Microalgae as a biofuel source are of great interest. The outdoor Regional Algal Feedstock Testbed (RAFT) was established to optimize algal biomass production in the U.S. sunbelt states. The phycosphere inhabitants, consisting in part of the bacterial community within algal cultures, are hypothesized to contribute to algal productivity. However, little is known about their identity or specific contributions. In this study the bacterial composition of the Chlorella sorokiniana phycosphere was

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... ermined over several production cycles in different growing seasons by 16S rRNA gene sequencing and identification using the SILVA database. Results The C. sorokiniana bacterial phycosphere was sampled during 41 RAFT testbed cycles in two consecutive growing seasons. Eight reactors were sampled twice daily to establish baseline community composition, determined by high-throughput sequencing of 16S ribosomal RNA amplicons. The baseline diversity of the phycosphere increased overall with time during each individual reactor run, based on an increase in Faith's phylogenetic diversity metric versus days post-inoculation (R = 0.66, p < 0.001). Bacterial assemblages were identified in samples during the stationary, growth, and death stages. During summer months, Vampirovibrio chlorellavorus, an obligate predatory bacterium, was prevalent. The possibility that a sequence correlated with biomass productivity, days postinoculation, dissolved oxygen, and weather measurements was assessed using a rankbased multinomial regression model. Bacterial sequences assigned to the Rhizobiales, Betaproteobacteriales, and Chitinophagales were positively associated with algal biomass productivity, the industry gold standard for establishing 'efficiency'. Applications of the general biocide, benzalkonium chloride, to a subset of RAFT experiments intended to