL^-1 vs 1.0x10^2 cells.mL^-1).

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Borowitzka, Michael
URI: http://researchrepository.murdoch.edu.au/id/eprint/51874
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