Effect of temperature on growth, photosynthesis and biochemical composition of Nannochloropsis oceanica, grown outdoors in tubular photobioreactors

Abstract

Temperature is an important factor affecting growth, photosynthetic rate and biomass composition. As such, this study focused on the effects of temperature on biomass yield and night-time biomass loss, as well as photochemical changes, using Nannochloropsis oceanica as model species, grown in two outdoor 50-L tubular photobioreactors (PBR). Microalgal biomass produced during daytime is partially lost overnight by respiration of its intracellular carbon reserves, which may have a considerable negative impact on industrial biomass productivity. In two independent trials, cultures were subjected to a diurnal light:dark cycle, under a constant temperature of 28 oC and, on the second trial, at 18 oC. Changes in culture performance were assessed by measuring growth and assessing lipid and fatty

acid composition of the biomass in the early morning and evening. Chlorophyll fluorescence quenching analysis was also used to better evaluate their physiological state. Our results revealed that N. oceanica shows a wide temperature tolerance with relevant night-time biomass loss, which decreased at lower temperatures, although at the expenses of daily productivity. Fluorescence measurements revealed reversible damage to photosystem II in cells growing in the PBR under optimal thermal conditions, whereas microalgae grown at suboptimal ones exhibited an overall lower photosynthetic activity. Total lipids were consumed overnight to support cell division and provide maintenance energy. Eicosapentaenoic acid (EPA) catabolism reached a maximum after the dark period, as opposed to their saturated counterparts; whereas lower temperatures led to higher EPA content which reached the maximum in the morning. These findings are relevant for industrial scale development.