Heterotrophic prokaryote distribution along a 2300 km transect in the North Pacific subtropical gyre during a strong La Ni?a conditions: relationship between distribution and hydrological conditions

摘要

The spatial distribution of heterotrophic prokaryotes was investigatedduring the Tokyo–Palau cruise in the western part of the North Pacificsubtropical gyre (NPSG) along a north–south transect between 33.60 and13.25° N. The cruise was conducted in three different hydrologicalareas identified as the Kuroshio region, the subtropical gyre area and thetransition zone. Two eddies were crossed along the transect: one cold-corecyclonic eddy and one warm-core anticyclonic eddy and distributions of theheterotrophic prokaryotes were recorded. By using analytical flow cytometryand a nucleic acid staining protocol, heterotrophic prokaryotes werediscriminated into three subgroups depending on their nucleic acid content(low, high and very high nucleic acid contents labelled LNA, HNA and VHNA,respectively). Statistical analyses performed on the data set showed thatLNA, mainly associated with low temperature and low salinity, were dominantin all the hydrological regions. In contrast, HNA distribution seemed to beassociated with temperature, salinity, Chl a and silicic acid. A latitudinalincrease in the HNA / LNA ratio was observed along the north–south transectand was related to higher phosphate and nitrate concentrations. However, theopposite relationship observed for the VHNA / HNA ratio suggested that thelink between nucleic acid content and oligotrophic conditions is not linear,underlying the complexity of the biodiversity in the VHNA, HNA and LNAsubgroups. In the Kuroshio Current, it is suggested that the highconcentration of heterotrophic prokaryotes observed at station 4 was linkedto the path of the cold cyclonic eddy core. In contrast, it is thought thatlow concentrations of heterotrophic prokaryotes in the warm core of theanticyclonic gyre (Sta. 9) are related to the low nutrient concentrationsmeasured in the seawater column. Our results showed that the highvariability between the various heterotrophic prokaryote cluster abundancesdepend both on the mesoscale structures and the oligotrophic gradient.