Impact of growth phases on photochemically produced reactive species in the extracellular matrix of algal cultivation systems

Abstract

There is growing interest in microalgal biotechnologies for biofuel production and nutrient recovery in wastewater treatment. Often overlooked is the exudation of extracellular organic matter (EOM) in algal reactors and its influence on dissolved organic carbon (DOC) characteristics and photochemical processes. This study reports photosensitized production of excited triplet state dissolved organic matter (³DOM*), hydroxyl radicals (HO˙), and singlet oxygen (¹O₂) under simulated sunlight in photobioreactor extracellular matrices. Reactive species were measured in irradiated supernatant from batch cultures of Chlamydomonas reinhardtii throughout lag (1.2–1.9 mg-C L⁻¹ as DOC_EOM), exponential (1.9–8.1 mg-C L⁻¹), early stationary (carbon accumulation; 8.1–21.4 mg-C L⁻¹), and late stationary (21.4–108 mg-C L⁻¹) growth phases, and in solutions amended with Suwannee river natural organic matter (SRNOM) as a benchmark. DOC_EOM represented 1.0–8.6% of fixed carbon (cellular organic carbon + EOM) across algal phases of growth. EOM solutions exhibited lower light absorption and reactive species production than SRNOM solutions per mg-C L⁻¹. However, photosensitized ³DOM* quantum yield coefficients in EOM solutions during all growth phases and HO˙ apparent quantum yields observed during exponential and early stationary phases were greater than in SRNOM solutions. Additionally, ¹O₂ apparent quantum yields in EOM solutions during exponential and late stationary phases were similar to SRNOM solutions. EOM solutions also photoproduced reactive species at levels comparable to natural waters. These results suggest sensitized photochemical processes involving EOM and growth medium constituents may contribute to DOM bleaching and mineralization, nutrient cycling, pathogen inactivation, and fate of trace organic contaminants in engineered algal systems.