Multistage optimization of growth and physiological condition of brackish green microalgae with the use of natural waters

We investigated the achievable maxima of optimisation of both growth and physiological condition of the preselected, Baltic green microalgae strains Chlorella vulgaris and Monoraphidium convolutum. The optimisation was divided into 3 consecutive stages of 8 days batch cultures, each focused on the effect of different set of factors on microalgae growth. The conditions identified as the most optimal served as a growth rate reference for the subsequent stage. We monitored the algal growth results spectrophotometrically (absorbance), gravimetrically (dry biomass DW) and biochemically (% protein, carbohydrates, lipids of DW). From the latter, it was possible to infer the physiological state of the microalgae and thus the presence or absence of a cellular stress factor. Stage 1 concerned the influence of basic culture parameters such as water salinity, medium type, and the use of natural waters in place of artificial media (i.e., fresh/brackish, environmental/artificial); stage 2 - the impact of the nitrogen source and its concentration (organic/inorganic, diluted/concentrated) and carbonates addition. Finally, stage 3 concentrated on the influence of the photoperiod and light spectral composition. The results show that among the different approaches to optimize algal growth, the most noticeable improvement was gained using environmental waters in place of artificial ones as a base for preparing the culture medium. Culture on natural, fresh river water yielded 61.1% and 57.4% higher biomass DW/L for C.vulgaris and M.convolutum respectively. For brackish waters, the use of natural bay water (6.7 PSU) in place of an artificial equivalent (distilled water + sea salt) resulted in 33.9% and 28.7% higher DW/L value. Selecting the optimal nitrogen source accelerated biomass production by another 10% for C.vulgaris, while no clear nitrogen source preference was observed for M.convolutum. Modifications to the photoperiod and wavelength of light proved efficient growth with continuous exposure to white light (24h) and highlighted achieved similar growth results with red or blue light in addition to white light in the 12h White/12h Colour photoperiod. In summary, with the use of multi-stage optimization, efficient and physiologically optimized growth was achieved, approximately 57% higher in final biomass DW/L regarding standard conditions, for both green algae strains. We discussed the the advantages of using natural waters and the opportunities presented by the physiological adaptation of brackish strains for growth on fresh and brackish water - the controlled application of salinity stress using local environmental waters in mass culture.