Productivity parameters of Tetraselmis viridis (Chlorophyta) under natural illumination with increased carbon supply

Abstract

The development of sustainable biotechnologies for microalgal biomass production requires optimization of cultivation conditions and evaluation of productivity under natural illumination, accounting for the natural dynamics of environmental factors. The aim of this study was to assess the effect of carbon supply on production characteristics, diurnal growth dynamics, and night biomass loss during pilot‑scale cultivation of the green microalga Tetraselmis viridis in southern Russia (mid‑latitudes). The experiment was conducted in batch mode in open ponds under natural solar illumination in spring. In the control treatment, no additional carbon source other than atmospheric air was used, while in the experimental treatment, CO₂ was supplied to the culture with pH maintained up to 8.5. Biomass concentration, cell number, average daily productivity, daily production, night biomass loss, and photosynthetic efficiency were determined. In the control treatment, batch growth was limited to 8 days, with maximum average daily productivity of 6.5 g m^-2 day^-1 and a total yield of 43.5 g m^-2; growth was constrained by a pH increase to 10.5–10.8. Supplementary CO₂ supply increased productivity 1.4‑fold (up to 9.5 g m^-2 day^-1), daily production 1.9‑fold (up to 14.6 g m^-2), maximum biomass to 1.12 g L^-1, and total yield 2.4‑fold (103 g m^-2). Meanwhile, night biomass loss decreased from 12.5 % to 9.5 % of the evening biomass, and photosynthetic efficiency increased from 7.1 % to 12 % on average (reaching up to 17 % on certain days). Under carbon‑sufficient conditions, night biomass loss was positively correlated with daily production. It was also shown that photosynthetic efficiency was negatively correlated with daily irradiance. Thus, CO₂ enrichment of T. viridis culture not only overcomes carbon limitation and extends the linear growth phase but also improves solar energy utilization and reduces night losses. These findings can serve as a basis for optimizing T. viridis cultivation technologies in open systems for biomass production intended for aquaculture, biofuel, and other biotechnological applications.