Microbial dynamics in acetate-enriched ballast water at different temperatures

The spread of invasive species through ships' ballast water is considered as a major ecological threat to the world's oceans. For that reason, the International Maritime Organization (IMO) has set performance standards for ballast water discharge. Ballast water treatment systems have been developed that employ either UV-radiation or 'active substances' to reduce the concentration of living cells to below the IMOs standards. One such active substance is a chemical mixture known as Peraclean (R) Ocean. The residual of Peraclean (R) Ocean is acetate that might be present at high concentrations in discharged ballast water. In cold coastal waters the breakdown of acetate might be slow, causing a buildup of acetate concentrations in the water if regularly discharged by ships. To study the potential environmental impact, microbial dynamics and acetate degradation were measured in discharge water from a Peraclean (R) Ocean treatment system in illuminated microcosms. In addition, microbial dynamics and acetate degradation were studied at 1, 4, 10, 15 and 25 degrees C in dark microcosms that simulated enclosed ballast water tanks.<br>Acetate breakdown indeed occurred faster at higher temperatures. At 25 degrees C the highest bacteria growth, fastest nutrient and oxygen consumption and highest DOC reduction occurred. On the other hand, at -1 degrees C bacterial growth was strongly delayed, only starting to increase after 12 days. Furthermore, at 25 degrees C the acetate pool was not depleted, probably due to nutrient and oxygen limitation. This means that not all acetate will be broken down in ballast water tanks, even during long voyages in warm waters. In addition, at low temperatures acetate breakdown in ballast water tanks and in discharged water will be extremely slow. Therefore, regular discharge of acetate enriched ballast water in harbors and bays may cause eutrophication and changes in the microbial community, especially in colder regions