Photosynthesis, competition, and wastewater treatment characteristics of the microalga Monoraphidium sp. Dek19 at cool temperatures

Monoraphidium sp. Dek19 (M. Dek19) is a versatile cold-adapted microalga suitable for treatment of municipal wastewater and production of biofuels from cellular lipids. This study investigated the growth and photosynthetic properties of M. Dek19 which allow it to phycoremediate effluent under cool conditions. Autotrophic cultures grown in tertiary effluent reached greater cell densities at 10 °C than at 22 °C. At both temperatures, this alga could completely deplete polluting PO₄³⁻ and NO₃²⁻ ions during growth to stationary phase. M. Dek19 also grew prolifically in primary effluent from an earlier water treatment stage. NH₄⁺ was removed as an alternate N-source, and algal photosynthesis had potential for energy-saving oxygenation of downstream aerobic treatments, including the activated sludge step. In mixed algal culture at a cool temperature (10 °C), M. Dek19 outcompeted Chlorella sp. However, at 22 °C this advantage was diminished, and the population of M. Dek19 cells declined. Photosynthetic O₂ evolution by log-phase M. Dek 19 cells at 22 °C showed light saturation at 100 μmol photons m⁻² s⁻¹. At 10 °C, photosynthetic rates were 50% lower, light saturation decreased to 60 μmol photons m⁻² s⁻¹, yet the cells grew to higher cell densities under cooler conditions. At stationary phase, maximal photosynthesis declined by 50%, indicating self-shading effects. Concurrently, cellular Chlorophyll (Chl) content increased 2-fold, and Chl a/b ratios increased from 1.7 to 3.72. This indicated an acclimation to lower light penetration as the culture matured. The potential for biofuel production was evident from the elevated neutral lipid content of M. Dek19 as nutrients were depleted from the wastewater. We conclude that M. Dek19 is well-adapted to low temperatures and light levels found in winter at many water treatment facilities in colder climates. Thus, M. Dek19 offers a means to phycoremediate municipal wastewater in geographical areas where outdoor growth of algae has previously been discounted as impractical.