Approaches for determining the effects of UV radiation on microorganisms in ballast water

To minimize the dispersal of aquatic nuisance species through shipping, ballast water can be treated to kill, remove, or inactivate organisms. Ultraviolet (UV) radiation is used in some ballast water management systems to address this goal. Because UV treatment renders cells nonviable (by sterilizing them, preventing reproduction) and does not necessarily kill organisms instantaneously, the efficacy of UV treatment has been verified historically by regrowth assays in which microorganisms are cultured (and thus, reproduce) under optimal growth conditions. Although regrowth assays are definitive, they are time consuming—lasting for days or weeks—and, importantly, are applicable only to organisms amenable to culturing. Furthermore, these estimates of cell concentrations are often accompanied by large error estimates. In this paper, several rapid alternatives to regrowth assays are described and evaluated. An ideal approach would shorten or simplify the analysis burden and, potentially, could be used for shipboard testing to determine compliance with national and international ballast water standards. Complicating this task is the requirement that compliance with the ballast water standards will be determined by quantifying the number of living organisms in ships’ ballast water, and while organisms may be living following UV treatment, they may not be viable (i.e., they may not reproduce). To address this dichotomy , alternatives to regrowth assays were categorized based upon the complexity of the analysis and the means used to determine the status of microorganisms (either as viable or living): 1. Instantaneous growth and cell replication, 2. Cell activity and metabolic rates, 3. Cell structural integrity, and 4. Biomolecule presence and status. With the suite of approaches currently available, it is not possible to determine the viability of organisms rapidly, that is, within minutes of collecting a ballast water sample. Measurements of the photosystem integrity via variable fluorescence and the presence of adenosine triphosphate (ATP) are currently the most promising for rapidly estimating concentrations of living cells in compliance testing of ballast water discharges; however, extensive validation is required to verify the applicability of these approaches for the complexity of real-world samples.