Heterotrophic prokaryote distribution along a 2300 km transect in the North Pacific subtropical gyre during a strong La Nina conditions: relationship between distribution and hydrological conditions
Résumé
The spatial distribution of heterotrophic prokaryotes
was investigated during the Tokyo–Palau cruise in the
western part of the North Pacific subtropical gyre (NPSG)
along a north–south transect between 33.60 and 13.25◦ N.
The cruise was conducted in three different hydrological areas
identified as the Kuroshio region, the subtropical gyre
area and the transition zone. Two eddies were crossed along
the transect: one cold-core cyclonic eddy and one warmcore
anticyclonic eddy and distributions of the heterotrophic
prokaryotes were recorded. By using analytical flow cytometry
and a nucleic acid staining protocol, heterotrophic
prokaryotes were discriminated 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
that LNA, mainly associated with low temperature and low
salinity, were dominant in all the hydrological regions. In
contrast, HNA distribution seemed to be associated with temperature,
salinity, Chl a and silicic acid. A latitudinal increase
in the HNA / LNA ratio was observed along the north–south
transect and was related to higher phosphate and nitrate concentrations.
However, the opposite relationship observed for
the VHNA / HNA ratio suggested that the link between nucleic
acid content and oligotrophic conditions is not linear,
underlying the complexity of the biodiversity in the VHNA,
HNA and LNA subgroups. In the Kuroshio Current, it is suggested
that the high concentration of heterotrophic prokaryotes
observed at station 4 was linked to the path of the cold
cyclonic eddy core. In contrast, it is thought that low concentrations
of heterotrophic prokaryotes in the warm core
of the anticyclonic gyre (Sta. 9) are related to the low nutrient
concentrations measured in the seawater column. Our
results showed that the high variability between the various
heterotrophic prokaryote cluster abundances depend both on
the mesoscale structures and the oligotrophic gradient.
Domaines
Océan, Atmosphère
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