Characterising key bacterial taxa of the benthic diatom Amphora sp. biofilms in a porous substrate photobioreactor

Microalgae-bacteria interactions have recently emerged as a potential approach to enhance microalgal growth and metabolite production. However, successful co-culturing relies on synergistic and metabolically complementary interactions between microalgae and associated microorganisms, which enhance culture stability and productivity. Rather than introducing exogenous bacteria, characterising native host-associated microbiomes represents an ecologically relevant strategy, as these communities may reflect long-standing mutualistic or syntrophic interactions. Despite increasing interest in biofilm-based cultivation, microbial dynamics within microalgal biofilms in bioreactors remain poorly understood compared to planktonic systems. In this study, we investigated bacterial succession in a monoalgal biofilm of the oleaginous benthic diatom Amphora sp. during its exponential growth phase in a porous substrate photobioreactor (PSBR), aiming to identify key bacterial taxa for future co-culturing experiments. The exponential phase was targeted due to its relevance for biomass accumulation and active cellular metabolism underpinning productivity in biofilm-based systems. Non-axenic Amphora sp. was grown in F/2-enriched artificial seawater, and bacterial community dynamics were assessed at Day 0, Day 3 (mid-exponential), and Day 6 (late-exponential) using PacBio Sequel II/IIe 16S rRNA amplicon sequencing. Distinct phase-dependent shifts in bacterial community composition were observed, with Alphaproteobacteria declining over time and concomitant increases in Flavobacteriia and Planctomycetota. Despite these changes, bacterial communities maintained relatively stable evenness, structured around a few pivotal taxa. Marivita , Sulfitobacter , Polaribacter , and Rhodopirellula were identified as key bacterial taxa dynamically associated with the exponential phase of Amphora sp. Overall, this study provides a foundational screening framework to identify native bacterial candidates for co-culturing with mono-microalgal biofilms. • The bioreactor ecology of mono-microalgal biofilms remains poorly understood. • Bacterial dynamics of Amphora biofilm during active growth in a PSBR were studied. • Phase-dependent shifts in bacterial community and predicted functions were observed. • Marivita , Polaribacter , Sulfitobacter and Rhodopirellula were key bacterial taxa.