Toward enhanced performance of integrated photo-bioelectrochemical systems: Taxa and functions in bacteria-algae communities

Growth parameters and responses of green algae across a gradient of phototrophic, mixotrophic and heterotrophic conditions

Many studies have shown that algal growth is enhanced by organic carbon and algal mixotrophy is relevant for physiology and commercial cultivation. Most studies have tested only a single organic carbon concentration and report different growth parameters which hampers comparisons and improvements to algal cultivation methodology. This study compared growth of green algae Chlorella vulgaris and Chlamydomonas reinhardtii across a gradient of photoautotrophic-mixotrophic-heterotrophic culture conditions, with five acetate concentrations. Culture growth rates and biomass achieved were compared using different methods of biomass estimation. Both species grew faster and produced the most biomass when supplied with moderate acetate concentrations (1-4 g L^-1), but light was required to optimize growth rates, biomass yield, cell size and cell chlorophyll content. Higher acetate concentration (10 g L^-1) inhibited algal production. The choice of growth parameter and method to estimate biomass (optical density (OD), chlorophyll a fluorescence, flow cytometry, cell counts) affected apparent responses to organic carbon, but use of OD at 600, 680 or 750 nm was consistent. There were apparent trade-offs among exponential growth rate, maximum biomass, and culture time spent in exponential phase. Different cell responses over 1-10 g L^-1 acetate highlight profound physiological acclimation across a gradient of mixotrophy. In both species, cell size vs cell chlorophyll relationships were more constrained in photoautotrophic and heterotrophic cultures, but under mixotrophy, and outside exponential growth phase, these relationships were more variable. This study provides insights into algal physiological responses to mixotrophy but also has practical implications for choosing parameters for monitoring commercial algal cultivation.

Enhanced metronidazole removal by binary-species photoelectrogenic biofilm of microaglae and anoxygenic phototrophic bacteria

High efficient removal of antibiotics during nutriments recovery for biomass production poses a major technical challenge for photosynthetic microbial biofilm-based wastewater treatment since antibiotics are always co-exist with nutriments in wastewater and resist biodegradation due to their strong biotoxicity and recalcitrance. In this study, we make a first attempt to enhance metronidazole (MNZ) removal from wastewater using electrochemistry-activated binary-species photosynthetic biofilm of Rhodopseudomonas Palustris (R. Palustris) and Chlorella vulgaris (C. vulgaris) by cultivating them under different applied potentials. The results showed that application of external potentials of -0.3, 0 and 0.2 V led to 11, 33 and 26-fold acceleration in MNZ removal, respectively, as compared to that of potential free. The extent of enhancement in MNZ removal was positively correlated to the intensities of photosynthetic current produced under different externally applied potentials. The binary-species photoelectrogenic biofilm exhibited 18 and 6-fold higher MNZ removal rate than that of single-species of C. vulgaris and R. Palustris, respectively, due to the enhanced metabolic interaction between them. Application of an external potential of 0V significantly promoted the accumulation of tryptophan and tyrosine-like compounds as well as humic acid in extracellular polymeric substance, whose concentrations were 7.4, 7.1 and 2.0-fold higher than those produced at potential free, contributing to accelerated adsorption and reductive and photosensitive degradation of MNZ.

Enhanced microalgae cultivation using wastewater nutrients extracted by a microbial electrochemical system

Cultivating algae using wastewater nutrients is a potential approach to realize resource recovery that can contribute to circular economy. However, growing algae directly in a wastewater has problems such as bacterial contamination and a low biomass density. To address those problems, we investigated microalgal cultivation in a photobioreactor (PBR) fed with the nutrients extracted from wastewater by a microbial nutrient recovery cell (MNRC). With an external voltage of 0.3 V, the MNRC-PBR system removed 96% of COD and recovered 44% of NH₄⁺-N and 39% of PO₄^3--P at a hydraulic retention time of 7.2 h. Microalgae cultivated in the nutrient recovery medium from the MNRC had 8.3-fold biomass density and 1.4-fold lipid contents, versus that cultivated in a food wastewater containing more nutrients. More significantly, 90% of biomass yielded from the MNRC-PBR system was microalgae, much higher than ∼30% in the food wastewater. A liquid exchange ratio of 30% achieved the highest microalgal density of 0.61 ± 0.06 g L^-1, comparable to that in a standard BG11 medium. There was a tradeoff between recycling PBR medium and microalgal growth. The accumulated salinity was observed in the extended operation of the MNRC-PBR system treating an actual food wastewater. The results of this study have demonstrated an effective approach to extract nutrients from wastewater for enhanced microalgal growth and improved biomass quality.