Bacterioplankton community composition and activity in the Atlantic Ocean

Abstract

Temporal and spatial patterns of bacterioplankton in six different provinces of the Atlantic
Ocean were examined between 1996 and 2004. The abundance and integrated biomass
of three prokaryote groups (Prochlorococcus spp., Synechococcus spp. and heterotrophic
bacteria) were used to detect standing stock changes and characterise community
structure in the Northern and Southern oligotrophic gyres and in the equatorial region.
There was no statistically significant inter-annual variability in Prochlorococcus or
Synechococcus abundance or integrated biomass in any of the provinces. The
abundance and biomass of the remaining prokaryoplankton was variable but this variation
could not be ascribed to seasonal differences and did not follow a clear inter-annual
trend.
The importance of the microbial loop in recycling organic nutrients in the upper Atlantic
Ocean was also studied by comparing ratios of bacterial to primary production in different
oceanic provinces. A proportionately higher rate of photosynthetically fixed carbon flowed
through the microbial loop in the Northern oligotrophic gyre (22 – 55 %) compared to the
other provinces studied. This indicates a difference in energy flow through the ecosystem
in different oceanic regions with a greater emphasis on energy flow through the microbial
loop in the Northern oligotrophic gyre probably due to reduced grazing of phytoplankton
and reduced export production compared to other Atlantic Ocean provinces.
The role of defined groups of bacteria in the cycling of nutrients was identified using a
combination of flow cytometric sorting with radiotracer uptake and CARD-FISH. The
SAR11 clade of bacteria were found to dominate the low nucleic acid group of
bacterioplankton and as such it was possible to quantify the activity and abundance of
these cells in the Atlantic Ocean. Despite their small genome size, SAR11 bacteria were
found to be generally as active as an average bacterioplankton cell and were responsible
for between 30 and 50 % of the total community methionine uptake.
This research has characterised bacterioplankton composition and activity in Atlantic
Ocean provinces thus enabling further understanding of the function and importance of
the microbial loop in the upper ocean.

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