Impact of daytime nitrogen limitation on structure and function of mixed microalgal communities derived from three geographically distinct sources

Increased nitrogen (N) and phosphorus (P) loadings to surface waters lead to algal blooms which cause eutrophication and hypoxia, threatening aquatic ecosystems, human health, and local economies. The adoption of increasingly stringent regulations for effluent N and P loadings, including from wastewater treatment plants (WWTPs), is a successful strategy for addressing this challenge. However, existing technologies for N and P removal are inconsistent with sustainability goals for nutrient recovery and reduced energy consumption. Eukaryotic microalgae are uniquely equipped to address these challenges, with the capacity to remove inorganic and organic N and P from growth media to the point of non-detection and to accumulate carbohydrate and lipid stores which may be further converted to biofuels. While algal wastewater treatment and biofuels are not new lines of research, much previous work has focused on either pure cultures or minimally engineered open systems. Algal biotechnology, using highly engineered open systems, seeks a middle road between these two approaches, offering greater control than open ponds but requiring fewer inputs than axenic systems. The work presented in this thesis explores nutrient dynamics, carbon storage, and community dynamics of mixed algal cultures within the context of algal biotechnology with an aim toward filling key knowledge gaps. Three parallel photobioreactors, each inoculated with mixed microbial communities sourced from WWTP clarifiers and/or surface water in one of three different geographic locations of the United States (Florida, North Carolina, Illinois) were operated in sequencing batch mode with an 8-day solids residence time (SRT) and 14hr:10hr light:dark cycle with nighttime N-feeding and daytime N-limitation. Metrics of community function included nutrient dynamics and biomass composition which were monitored over the course of 82 days and during a subsequent 10-day batch experiment to determine kinetic parameters. Community structure over 82 days was monitored via sequencing of the V4 and V8-V9 regions of the 18S rRNA gene. Structural and functional metrics showed complementary patterns. Functional metrics transitioned from dynamic to stable performance through time while community structure showed a departure from, followed by a return to, a community resembling the initial community. Although the communities in the three reactors remained distinct from each other through time, the most dominant OTUs were shared between all three reactors. The data presented in this experiment demonstrate how system design can induce communities which differ in structure to follow similar extant performance patterns. However, intrinsic storage parameters for carbohydrates and lipids differed widely between reactors, suggesting that more research is needed to elucidate structure-function relationships in microalgal communities to maximize carbon storage potential