The past, present and future of algal continuous cultures in basic research and commercial applications

Photoautotrophic continuous cultures (chemostat, turbidostat or exponential fed-batch operation) of algae began seventy-five years ago in the laboratory and nearly sixty years ago in large-scale outdoor wastewater treatment. Despite this deep and rich history, continuous culture approaches have been used in a small fraction of publications involving algal culturing over the last fifty-five years. Algae grown in continuous cultures are indefinitely in exponential balanced growth, poised at a particular physiological steady state, or a cyclical steady state under a light-dark cycle (cyclostat). This enables mechanistic linking of culture conditions to physiology much more precisely than in batch cultures, where conditions (light, nutrient and possibly CO2 availability), physiological state and specific growth rate change progressively. Continuous cultures also yield consistent biomass composition over time and have the potential to exhibit higher time-averaged biomass production than batch cultures, because growth rate does not decelerate. Commercial algal culture for various products has been studied almost exclusively in batch culture at all scales. The prevailing dogma is that elevated TAG content (the most extensively studied product) occurs only when cell division ceases due to nutrient depletion. Limited recent experimental evidence from continuous cultures suggests that this is not necessarily the case, thus warranting further investigation. Continuous cultures may be particularly useful for customizing biomass composition in favor of desirable high value bioproducts. Innovative continuous culture approaches, including hybrid designs and manipulated steady state, therefore promise to advance basic algal research and, pending scaled-up experimentation, may be useful for commercial biomass production in some contexts. There is insufficient information to date to conclude whether continuous culture (or hybrid) approaches can be commercially successful for any application, thus it is premature to dismiss commercial continuous cultures as impracticable or to assert them as suitable or superior.