Control of the pH for marine microalgae polycultures: A key point for CO2 fixation improvement in intensive cultures.

• The highest conversion of CO 2 into carbon biomass was reached at pH 7. • The theoretical maximal biological conversion of CO 2 was estimated to be ca. 60 %. • Lower and higher pH favoured respectively the amoebae and copepod development. • pH can be a tool for favouring one microalgal species among a natural assemblage. Recently, CO 2 recycling for the production of valuable microalgae has acquired substantial interest. Most studies investigating CO 2 conversion efficiency in algal cultures were based on single species, although a stabilising effect of algal diversity on biomass production was recently highlighted. However, addition of CO 2 into polyalgal cultures requires a careful control of pH; performance of CO 2 conversion, growth and carbon biomass production are affected by pH differently, depending on the species of microalgae. This study investigates the efficiency of CO 2 conversion by natural marine algal assemblage cultivated in open, land-based raceways (4.5 m³, 10 m²), working as high rate algal ponds (HRAP). Ponds were enriched with nitrogen and phosphate, pure CO 2 was added and algal cultures were grown under three different fixed pH levels: pH 6, 7 and 8. The highest conversion of photosynthetically fixed CO 2 into carbon biomass (40 %) was reached at pH 7, an intermediate level, due to the partial CO 2 asphyxiation of algal predators (copepods, ciliates), while being under the suboptimal conditions for the development of marine amoebae. Under this pH, the theoretical maximal biological conversion of available CO 2 into carbon biomass was estimated to be 60 % in naturally inoculated open ponds. [ABSTRACT FROM AUTHOR]