Sustainable cultivation of marine, halotolerant and halophilic miccroalgae

There has been a worldwide interest in mass microalgal production. As fresh water is a limited resource, seawater must be used for sustainable production of microalgal biomass. If an open pond cultivation system relies only on seawater, the salinity of the growth media will rise over time. It is not possible for any microalgal species to achieve high biomass under continuous salinity increase (from low salinity up to salt saturation). Therefore, cultivation of microalgae with different salinity optima would be the best possible approach to produce high biomass under continuously increased salinity. In this study, the effect of increased salinity on microalgal biomass productivity was assessed. Main targeted products were biofuel (low priced commodity) and fucoxanthin (high-value pigment). To identify high biomass producing species under constantly increased salinity, nine microalgae of three different salinity ranges were selected. Of them six are marine or low saline microalgae (Chrysotila carterae, Chaetoceros muelleri, Nannochloropsis sp., Pheodactylum tricornutum, Tisochrysis lutea and Tetraselmis suecica), two are halotolerant or mid saline microalgae (Amphora sp. and Navicula sp.), and a halophilicor hypersaline microalgae (Dunaliella salina). All these species have already been successfully cultivated in outdoor condition and are also commercially important. Among six marine microalgae, Tetraselmis suecica showed the widest salinity range, i.e., 35 to 109 ppt (parts per thousand) with overall high biomass and lipid productivity 32 mg L-1 d-1 and 13.6 mg L-1 d-1 of AFDW (ash-free dry weight), respectively. Between two halotolerant microalgae, Navicula sp. showed 3% higher biomass productivity than Amphora sp. However, Amphora sp. was chosen for the further experiment since Navicula sp. have been reported to achieve high growth only in winter. Halophilic miii croalgae Dunaliella salina showed widest salinity range (35 and 233 ppt) among the selected species with highest lipid content 56.2% of AFDW; hence, these three species were selected for further study to produce high biomass and lipid under increased salinity. Among the nine selected species, six marine species, e.g., Chrysotila carterae, Chaetoceros muelleri, P. tricornutum, and Tisochrysis lutea and; two halotolerant species Amphora sp. and Navicula sp. are able to produce fucoxanthin, a high value pigment. Their fucoxanthin producing ability wastested under incremental salinity increase. The results showed that at salinity below 55 ppt marine microalgae Chaetoceros muelleri produced higher fucoxanthin than any other marine species. Both halotolerant microalgae Amphora sp. and Navicula sp. were able to produce high fucoxanthin at salinity above 55 ppt; however, Amphora sp. was selected for further study as Navicula sp. was reported to show good growth only in winter. The results indicated that the highest biomass, lipid, and fucoxanthin production could only be achieved at the optimal salinity ranges of tested microalgae. However, the optimal salinity ranges of marine, halotolerant and halophilic microalgae are not continuous. There are non-optimal salinity zones in between the optimal salinity ranges where the biomass productivity is found low. Therefore, to produce high biomass at nonoptimal salinity zone, two cultivation methods namely co-cultivation and stepwise cultivation were performed. The overall biomass productivity of Tetraselmis suecica and Amphora sp.co-culture showed no significant differences with the overall biomass productivity of Tetraselmis suecica and Amphora sp. monocultures. However, the lipid productivity in Amphora sp. monoculture was found 23% higher than the lipid productivity of co-culture. Similarly, no significant difference was also observed between the biomass productivity of Amiii phora sp. and D. salina co-culture and their monocultures. The overall lipid productivity in co-culture was approximately 40% less than that of D. salina monoculture. In addition, fucoxanthin producing species, i.e., Chaetoceros muelleri and Amphora sp. co-culture showed 38% and 50% less overall biomass and fucoxanthin productivity, respectively than the overall biomass and fucoxanthin productivity of Amphora sp. monoculture. As co-cultivation showed low biomass, lipid and fucoxanthin productivity, stepwise cultivation was performed to check whether it helped to improve the biomass, lipid and fucoxanthin productivity. During stepwise culture, halotolerant microalgae were grown in the filtrate of marine microalgae and halophilic species was cultivated in the filtrate of halotolerant microalgae. No negative effect of recycled media was observed on the growth and biochemical content of microalgae. The biomass and lipid productivity in Tetraselmis suecica and Amphora sp. stepwise culture was found 10% and 30%, respectively higher than that of Tetraselmis suecica and Amphora sp. co-culture within the same salinity range. No significant differences were observed between the biomass productivity of Amphora sp. and D. salina coculture and stepwise culture. However, the lipid productivity in Amphora sp. and D. salina stepwise culture was found 40% higher than that of their co-culture within the same salinity range. In stepwise culture, fucoxanthin producing Chaetoceros muelleri and Amphora sp. showed 63% and 46.6% higher biomass and fucoxanthin productivity, respectively than that of their co-culture at the same salinity range. A preliminary economic assessment was also carried out to estimate the biomass and fucoxanthin production cost. The results indicated that monoculture integrated with stepwise culture offered lowest microalgal biomass production cost (≈1.37 Aus $kg-1 of dry biomass). This biomass cost was 9$ kg-1) was also achieved using monoculture integrated with stepwise culture system. The outcome of this study clearly indicated that an integration of monoculture with stepwise culture is the most effective approach for producing sustainable and high biomass, lipid and fucoxanthin under incremental salinity increase

Biomass production of marine microalga Tetraselmis suecica using biogas and wastewater as nutrients

Anaerobic digestion is a suitable method for treating organic wastes and generating biogas. This biogas contains significant amount of CO2 and some other contaminants. The coupling of wastewater treatment with biogas purification using saline microalgae could effectively upgrade biogas (through photosynthetic CO2 fixation) and concurrently remove nutrients from the effluent, while producing valuable algal biomass. In this context, Tetraselmis suecica biomass production with the use of an impurity (CO2) in biogas to supply carbon, and nutrients (nitrogen and phosphorus) from anaerobically-digested piggery effluent (ADPE) was investigated at four operating pH set points (6.5, 7.5, 8.5 and 9.5). Results showed that pH 7.5 produced the optimum conditions for T. suecica growth and biogas-based CO2 removal, with the maximum biomass (59.8 mg L−1 d−1), lipid (25 mg L−1 d−1) and carbohydrate (6.5 mg L-1 d-1) productivities. Under this condition, CO2, total nitrogen and phosphorus removal efficiencies were 94.7%, 96% and 72%, respectively. Furthermore, the results showed no inhibitory effect of dissolved CH4 on the growth of T. suecica at pH 7.5, suggesting the technical feasibility of harnessing marine T. suecica for simultaneous nutrients removal from wastewaters, biogas upgrading, and production of energy-rich algal biomass. This process clearly harnesses anaerobically-digested piggery effluent not only as an asset but also uses an impurity (CO2) in biogas to produce valuable algal biomass