Putative functional redundancy in cyanobacterial microbiomes allows stable bioplastic production despite shifts in dominant populations.

This study investigates whether a single initial cyanobacterial microbiome, originating from a mixed community, retains its capacity for polyhydroxybutyrate (PHB) production under non-sterile long-term operation when subjected to cultivation conditions that promote different dominant cyanobacterial populations. Distinct selective pressures were applied in two photobioreactors: one subjected to high pH (>10) and nutrient limitation, which favored the dominance of Synechococcus sp., and one operating under controlled pH (8.0-8.25) and sufficient nutrient supplementation, which promoted the enrichment of Synechocystis sp. Despite these shifts in cyanobacterial population dominance, the microbiome retained the capacity to accumulate PHB over 31 days, indicating functional stability and potential functional redundancy within mixed cyanobacterial consortia. Differences in productivity between photobioreactors, with the Synechocystis sp.-dominated culture achieving a maximum PHB content of 36 %dcw compared to 17 %dcw in the Synechococcus sp.-dominated culture, were more likely associated with the distinct operational conditions imposed rather than with the dominant cyanobacterial population, though the contribution of population composition cannot be fully excluded. Daily acetate addition (100 mg·L-1) proved to be an effective supplementation strategy for sustaining PHB accumulation in both systems. These findings suggest that bioplastic production can be maintained despite changes in cyanobacterial population dominance within a shared microbiome, consistent with putative functional redundancy, and that operational parameters may be more decisive than species dominance in determining PHB productivity.