Stepwise culture approach optimizes the biomass productivity of microalgae cultivated using an incremental salinity increase strategy

Highlights: • Microalgal stepwise and co-cultivation were tested. • Stepwise cultivations resulted in higher biomass and lipid productivities. • Fv/Fm showed the same trend as growth and productivity in all cultures. -- Abstract: If our target is to produce low value commodity product, seawater needs to be used to generate sustainable microalgal biomass. In most areas of the world, open ponds are the preferred microalgal cultivation system. If seawater is used for filling up the pond and as make up for evaporative loss, the salinity of the growth media will gradually increase. Every saline microalga produces high biomass only within its optimal salinity range and the optimal salinity ranges of marine, halotolerant and halophilic microalgae are not continuous. The presence of non-optimal salinities reduces the overall biomass productivity under increased salinity. To achieve high biomass at non-optimal salinities, in present study, co-cultivation and stepwise cultivation of marine (Tetraselmis suecica), halotolerant (Amphora sp.) and halophilic (Dunaliella salina) microalgae were tested. Stepwise cultivation using T. suecica and Amphora sp. showed significantly higher biomass and lipid productivities (4.7% and 38.4%, respectively) compared to co-cultivation. Similarly, Amphora sp. and D. salina stepwise culture showed 54.8% extra lipid productivity compared to their co-culture. No significant difference was found between the maximum quantum yield of any of the treatments. Compared to co-cultivation, the outcome of this study confirmed that stepwise cultivation is a better way for maintaining the growth of selected species when salinity is increasing. This is indeed a suitable method for production of microalgal biomass and lipid when grown at suboptimal salinity zone. Further, stepwise process demonstrates the feasibility of a cultivation system based on seawater (to cultivate and replenish the pond), rather than potable fresh water.