Nutrient-driven algal-bacterial dynamics in semi-continuous, pilot-scale photobioreactor cultivation of Nannochloropsis salina CCMP1776 with municipal wastewater nutrients

Algal cultures at production scales are de-facto xenic cultures that integrate diverse microbial communities. These microbes interact directly or indirectly with the algal primary producers in the system. The extent and direction of the interactions are determined by key algal growth parameters that are typically dominated by temperature, light and nutrient availability. The population dynamics of the microbial community relative to the growth conditions for Nannochloropsis salina (CCMP 1776) were investigated. The experiment was carried out in f/2 growth medium or secondarily treated wastewater in enclosed Solix BioSystems Lumian AGS4000 photobioreactor panels. Bacterial abundance and taxonomic profiles were assessed through amplicon metagenomics. Escherichia coli, as a pathogen indicator, was assessed through standard and genomic approaches. The microbial community responded to changes in the availability of macronutrients. Responses were also correlated to changes in the algal density and relative chlorophyll content. Stress conditions, associated with rapid algae growth enhance bacterial diversity, while having a non-uniform impact on abundance across taxa. The taxonomically rich Proteobacteria phylum reacted to nutrient enhancement with an increase in abundance across numerous genera. Bacteroidetes genera showed a variable response to changes in both nutrient status and other growth parameters, including substrate type, indicating a more complex response to changes in environmental factors. Nevertheless, while notable shifts were observed in the microbial community composition, the community structure has shown resilience and evident capacity for rapid adaptation to changes in the environmental conditions. Pathogen indicator organisms, while declining during rapid growth of algae, may survive and can recover in numbers and viability as algae enter senescence under nutrient stress conditions.