Time-Dependent Inhibition of Photosynthetic Activity in Filamentous Alga <i>Oedogonium</i> and Its Implications for Outdoor Cultures

The freshwater filamentous alga Oedogonium has the ability to form stable floating mats on the surface of ponds, with potential for wastewater bioremediation. While this trait facilitates ease of harvesting, the static nature of the filamentous mats exposes the algal cells to high irradiance levels for a prolonged period which could exacerbate the damage due to photoinhibition. This study investigated the time-dependent inhibition of photosynthetic activity (via the O2 evolution measurements) in Oedogonium. Under all but the mildest test conditions, photosynthetic activity progressively declined during exposure to constant light, with the degree of decline increasing at higher irradiance and temperature. Time-dependent inhibition was highest when both photooxidative and heat stresses were present. Oedogonium acclimated under low-stress conditions initially exhibited higher photosynthetic efficiency but suffered more rapid time-dependent inhibition than cells acclimated to high-stress conditions, with a 13-fold increase in the rate constant. Therefore, the influence of photosynthetic decline and delayed recovery will be critical to consider for filamentous algae in static mats or biofilms that result in prolonged exposure to stress. This study emphasizes the importance of considering the time-dependent inhibition of photosynthetic activity for accurate predictions of the performance of outdoor algae cultures in both engineered systems and natural ecosystems.