Physiological responses of carbon-sequestering microalgae to elevated carbon regimes

In order to identify a high carbon-sequestering microalgal strain, the physiological effect of different concentrations of carbon sources on microalgae growth was investigated. Five indigenous strains (I-1, I-2, I-3, I-4 and I-5) and a reference strain (I-0: Coccolithus pelagicus 913/3) were subjected to CO2 concentrations of 0.03-15% and NaHCO3 of 0.05-2 g CO2 l(-1). The logistic model was applied for data fitting, as well as for estimation of the maximum growth rate (mu(max)) and the biomass carrying capacity (B-max). Amongst the five indigenous strains, I-3 was similar to the reference strain with regards to biomass production values. The B-max of I-3 significantly increased from 214 to 828 mg l(-1) when CO2 concentration was increased from 0.03 to 15% (r = 0.955, P = 0.012). Additionally, the B-max of I-3 increased with increasing NaHCO3 (r = 0.885, P = 0.046) and was recorded at 153 mg l(-1) (at 0.05 g CO2 l(-1)) and 774 mg l(-1) at (2 g CO2 l(-1)). Relative electron transport rate (rETR) and maximum quantum yield (F-v/F-m) were also applied to assess the impact of elevated carbon sources on the microalgal cells at the physiological level. Isolate I-3 displayed the highest rETR confirming its tolerance to higher quantities of carbon. Additionally, the decline in F-v/F-m with increasing carbon was similar for strains I-3 and the reference strain. Based on partial 28s ribosomal RNA gene sequencing, strain I-3 was homologous to the ribosomal genes of Chlorella sp.