Global comparison of the bacterial communities of bilge water, boat surfaces, and external port water.
In the past, ballast water has been a key vector in the ship-mediated dispersal of invasive species. Here, we evaluate the potential for port microorganisms to enter and colonize the hull and bilge water of ships. Due to the small size and ubiquitous nature of bacteria, they also have the potential to be spread through hull fouling and bilge water discharge. The goal of this study was to identify the extent to which the boat microbial community is shaped by the microbial community in the port water where the boat spends most of its time. Here, we compared the microbial communities of the hull and bilge compartments of 20 boats to those of the port water in 20 different ports in five regions around the world. We found that there was a significant difference in microbial diversity between boat and port microbial communities. Despite these differences, we found that Cyanobacteria were present at high abundances in the bilge water of most vessels. Due to the limited light in the bilge, the presence of Cyanobacteria suggests that port microorganisms can enter the bilge. Using source-tracking software, we found that, on average, 40% of the bilge and 52% of the hull microbial communities were derived from water. These findings suggest that the bilge of a vessel contains a diverse microbial community that is influenced by the port microbial community and has the potential to serve as an underappreciated vector for dispersal of life.