Effect of UV Irradiation on Viability of Micro Scale and Resistant Forms of Marine Organisms: Implications for the Treatment of Ships’ Ballast Water
D. Oemcke, N. Parker, and D. Mountfort
The efficacy of UV in killing zoospores of the seaweed, Undaria pinnatifida, a marine bacterium (Vibrio alginolyticus), and cysts of Gymnodinium catenatum have been determined in two laboratories. In experiments performed in New Zealand employing a flowthrough UV reactor in which the lamp was positioned centrally along the length of the reactor system between the inflow and outflow, plots of log of viability versus dose (dose range, 50-350 mWscm-2) were negative log linear with a maximum kill of 40% achieved in the case of G. catenatum cysts. Over a lower dose range (10-65 mWscm-2), negative log linear responses were also obtained with cultures of Vibrio alginolyticus and with zoospores of U. pinnatifida, in which kills of 99% (zoospores) and 99.9% (V. alginolyticus) were achieved at doses of 60 and 37.5 mWs cm-2 respectively. Investigations in Australia on the effects of UV irradiation on cysts of G. catenatum using collimating beam and continuous flow apparatus and semi-quantitative and quantitative methods to assess kill confirmed the above results by showing survival (maximum kill, 70%) as determined by hatching of planomeicytes even after irradiations up to 1,600 mWscm-2. This contrasts with the findings with gametes of the same organism which showed that doses of 110-220 mWscm-2 were sufficient to produce kills of > 99%. Lower doses (45 mWscm-2) could produce the same effect if cells were stored for a period of time in the dark after treatment or heated to 23oC. The results of these studies demonstrate that while UV irradiation may be an effective option for the treatment of bacteria, the asexual forms of micro-algae, and the motile forms of macro-algae it is unlikely to be effective in the treatment of dinoflagellate cysts. On this basis our recommendation would be a dose of 60 to 120 mWscm-2 (excluding cysts) for UV treatment ballast water. Because the dose range would be ineffective against the resistant forms of microscale organisms, and larger macro-ballast organisms, these organisms would need to be removed via an effective primary treatment. To date no method has been demonstrated to be effective in this respect.