Characterisation and comparison of bacterial communities on reverse osmosis membranes of a full-scale desalination plant by bacterial 16S rRNA gene metabarcoding

Microbiomes of full-scale seawater reverse osmosis membranes are complex and subject to variation within and between membrane units. The pre-existing bacterial communities of unused membranes before operation have been largely ignored in biofouling studies. This study is novel as unused membranes were used as a critical benchmark for comparison. Fouled seawater reverse osmosis membrane biofilm communities from an array of autopsied membrane samples, following a 7-year operational life-span in a full-scale desalination plant in Western Australia, were characterised by 16S rRNA gene metabarcoding using the bacterial primers 515F and 806R. Communities were then compared based on fouling severity and sampling location. Microbiomes of proteobacterial predominance were detected on control unused membranes. However, fouled membrane communities differed significantly from those on unused membranes, reflecting that operational conditions select specific bacteria on the membrane surface. On fouled membranes, Proteobacteria were also predominant but families differed from those on unused membranes, followed by Bacteriodetes and Firmicutes. Betaproteobacteria correlated with stable, mature and thick biofilms such as those in severely fouled membranes or samples from the feed end of the membrane unit, while Alpha and Gammaproteobacteria were predominantly found in biofilms on fouled but visually clean, and moderately fouled samples or those from reject ends of membrane units. Gammaproteobacteria predominated the thin, compact biofilms at the mid-feed end of membrane units. The study also supported the importance of Caulobacterales and glycosphingolipid-producing bacteria, namely Sphingomonadales, Rhizobiales and Sphingobacteriia, in primary attachment and biofilm recalcitrance. Nitrate-and-nitrite-reducing bacteria such as Rhizobiales, Burkholderiales and some Pseudomonadales were also prevalent across all fouled membranes and appeared to be critical for ecological balance and biofilm maturation. The diverse microbial populations on seawater desalination plant membranes have been characterised after full operational lifecycles. The membranes were used for seven years to purify water by reverse osmosis. Biofouling can seriously impair the efficiency of the membranes but the problem has not previously been well characterised, especially after a full life-span of membrane operation. Veena Nagaraj and colleagues at Murdoch University in Australia investigated biofilms and used genetic analysis to identify the bacteria growing on 14 used membranes, and compared the results with pre-existing contamination on unused membranes. The research revealed that operational conditions favour the growth of specific bacterial populations, predominantly Proteobacteria, but also Bacteriodetes and Firmicutes. The results should assist research to devise new methods to prevent and alleviate the biofouling of desalination plant membranes and maximise the efficiency of their operation.