Competitive partitioning of denitrification pathways during arrested methanogenesis: Implications in ammonium recovery, N2O emission, and volatile fatty acid production.

The complex interaction between nitrate (NO3-) reduction and fermentation is poorly understood when high levels of NO3- are introduced into anaerobic systems. This study investigated the competitive distribution between conventional denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA) during simultaneous denitrification and fermentation in arrested methanogenesis. Up to 62% of initial NO3- (200 mg-N/L) was retained as ammonium through DNRA at a chemical oxygen demand (COD)/N ratio of 25. Significant N2O emission occurred (1.7 - 8.0% of the initial NO3-) with limited carbon supply (≤1600 mg COD/L) and sludge concentration (≤3000 mg COD/L). VFA composition shifted predominantly towards acetic acid (>50%) in the presence of nitrate. A novel kinetic model was developed to predict DNRA vs. DEN partitioning and NO2- accumulation. Overall, NO3- input, organic loading, and carbon source characteristics independently and collectively controlled competitive DNRA vs. DEN partitioning.

Cyanobacterial community succession and associated cyanotoxin production in hypereutrophic and eutrophic freshwaters.

Cyanobacterial harmful algal blooms (cyanoHABs) in freshwater bodies are mainly attributed to excess loading of nutrients [nitrogen (N) and phosphorus (P)]. This study provides a comprehensive review of how the existing nutrient (i.e., N and P) conditions and microbial ecological factors affect cyanobacterial community succession and cyanotoxin production in freshwaters. Different eutrophic scenarios (i.e., hypereutrophic vs. eutrophic conditions) in the presence of (i) high levels of N and P, (ii) a relatively high level of P but a low level of N, and (iii) a relatively high level of N but a low level of P, are discussed in association with cyanobacterial community succession and cyanotoxin production. The seasonal cyanobacterial community succession is mostly regulated by temperature in hypereutrophic freshwaters, where both temperature and nitrogen fixation play a critical role in eutrophic freshwaters. While the early cyanoHAB mitigation strategies focus on reducing P from water bodies, many more studies show that both N and P have a profound contribution to cyanobacterial blooms and toxin production. The availability of N often shapes the structure of the cyanobacterial community (e.g., the relative abundance of N2-fixing and non-N2-fixing cyanobacterial genera) and is positively linked to the levels of microcystin. Ecological aspects of cyanotoxin production and release, related functional genes, and corresponding nutrient and environmental conditions are also elucidated. Research perspectives on cyanoHABs and cyanobacterial community succession are discussed and presented with respect to the following: (i) role of internal nutrients and their species, (ii) P- and N-based control vs. solely P-based control of cyanoHABs, and (iii) molecular investigations and prediction of cyanotoxin production.

Harnessing Solar Energy using Phototrophic Microorganisms: A Sustainable Pathway to Bioenergy, Biomaterials, and Environmental Solutions.

Phototrophic microorganisms (microbial phototrophs) use light as an energy source to carry out various metabolic processes producing biomaterials and bioenergy and supporting their own growth. Among them, microalgae and cyanobacteria have been utilized extensively for bioenergy, biomaterials, and environmental applications. Their superior photosynthetic efficiency, lipid content, and shorter cultivation time compared to terrestrial biomass make them more suitable for efficient production of bioenergy and biomaterials. Other phototrophic microorganisms, especially anoxygenic phototrophs, demonstrated the ability to survive and flourish while producing renewable energy and high-value products under harsh environmental conditions. This review presents a comprehensive overview of microbial phototrophs on their (i) production of bioenergy and biomaterials, (ii) emerging and innovative applications for environmental conservation, mitigation, and remediation, and (iii) physical, genetic, and metabolic pathways to improve light harvesting and biomass/biofuel/biomaterial production. Both physical (e.g., incremental irradiation) and genetic approaches (e.g., truncated antenna) are implemented to increase the light-harvesting efficiency. Increases in biomass yield and metabolic products are possible through the manipulation of metabolic pathways and selection of a proper strain under optimal cultivation conditions and downstream processing, including harvesting, extraction, and purification. Finally, the current barriers in harnessing solar energy using phototrophic microorganisms are presented, and future research perspectives are discussed, such as integrating phototrophic microorganisms with emerging technologies.