Inorganic carbon sources and biomass regulation in intensive microalgal cultures

Three freshwater and one marine algal species were grown under inorganic carbon limitation in laboratory continuous cultures. Comparisons were made between HCO₃^? alkalinity and bubbled CO₂ as carbon sources. HCO₃^? alkalinity was an excellent source of inorganic carbon below specific pH levels, but chemical precipitation at high pH placed an upper limit on productivity that was far lower than potential light-limiting levels. With bubbled CO₂ it was possible to achieve light limitation. The main factor controlling productivity was the mass flux of inorganic carbon added to the culture, which is the product of gas flow rate and influent P level. Small bubbles were more efficient than large bubbles at low gas flow rates and P levels, but led to froth flotation of algal cells and concomitant reductions in productivity at high bubble rates. At 1% CO₂ productivity was still dependent on mass fluxes of added carbon, but was independent of bubble size. At high bubble rates with 1% CO₂ narcosis was evident. Maximum yields occurred at intermediate dilution rates when inorganic carbon was supplied via bubbled gas.