Cultivation and photophysiological characteristics of desmids in moderately saline aquaculture wastewater

Although desmids typically inhabit freshwater environments characterized by low amounts of nutrients and low salinity, several desmid species have been recorded in eutrophic waters, indicating their adaptation to elevated pollution and conductivity. This study aimed to determine whether desmids could be used for remediation of moderately saline aquaculture wastewater (AWW) from a fish farm situated in the southeast of Sweden. Fourteen desmid strains isolated from different climates (tropical to polar) and trophic conditions (oligo- to eutrophic) were cultivated in diluted AWW and we estimated their growth rates, biomass, nutrient removal efficiency, chlorophyll fluorescence parameters, and cellular C, N, and P quotas. Despite being grown at moderate salinity, unfavourable N:P ratio, and relatively low light/temperature regime the eutrophic strains, Cosmarium humile, C. laeve, and a meso-oligotrophic species C. impressulum, completely absorbed nitrate and phosphate from AWW media after 7 days, indicating their potential for remediation of fish effluents in colder climates. These species, along with the typical eutrophic species, C. meneghinii and Staurastrum chaetoceras, had biomass in the range 0.45 to 1.19 g · L-1 while maximum growth rates ranged from 0.36 - 0.51 · d-1 , similar to published rates for several fast-growing green microalgae cultivated in various AWW types. Tropical desmids had distinctly high values of saturating irradiance (Ik > 1000 µmol photons · m-2 · s-1 ), and, along with eutrophic desmids, had high potential electron transport (rETRmax > 155 rel. units). Hence, the desmids studied demonstrated inherent photophysiological responses corresponding to their climate and trophic origin under the suboptimal growth conditions.