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Ballast tank biofilms pose an additional risk of microbial invasion if sloughed off during ballasting operations, yet their significance and invasion biology is poorly understood. In this study, biofilms developed in marine and freshwater locations were exposed to prolonged darkness and aging by mimicking ballast water conditions in the laboratory. Upon prolonged darkness, the decay of phytoplankton, as indicated by the decrease in chlorophyll a in marine biofilms, led to remineralization and enhanced bacterial and protist populations. However, the same trend was not observed in the case of freshwater biofilms wherein the microbial parameters (i.e., bacteria, protists) and chlorophyll a decreased drastically. The bacterial community structure in such conditions was evaluated by real-time quantitative PCR (qPCR), and results showed that the biofilm bacterial communities changed significantly over a period of time. α-Proteobacteria was the most stable taxonomic group in the marine biofilms under dark conditions. However, β-proteobacteria dominated the freshwater biofilms and seemed to play an important role in organic matter remineralization. γ-Proteobacteria, which includes most of the pathogenic genera, were affected significantly and decreased in both the types of biofilms. This study revealed that marine biofilm communities were able to adapt better to the dark conditions while freshwater biofilm communities collapsed. Adaptation of tolerant bacterial communities, regeneration of nutrients via cell lysis, and presence of grazers appeared to be key factors for survival upon prolonged darkness. However, the fate of biofilm communities upon discharge in the new environment and their invasion potential is an important topic for future investigation.

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