Growth and specific P-uptake rates of bacterial and phytoplanktonic communities in the Southeast Pacific (BIOSOPE cruise)

International audiencePredicting heterotrophic bacteria and phytoplankton specific growth rates (µ ) is of great scientific interest. Many methods have been developed in order to assess bacterial or phytoplankton µ. One widely used method is to estimate µ from data obtained on biomass or cell abundance and rates of biomass or cell production. According to Kirchman (2002), the most appropriate approach for estimating µ is simply to divide the production rate by the biomass or cell abundance estimate. Most methods using this approach to estimate µ are based on carbon (C) incorporation rates and C biomass measurements. Nevertheless it is also possible to estimate µ using phosphate (P) data. We showed that particulate phosphate (PartP) can be used to estimate biomass and that the P uptake rate to PartP ratio can be employed to assess µ. Contrary to other methods using C, this estimator does not need conversion factors and provides an evaluation of µ for both autotrophic and heterotrophic organisms. We report values of P-based µ in three size fractions (0.2–0.6; 0.6–2 and >2 µm) along a Southeast Pacific transect, over a wide range of P-replete trophic status. P-based µ values were higher in the 0.6–2 µm fraction than in the >2 µm fraction, suggesting that picoplankton-sized cells grew faster than the larger cells, whatever the trophic regime encountered. Picoplankton-sized cells grew significantly faster in the deep chlorophyll maximum layer than in the upper part of the photic zone in the oligotrophic gyre area, suggesting that picoplankton might outcompete >2 µm cells in this particular high-nutrient, low-light environment. P-based µ attributed to free-living bacteria (0.2-0.6 µm) and picoplankton (0.6–2 µm) size-fractions were relatively low (0.11±0.07 d-1 and 0.14±0.04 d-1, respectively) in the Southeast Pacific gyre, suggesting that the microbial community turns over very slowly