Synergistic effect of UV radiation and nutrient limitation on chlorella fusca (Chlorophyta) cultures grown in outdoor cylindrical photobioreactors

This study assessed the interactive effects of UVR and nutrient depletion on Chlorella fusca cultures on the production and accumulation of particular biomolecules. To accomplish this, algae were grown for 5 d in outdoor thin-layer cascade cultivators under 3 nutrient treatments (full nutrients, -N and -S) and then transferred to outdoor cylindrical photobiore-actors for another 5 d. Cultures were then exposed to full solar radiation (PAB) and decreased UVR. During the last 5 d, bio-optical properties, photosynthetic activity, pigments, biochemical composition and oxidative stress were assessed. Initially, nutrient depletion caused changes in productivity and cell number in a manner that affected biochemical composition. After 3 d, the percentage of lipids in the cultures under N deprivation reached values appropriate for being used as feed or food additives or for energy applications (35% of lipid content), regardless of the light conditions. A longer exposure (5 d) resulted in interactive effects of light and nutrient conditions. Specifically, PAB increased lipid content in all cases (1.3- to 2.3-fold), but particularly under S deprivation. Longer exposure to PAB also increased oxidative stress in UVR and nutrient-limited treatments (-N and -S). These results showed that the benefits expected from nutrient depletion (increase in biomolecule content e.g. lipids, carbohydrates and pigments) were modulated by the negative effects of algal UVR acclimation costs

Light acclimation and pH perturbations affect photosynthetic performance in <i>Chlorella</i> mass culture

Chlorella spp. are robust chlorophyte microalgal species frequently used in mass culture. The pH optimum for growth is close to neutrality; at this pH, theoretically little energy is required to maintain homeostasis. In the present study, we grew Chlorella fusca cells in an open, outdoor, thin-layer cascade photobioreactor (TLC), under ambient photon flux at the theoretically preferred pH (7.2), and let the culture pass the exponential growth phase. Using pH drift experiments, we show that an alkalization to pH 9 supported photosynthesis in the TLC. The increased photosynthetic activity under alkaline conditions was a pH-dependent effect, and not a dissolved inorganic carbon (DIC) concentration- or light intensity-dependent effect. Re-acidification (in one step or in increments) lowered gross oxygen production and increased non-photochemical quenching in short-term experiments. Gross oxygen production and electron transport rates in PSII were uncoupled during the pH perturbation experiments. Electron transport rates were only marginally affected by pH, whereas oxygen production rates decreased with acidification. Alternative electron pathways, electron donation at the plastid terminal oxidase and state-transitions are discussed as a potential explanation. Because cell material from the TLC was not operating at maximal capacity, we propose that alkalization can support photosynthesis in challenged TLC systems