Carbon and phosphate incorporation rates of microbial assemblages in contrasting environments in the Southeast Pacific

Abstract

Size-fractionated chlorophyll a (chl a) was determined along with carbon (C) and phosphate (P) incorporation rates over a west-east transect in the Southeast Pacific (between 146.36°W and 72.49°W). A clear longitudinal gradient was observed for both chl a and C and P incorporation rates from the productive areas, near the Marquesas Islands and the Chilean upwelling, to the hyperoligotrophic area associated with the central part of the Southeast Pacific Gyre (SPG). The 0.2-0.6 µm fraction represented 15 to 43% (29 ± 7% mean ± SD, n = 24) of the P incorporation rate along the transect, suggesting that heterotrophic bacteria were important factors in the P cycle as dissolved inorganic P consumers. Small-sized cells (< 2 µm) formed the bulk of chl a biomass (69 ± 4%) and accounted for 49 ± 7% of total primary production in the SPG. In contrast, large phytoplankton (> 2 µm) accounted for a significantly larger fraction of the total primary production, despite having lower chl a biomass and P incorporation rates. Consequently, larger cells presented higher photosynthetic efficiency values than the smaller phytoplankton. The C:P incorporation rate ratios exhibited increasing deviation from the canonical stoichiometric ratios of 106 and 50 (for phytoplankton and heterotrophic bacteria, respectively) towards the centre of the SPG. We propose hypotheses to explain these deviations.

Evidence for Coupling of the Carbon and Phosphorus Biogeochemical Cycles in Freshwater Microbial Communities

“…Free phosphate is typically assimilated much more rapidly by bacteria than heterotrophic eukaryotic microbes and bacteria may be one of the more important contributors to the phosphorous cycle in marine ecosystems (Duhamel and Moutin, 2009;Popendorf and Duhamel, 2015). Moreover, among lake microplankton, bacteria may account for as much as 94% of the APA (Chróst et al, 1984).…”

“…Although there is substantial published research on the role of organic C in microbial community dynamics, there is less research on the coupling of organic C with the phosphorus cycle, including production of microbial phosphatases, a potentially significant source of phosphate remineralization, especially in aquatic environments (Gibson et al, 1996;Dyhrman et al, 2007;Duhamel and Moutin, 2009;Ruttenberg and Dyhrman, 2012;Duhamel et al, 2014). Because of the potential threats from global warming, considerable research has been done on the coupling of nitrogen and carbon cycles, largely in relation to the effect of nitrogen on primary production, thus enhancing CO 2 sequestration (Esser et al, 2011;Zaehle, 2013).…”

Seasonal phosphorus and carbon dynamics in a temperate shelf sea (Celtic Sea)

“…Previous studies of planktonic P i -uptake have differentiated bacterial and algal P-uptake using different pore-sized filters, for example considering bacterial uptake as from cells less than 0.6 µm and algal uptake from cells greater than 0.6 µm (e.g., Duhamel and Moutin, 2009). However, both bacterial and algal cell sizes are variable with taxonomy and physiological status (ref) and may overlap in sizedistribution; for example, the cyanobacteria Synechococcus, which is dominant in the Celtic Sea in summer (Hickman et al, 2012), may range in size from 0.4 to 0.8 µm (ref).…”

“…The low NPP:P i -uptake ratios (P-rich) in July are not associated with rapid phytoplankton growth (Table 3), but rather with high bacterial growth rates and a stronger bacterial influence on C:P uptake (and retention). Heterotrophic bacteria are recognised as strong competitors for P i under nutrient depleted conditions (Thingstad et al, 1993(Thingstad et al, , 1996Duhamel and Moutin, 2009). Whilst phytoplankton cellular C:P stoichiometry is near, or slightly lower, than the canconical Redfield ratio (Geider and LaRoche, 2002;Ho et al, 2003), bacterial cellular C:P ratios are significantly more P-rich (e.g., ~50; Fagerbakke et al, 1996;Sterner and Elser, 2002;Hessen et al, 2004;Duhamel and Moutin, 2009; see also Scott et al, 2012).…”

“…Planktonic micro-organisms, such as heterotrophic bacteria and phytoplankton, have rapid growth rates and hence can exert a strong influence on the turnover of different C and P pools (Arrigo, 2005). Both phytoplankton and heterotrophic bacteria consume P, though the two have contradictory roles in the marine C-cycle as primary producers and remineralisers of organic material, respectively (Duhamel and Moutin, 2009), and compete strongly for available P (Thingstad et al, 1993(Thingstad et al, , 1996.…”