Adjustments of the photosynthetic unit and compensation mechanisms of tolerance to high ammonia concentration in Chlorella sp. grown in food waste digestate

Abstract

Anaerobically digested food waste (ADF) is a waste effluent produced from anaerobic digestion of food wastes. It is difficult to treat but also holds high potential for valorization. Microalgae cultivation is a promising path for its treatment and valorization. However, ADF is very turbid and has toxic concentration of ammonia nitrogen (NH3-N) (up to 5000 mg L−1 NH3-N). Here we looked at the physiological responses and acclimation patterns of Chlorella sp., Mur 269 to toxicity and turbidity in ADF, compared to bolds basal medium (BBM), under different irradiances. Indicators obtained via pulse amplitude modulation (PAM) fluorometry such as electron transport rate (ETR), fv′/fm′ and alpha (α), alongside oxygenesis as an indicator of photosynthesis, were used to study the state of the photosystem of the organism. Pigment (chlorophylls and carotenoids) ratios were studied as indicators of adjustments in the photosynthetic unit and growth rates and productivities were used to monitor cell growth. μmax and biomass productivities were 63 and 47% higher in ADF than in BBM. μmax and biomass productivity of Chlorella sp., Mur 269, remained close to their optimal values at 0.681 ± 0.03 d−1 and 165 ± 8 mg L−1 d−1 even at high irradiance intensities of 1500 μmol m−2 s−1, indicating applicability of this species in outdoor cultures at high irradiance. On the other hand, the results showed that in comparison with BBM, the efficiency of Chlorella sp., Mur 269 photosystem II at optimum irradiance, as reflected in Fv′/Fm′ values, was reduced by 16% in ADF. A critical look at the photosynthesis of the species by PAM fluorometry revealed that the adaptive methods of this microalgae to toxicity involves adjustment of the photosynthetic unit to maximize absorption of light and compensation mechanisms for reductions in PSII activity including switching to mixotrophic growth mode.