Microalgal photosynthetic inhibition and mixotrophic growth in Post Hydrothermal Liquefaction Wastewater (PHW)

Hydrothermal Liquefaction converts wet organic biomass into renewable biocrude oil and simultaneously generates a toxic wastewater (PHW) that is, however, rich in nutrients and organic compounds. While inhibition of algae in diluted PHW has been reported, the underlying reasons are still unclear. The present research explores, for the first time, the effects of PHW on the growth, photoautotrophic functions, and biomass characteristics of four freshwater green microalgae. Chlorolobion braunii, Chlorella sorokiniana, Chlorella vulgaris, and Scenedesmus quadricauda were exposed to four PHW concentrations (0, 0.5, 1, and 2% v/v) and their physiological responses comprehensively investigated. Based in pulse amplitude fluorometry (PAM), a general inhibitory effect in photosynthesis was revealed, indicating diminished photoautotrophic activity. Rapid light curve (RLC) showed that different photosynthetic stages were affected and maximum quantum yield decreased, while Gompertz modeling of population growth highlighted differences in both growth rates and lag phases. Physiological adjustment is suggested by reduced cell viability, increased cell size, and modification in pigment profiles. C. sorokiniana stood out based on its limited inhibitory effects and biomass generation in PHW. The combination of diminished photosynthetic performance with increased growth rate affirmed mixotrophy. These findings add to a growing body of literature on the recycling of PHW and similar wastewaters by providing new insights into inhibition mechanisms considering algal photobiology and potential for enhanced biomass production through mixotrophic metabolism.