Nutrient uptake, chlorophyll a and carbon fixation by Rhodomonas sp. (Cryptophyceae) cultured at different irradiance and nutrient concentrations

The goal of this research was to study biomass production, nitrate and phosphate uptake and carbon fixation in batch culture of the marine microalgae Rhodomonas sp., which is used in aquaculture as food for commercially reared invertebrates. Cultures were grown for 7 days under four irradiance levels (52, 68, 103 and 142 μmol photon m−2 s−1) and at three nitrate and phosphate concentrations (661-29, 882-39, 1323-58.5 μM of NaNO3 and NaH2PO4, respectively) establishing 12 experimental treatments. During the exponential phase, growth was directly correlated with irradiance and the initial nitrate and phosphate concentrations. Significant differences were found in the low growth phase as a function of nutrient concentration, but not for irradiance. For the low nutrient concentration treatments, growth was limited after 2 days. Final cell density was influenced by the initial concentration of nutrients, independently of the irradiance level. The average maximum biomass production was reached in 7 days in cultures with high nutrients (1.53 ± 0.07 × 106 cells mL−1). The total chlorophyll a content was directly related to cellular density and indirectly to irradiance level and concentration of nutrients. In conclusion, the optimal growing condition for Rhodomonas was up to the fourth day. However, after this time the cellular density, chlorophyll content and carbon uptake changed with the nutrient concentration and irradiance. Therefore we suggest that Rhodomonas culture could be used at the fourth day, either as inoculums for higher volume or as nourishment for invertebrate animals in marine aquaculture program.