Ultraviolet radiation as a ballast water treatment strategy: Inactivation of phytoplankton measured with flow cytometry.

This study investigates different UV doses (mJ/cm(2)) and the effect of dark incubation on the survival of the algae Tetraselmis suecica, to simulate ballast water treatment and subsequent transport. Samples were UV irradiated and analyzed by flow cytometry and standard culturing methods. Doses of ≥400 mJ/cm(2) rendered inactivation after 1 day as measured by all analytical methods, and are recommended for ballast water treatment if immediate impairment is required. Irradiation with lower UV doses (100-200 mJ/cm(2)) gave considerable differences of inactivation between experiments and analytical methods. Nevertheless, inactivation increased with increasing doses and incubation time. We argue that UV doses ≥100 mJ/cm(2) and ≤200 mJ/cm(2) can be sufficient if the water is treated at intake and left in dark ballast tanks. The variable results demonstrate the challenge of giving unambiguous recommendations on duration of dark incubation needed for inactivation when algae are treated with low UV doses.

Flow cytometric applicability to evaluate UV inactivation of phytoplankton in marine water samples.

Disinfection of microbes is of importance to prevent the spread of pathogens and non-indigenous species in the environment. Here we test the applicability of using flow cytometry (FCM) to evaluate inactivation of the phytoplankter Tetraselmis suecica after UV irradiation and labeling with the esterase substrate 5-carboxyfluorescein diacetate acetoxymethyl ester (CFDA-AM). Non-irradiated and UV irradiated samples were analyzed with the plate count technique and FCM for 24 days. The numbers of colony forming units were used as a standard to develop a FCM protocol. Our protocol readily distinguishes live and dead cells, but challenges were encountered when determining whether UV damaged cells are dying or repairable. As damaged cells can represent a risk to aquatic organisms and/or humans, this was taken into account when developing the FCM protocol. In spite of the above mentioned challenges we argue that FCM represents an accurate and rapid method to analyze T. suecica samples.