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. The Serial Dilution Culture-Most Probable Number (SDC-MPN) method, long used to enumerate fragile phytoplankton from natural communities, is appropriate for counting viable phytoplankton. We developed QA/QC “best practice” criteria for its application as a robust and repeatable assay of viable cells in cultures of phytoplankton before and after experimental treatment, then constructed dose-response curves for UVC-induced loss of viable cells in 12 species of phytoplankton from seven divisions. Sensitivity to UVC, expressed as the dose required to reduce viability by 99%—the criterion for type approval of treatment systems—varied more than 10-fold and was not correlated with cell size. The form of the dose-response curves varied between taxa, with most having a threshold dose below which there was no reduction in viability. Analysis of the patterns of growth indicates that if recovery from treatment occurred, it was complete in 1 or 2 days in > 80% of cases, long before the assays were terminated. We conclude that the SDC-MPN assay as described is robust and adaptable for use on natural phytoplankton.

The effects of freezing, drying, ultraviolet irradiation (UV), chlorine, and a quaternary ammonium compound on the infectivity of the myxospore stage of Myxobolus cerebralis (the causative agent of whirling disease) for Tubifex tubifex were examined in a series of laboratory trials. Freezing at either -20 degrees C or -80 degrees C for a period of 7 d or 2 months eliminated infectivity as assessed by the absence of production of the actinospore stage (triactinomyxons [TAMs]) from T. tubifex cultures inoculated with treated myxospores over a 4-5-month period. Myxospores retained infectivity when held in well water at 5 degrees C or 22 degrees C for 7 d and when held at 4 degrees C or 10 degrees C d for 2 months. In contrast, no TAMs were produced from T. tubifex cultures inoculated with myxospores held at 20 degrees C for 2 months. Drying of myxospores eliminated any evidence of infectivity for T. tubifex. Doses of UV from 40 to 480 mJ/cm2 were all effective for inactivating myxospores of M. cerebralis, although a few TAMs were detected in one replicate T. tubifex culture at 240 mJ/cm2 and in one replicate culture at 480 mJ/cm2. Treatments of myxospores with chlorine bleach at active concentrations of at least 500 mg/L for 15 min largely inactivated myxospore infectivity for T. tubifex. Likewise, there was no evidence of TAMs produced by T. tubifex inoculated with myxospores treated with alkyl dimethyl benzyl ammonium chloride (ADBAC) at 1,500 mg/L for 10 min. Treatments of myxospores with 1,000-mg/L ADBAC for 10 min reduced TAM production in T. tubifex cultures sevenfold relative to that in cultures inoculated with an equal number of untreated myxospores. These results indicate that myxospores of M. cerebralis demonstrate a selective rather than broad resistance to selected physical and chemical treatments, and this selective resistance is consistent with conditions that myxospores are likely to experience in nature.

Disinfection of SARS-CoV-2 is of particular importance due to the global COVID-19 pandemic. UV-C irradiation is a compelling disinfection technique because it can be applied to surfaces, air, and water and is commonly used in drinking water and wastewater treatment facilities.

The ability of a range of doses of ultraviolet irradiation (UV) to inactivate the waterborne actinospore or triactinomyxon stages (TAMs) of Myxobolus cerebralis was evaluated by infectivity for juvenile rainbow trout Oncorhynchus mykiss. TAMs were UV-irradiated using a low pressure mercury vapour lamp collimated beam apparatus. All doses 40, 80, 120 and 160 mJ cm -2 were found to completely inactivate the TAMs as demonstrated by the absence of microscopic lesions, myxospores and parasite DNA detected by quantitative PCR (qPCR) among rainbow trout 5 mo post-exposure. In contrast, rainbow trout receiving the same concentrations of untreated TAMs (1000 fish

To mitigate the potential spread of invasive species and pathogens, the International Maritime Organization and U.S. Coast Guard (USCG) adopted discharge performance standards for ballast water management that will take full effect in 2017. The USCG requires staining methods for enumerating ballast water treatment system (BWTS) efficacy. These stains do not detect DNA damage, the primary mechanism for ultraviolet (UV) disinfection, and neglect potential DNA repair after discharge. These factors necessitate investigation of enumeration methods for accurate validation and approval of UV-based BWTSs. To molecularly assess DNA damage and repair kinetics, UV-induced DNA lesions were quantified by an enzyme-linked immunosorbent assay in Tetraselmis suecica control and UV-treated samples that were cultured and tested over time. Most DNA repair occurred within 6 h, was essentially complete within 24 h, and was insensitive to light or nutrient conditions during incubation. Asymptotic repair kinetics indicated a maximum of 67% of DNA damage inflicted by 300 mJ/cm2 was repairable. These data provide a novel UV dose response for DNA damage in T. suecica and indicate that enzymatic DNA repair kinetics are not affected by culture conditions. Because DNA is rapidly photorepaired, culture-based enumeration can be used to accurately validate UV BWTSs.