Flow cytometry, microscopy, and DNA analysis as complementary phytoplankton screening methods in ballast water treatment studies

Abstract

Ballast water is the main vector for marine invasions. To minimize the spread of invasive species, the International Maritime Organization (IMO) has adopted the Ballast Water Management Convention which requires the installation of shipboard ballast water treatment systems (BWTS). During BWTS tests, the phytoplankton abundance and species composition were followed after treatment with both filtration and ultraviolet radiation. Although the installation fulfilled the IMO criteria after a 5-day holding time in a model ballast tank, the ultimate effectiveness of the treatment was further tested in long-term (20 days) incubation experiments under optimal phytoplankton growth conditions. Application of flow cytometry, microscopy, and DNA sequencing to these incubation samples gave an indication of the phytoplankton species that might be introduced by ballast water discharge—despite treatment. Phytoplankton was reliably quantified using flow cytometry, while fast identification was best done using microscopy. Some groups that contained potentially toxic species could not be identified at species level using microscopy; for these species, identification using genetic techniques was necessary. It is concluded that if long-term incubation experiments are used as an additional tool in testing BWTS effectiveness, a combination of phytoplankton screening methods can be applied depending on the detail of information that is required.