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Growth, calcification and photosynthesis in the coccolithophorid chrysotila carterae

Webb, Jason (2015) Growth, calcification and photosynthesis in the coccolithophorid chrysotila carterae. PhD thesis, Murdoch University.

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The mass culture of microalgae for the commercial production of a) low value commodities such as biofuel and food and b) high value products such as polyunsaturated fatty acids, carotenoids, and nano-scaffolds is becoming increasingly attractive. Coccolithophorid algae have been investigated as potential candidates for both low and high value products. This thesis provides data on the specific nutrient and growth requirements in the coccolithophorid, Chrysotila carterae (previously Pleurochrysis carterae). Via the use of oxygen evolution techniques and PAM fluorometry, it is shown that C. carterae is just as susceptible to photoinhibition as some other microalgae with photoinhibition occurring at around 1100-1500 μmol photon m2 s-1. C. carterae also has the ability to recover from short periods of acidification, with recovery from pH 5 when there was no organic carbon assimilation to pH 9 after 20 minutes, Carbon assimilation increased from almost 0, to 3.01 pg CORG cell-1 h-1 . This microalga has a fundamental requirement for selenium, with specific growth rates falling from a μmax of 0.6 d-1, with selenium to 0.1 d-1 in selenium-limited culture. Selenium is also required for coccolith production. In Se-limited culture coccolith production was almost reduced by half, from 70 x105 coccoliths mL-1 to 3.8 x 105 coccoliths mL-1. Diurnal studies of organic and inorganic carbon assimilation showed that C. carterae CCMP647 synthesises coccoliths during the day, and then extrudes them onto the cell surface during the last hours of the dark cycle.

Investigations into the effect of various nitrogen sources indicated that with unregulated pH, nitrate achieved the greatest cell density and stable growth: The maximum cell densities reached were nitrate (66.61 x 104 ± 8.2 x 103 cells mL-1) > urea (34.0 x 104 ± 6.2 x 103 cells mL-1) = ammonium (36.08 x 104 ± 4.2 x 103 cells mL-1). Nitrate had the greatest effect on the culture medium ΔpH, (NO3- (0.134 ± 0.003) > urea (0.111 ± 0.003) > NH4+ (0.043 ± 0.001) and urea increased the growth rate of C. carterae by 150 % from 0.17 0.002 d-1 on NO3- to 0.44 ± 0.001 d-1 on urea. However, coccolith production increased with NO3- (73.81± 3.51 ng CaCO3 cell-1> NH4+ 55.18 ± 0.61 ng CaCO3 cell-1 > urea at 12.88 1.62 ng CaCO3 cell-1. Organic carbon (CORG) assimilation using NO3- far exceeded that on NH4+ and urea (CORG assimilated with NO3- = 7 x103 pg CORG cell-1 h-1 vs Urea at 6 x103 pg CORG cell-1 h-1 and NH4+ 5 x103 pg CORG cell-1 h-1 . Inorganic carbon assimilation (CINORG) was also elevated with NO3- producing 3 x103 pg CINORG cell-1 h-1 vs urea at 2 x103 pg CINORG cell-1 h-1 and NH4+ at 2 x103 pg CINORG cell-1 h-1. Thus, nitrate provides long term, stable growth with the highest cell overall cell density under unregulated pH.

Under elevated medium pH, urea and ammonium had the highest rate of carbon assimilation far in excess of NO3- for both CORG (Urea 44921.73 ± 2191.08 pg CORG cell-1 h-1 > NH4+ 22006.22 ± 640.39 pg CORG cell-1 h-1 > NO3- 773.59 ± 14.8 pg CORG cell-1 h-1) and CINORG, Urea 773.59 ± 14.8 pg CINORG cell-1 h-1 NO3- 569.44 ± 31.4 pg CINORG cell-1 h-1. Although carbon assimilation rates were elevated under urea and NH4+ at higher pH levels, NO3- at pH 8 had the highest Calcifaction to photosynthsdis ratio (C:P) ratio of 0.158, while closely followed by urea at pH 9 (C:P = 0.150).

With enhanced carbon assimilation at pH levels exceeding the pKa of CO2 in the medium pH indicated that this species must be using HCO3- as a carbon source, as cell growth and calcification were elevated at pH levels at which there is a greatly reduced level of CO2 in the medium which is typically in air equilibrated water approximately 10 μmol L-1.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Borowitzka, Michael and Moheimani, Navid
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