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Abstract

Eastern boundary upwelling systems (EBUSs) are among the most productive
marine environments in the world. The Canary Current upwelling system
off the coast of Mauritania and Morocco is the second most productive of
the four EBUS, where nutrient-rich waters fuel perennial phytoplankton
blooms, evident by high chlorophyll a concentrations off Cape Blanc,
Mauritania. High primary production leads to eutrophic waters in the
surface layers, whereas sinking phytoplankton debris and horizontally
dispersed particles form nepheloid layers (NLs) and hypoxic waters at
depth. We used Catalyzed Reporter Deposition Fluorescence In Situ
Hybridization (CARD-FISH) in combination with fatty acid (measured as
methyl ester; FAME) profiles to investigate the bacterial and archaeal
community composition along transects from neritic to pelagic waters
within the "giant Cape Blanc filament" in two consecutive years (2010
and 2011), and to evaluate the usage of FAME data for microbial
community studies. We also report the first fatty acid profile of
Pelagibacterales strain HTCC7211 which was used as a reference profile
for the SAR11 clade. Unexpectedly, the reference profile contained low
concentrations of long chain fatty acids 18:1 cis11, 18:1 cis11
11methyl, and 19:0 cyclo11-12 fatty acids, the main compounds in other
Alphaproteobacteria. Members of the free-living SAR11 clade were found
at increased relative abundance in the hypoxic waters in both years. In
contrast, the depth profiles of Gammaproteobacteria (including
Alteromonas and Pseudoalteromonas), Bacteroidetes, Roseobacter, and
Synechococcus showed high abundances of these groups in layers where
particle abundance was high, suggesting that particle attachment or
association is an important mechanisms of dispersal for these groups.
Collectively, our results highlight the influence of NLs, horizontal
particle transport, and low oxygen on the structure and dispersal of
microbial communities in upwelling systems.