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

Abstract

Abstract. Predicting 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 Correspondence to: S. Duhamel (solange.duhamel@univmed.fr) 0.14±0.04 d −1 , respectively) in the Southeast Pacific gyre, suggesting that the microbial community turns over very slowly.

Accumulation of DOC in Low Phosphate Low Chlorophyll (LPLC) area: is it related to higher production under high N:P ratio?

“…5a and 5b), our size assumption combined with the cellular abundance selected for our simulations seems to give a satisfying first approximation of the expected nutrient fluxes for the western basin at least. Concerning cellular growth rate, values are in the range of reported values for osmotrophs in the oceans (Duhamel et al, 2007). Although the estimation of osmotrophs growth rate in oceanic environment is still subject to debate, it has been suggested that phytoplankton growth rate are well below their maximum growth rates and that values below 0.5 d −1 are consistent with the existing knowledge on the relationship between elemental composition and physiological state in phytoplankton (Maranon, 2005).…”

Small pigmented eukaryotes play a major role in carbon cycling in the P‐depleted western subtropical North Atlantic, which may be supported by mixotrophy

“…Indeed, on a per cell level, P‐Euk presented CO 2 fixation rates roughly an order of magnitude higher than picocyanobacteria and this relationship held after normalizing to biovolume, although results strongly depended on cell size estimates. Actually, data from the literature vary widely, and cell size estimates obtained using flow cytometry are to be taken with caution (Duhamel et al ; Tzur et al ). Using calibration beads, our estimates indicate that the populations of protists sorted here were smaller than 5 μ m (Supporting Information Fig.…”

“…The isometric scaling of phytoplankton photosynthesis with cell size was first demonstrated using size fractionated measurements of CO 2 fixation rates (Marañón et al ; Duhamel and Moutin ) and later verified using measurements in flow‐cytometry–sorted groups of phytoplankton similar to this study (Jardillier et al ; Grob et al ; Rii et al ). Calculated biomass‐normalized rates of CO 2 fixation, a proxy for growth rates (Marañón ; Duhamel et al ), using conversion factors in Hartmann et al (), were 0.56 ± 0.11, 0.78 ± 0.27, 0.58 ± 0.06, and 0.44 ± 0.08 d −1 for Prochlorococcus , Synechococcus , P‐Euk_A, and P‐Euk_B, respectively. Although comparable to values previously reported (Jardillier et al ; Rii et al ), these estimates should be taken with caution as they are highly dependent on volume‐specific C conversion factors and equations that are used to calculate biovolume and C content (Duhamel et al ), particularly considering the much wider size range for P‐Euk compared to picocyanobacteria.…”

Microbial response to enhanced phosphorus cycling in the North Pacific Subtropical Gyre

“…Growth rates calculated based on changes in average partP concentration (i.e. in P biomass) between T0 and T144 were 0.02, 0.20 and 0.16 d −1 in the control, NP32 and NP50 treatments, respectively, while growth rates based on average specific Pi uptake rate (ratio of Pi uptake rate to partP; Duhamel et al 2007) at T144 were 0.14 ± 0.03, 2.11 ± 0.53 and 2.94 ± 0.85 d −1 in the control, NP32 and NP50 treatments, respectively. The higher growth rates based on specific Pi uptake rates compared to changes in P biomass (or cell abundance) suggests that a large fraction of the Pi taken up by the cells was not transformed into biomass and must have been excreted as both Pi and DOP.…”