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Abstract

Ocean frontal systems are widespread hydrological features defining the
transition zone between distinct water masses. They are generally of
high biological importance as they are often associated with locally
enhanced primary production by phytoplankton. However, the composition
of bacterial communities in the frontal zone remains poorly understood.
In this study, we investigate how a coastal tidal front in Brittany
(France) structures the free-living bacterioplankton communities in a
spatio-temporal survey across four cruises, five stations and three
depths. We used 16S rRNA gene surveys to compare bacterial community
structures across 134 seawater samples and defined groups of co-varying
taxa (modules) exhibiting coherent ecological patterns across space and
time. We found that bacterial communities composition was strongly
associated with the biogeochemical characteristics of the different
water masses and that the front act as an ecological boundary for
free-living bacteria. Seasonal variations in primary producers and their
distribution in the water column appeared as the most salient parameters
controlling heterotrophic bacteria which dominated the free-living
community. Different dynamics of modules observed in this environment
were strongly consistent with a partitioning of heterotrophic
bacterioplankton in oligotroph and copiotroph ecological strategies.
Oligotroph taxa, dominated by SAR11 Clade members, were relatively more
abundant in low phytoplankton, high inorganic nutrients water masses,
while copiotrophs and particularly opportunist taxa such as
Tenacibaculum sp. or Pseudoalteromonas sp. reached their highest
abundances during the more productive period. Overall, this study shows
a remarkable coupling between bacterioplankton communities dynamics,
trophic strategies, and seasonal cycles in a complex coastal
environment.