Community structure and function of mixed microalgal communities for nutrient recovery and the selection of lipid and carbohydrate accumulators for enhanced feedstock production in wastewater treatment

Phototrophic systems are uniquely positioned to meet 21st century nutrient removal needs from wastewater – they have the potential to offer low-cost, low-input nitrogen and phosphorus recovery with effluent nutrient concentrations below the current limit-of-technology while simultaneously producing carbon-rich biomass. Despite this potential, current systems are operated empirically and suffer from unpredictable community dynamics that hinder performance. In order to create engineered systems that experience reliable and predictive behavior, there is a critical need to understand how design and operational parameters influence community structure, nutrient, and carbon (i.e., carbohydrate and lipid) dynamics. In addition to systematic process performance evaluations, elucidating this relationship requires a comprehensive examination of algal community structure, the molecular tools for which are underdeveloped. This works used process design to target specific functions in microalgae, with a focus on creating selective environments that recover nutrients and produce biomass rich in carbohydrates and lipids. Specifically, this work leveraged solids residence time (SRT) as a parameter that influences system wide performance and examined (1) nutrient (i.e., nitrogen and phosphorus) recovery via microalgal biomass assimilation, (2) eukaryotic and bacterial community structure, and (3) carbon dynamics of mixed algal-bacterial communities treating secondary effluent from a water resource recovery facility (a.k.a., wastewater treatment plant). Results are presented from long-term experiments using continuously-fed photobioreactors performed to test the efficacy of nutrient uptake in microalgal-wastewater systems and to elucidate the complex relationship between nutrient uptake, community structure and function, and carbon dynamics across long- term and diel time scales. Additionally, in order to overcome a core barrier to innovation in algal technology development, 18S rRNA gene-specific primers were designed and evaluated for use with the Illumina MiSeq second generation sequencing platform, a significant advancement that has broad interdisciplinary impacts and improves the current understanding of eukaryotic sequencing across the fields of environmental engineering, limnology, and oceanography. Additional significant findings indicate: (1) SRT meaningfully influences community structure and function, with higher SRTs exhibiting increased community stability but more variable system performance (i.e., nutrient recovery), and lower SRTs experiencing higher diversity and more dynamic community structure, but with more resilient system performance; (2) SRT drives differences in carbon dynamics across mixed communities, with dynamic carbohydrates and steady-state extant carbohydrate production increasing at lower SRTs; and (3) high SRTs successfully select for microalgae with higher intrinsic carbon accumulation rates, indicating SRT may be successfully leveraged to generate bioenergy feedstocks with a range of biochemical compositions and energy contents