Enumerating viable phytoplankton using a culture-based Most Probable Number assay following ultraviolet-C treatment.

Ballast water management systems (BWMS) must be tested to assess their compliance with standards for the discharge of organisms, for example in the ≥ 10- and < 50-μm size category, which is dominated by phytoplankton. Assessment of BWMS performance with the vital stains fluorescein diacetate + 5-chlorofluorescein diacetate, required by regulations in the USA, is problematic in the case of ultraviolet-C (UVC) radiation. This is because UVC targets nucleotides-and thus reproduction, hence viability-rather than membrane integrity, which is assayed by the stains.