Elucidating biodiversity shifts in ballast water tanks during a cross-latitudinal transfer: complementary insights from molecular analyses

Abstract

In this study, the evolution of ballast water (BW) assemblages across different trophic levels was characterized over a21 day cross-latitudinal vessel transit using a combination ofmolecular methods. Triplicate BW samples were collected everysecond day and size-fractionated (<2.7, 10, >50 μm). Measure-ments of adenosine triphosphate (ATP) and metabarcoding ofenvironmental nucleic acid (DNA and RNA) analyses, comple-mented by microscopy and flow cytometry, were performed oneach sample. Measured ATP concentrations exhibited high variancebetween replicates and a strong negative trend in the large (≥50μm) fraction over the voyage. In concert with microscopy, themetabarcoding data indicated a die-off of larger metazoans duringthe first week of study and gradual reductions in dinoflagellates andochrophytes. The ATP and metabarcoding data signaled persistent or increased cellular activity of heterotrophic bacteria and protistsin the BW, which was supported by flow cytometry. The metabarcoding showed the presence of active bacteria in all size fractions,suggesting that the sequential filtration approach does not ensure taxonomical differentiation, which has implications for BW qualityassessment. Although our data show that ATP and metabarcoding have potential for indicative BW screening for BW compliancemonitoring, further research and technological development is needed to improve representativeness of sampling and deliver theunequivocal response criteria required by the international Ballast Water Management Convention.