In Vivo Evaluation of Oxygenic Photogranules’ Photosynthetic Capacity by Pulse Amplitude Modulation and Phototrophic–Irradiance Curves

Abstract

The commingled microbial moiety of oxygenic photogranules (OPGs) facilitates aeration-free wastewater treatment. Embedded in an extracellular polymeric substances (EPS) matrix, microbial producers and consumers of oxygen occupying granular niches exchange substrates among themselves and with the bulk fluid. An assessment of the OPG’s phototrophic potential or functional capacity may require combining different photoactivity signals. The photosynthetic capacity was evaluated using photosynthetic oxygen evolution (POE) and chlorophyll fluorescence (rapid light curves, RLC) measurements using OPGs grown at different light intensities.. A maximum oxygen generating capacity for optimal OPGs and reactor conditions was determined to be 284.4 mgO2 gVSS–1 h–1 The OPGs exhibited photoelasticity, with higher photosynthetic capacity in high light (HL) compared to that in low light (LL) adapted samples. Saturation irradiances before the onset of photoinhibition for LL and HL samples were 1000 and 1200 μmol m–2 s–1, respectively using POE signals, and 478 μmol m–2 s–1 and 611 μmol m–2 s–1 using RLC signals. Moreover, HL adapted samples had higher nonphotochemical quenching rates which allude to the OPG’s photoelastic potential. The correlation coefficients (κ) between POE and RLCs were lower than reported values for pure microbial cultures reflecting the enhanced contribution from different photosynthetic clades with a variety of light-harvesting pigments present in OPGs. In an OPG reactor, the photochemical activity can be influenced by the granular size, granular ecology, and reactor operation metrics related to irradiance interactions such as mixing, self-shading, light intensity, photoperiods, and reactor depth. This presents opportunities for design of intensive wastewater resource recovery using phototrophic granular biomass.