| Summary: | 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.
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