Bacterial carbon dependence on freshly produced phytoplankton exudates under different nutrient availability and grazing pressure conditions in coastal marine waters

Abstract

The effects of grazing pressure and inorganic nutrient availability on the direct carbon transfer from freshly produced phytoplankton exudates to heterotrophic bacteria biomass production were studied in Mediterranean coastal waters. The short-term incorporation of ¹³C (H¹³CO₃) in phytoplankton and bacterial lipid biomarkers was measured as well as the total bacterial carbon production (BP), viral lysis and the microbial community structure under three experimental conditions: (1) High inorganic Nutrient and High Grazing (HN + HG), (2) High inorganic Nutrient and Low Grazing (HN + LG) and (3) under natural in situ conditions with Low inorganic Nutrient and High Grazing (LN + HG) during spring. Under phytoplankton bloom conditions (HN + LG), the bacterial use of freshly produced phytoplankton exudates as a source of carbon, estimated from ¹³C enrichment of bacterial lipids, contributed more than half of the total bacterial production. However, under conditions of high grazing pressure on phytoplankton with or without the addition of inorganic nutrients (HN + HG and LN + HG), the ¹³C enrichment of bacterial lipids was low compared with the high total bacterial production. BP therefore seems to depend mainly on freshly produced phytoplankton exudates during the early phase of phytoplankton bloom period. However, BP seems mainly relying on recycled carbon from viral lysis and predators under high grazing pressure.

Seasonal and interannual variability of the marine bacterioplankton community throughout the water column over ten years

“…Chlorophyll a concentrations likely relate to community structure because bacteria consume phytoplankton exudates (Obernosterer and Herndl, 1995;Fouilland et al, 2014), live symbiotically with phytoplankton (Caron, 2000;Aota and Nakajima, 2001) and/or respond to unmeasured variable(s) along with phytoplankton (Steele et al, 2011;Chow et al, 2014). Community structure may relate to leucine incorporation because some OTUs are more metabolically active than others.…”

Local differences in phytoplankton-bacterioplankton coupling in the coastal upwelling off Galicia (NW Spain)

“…The effect of temperature and inorganic nutrient availability on BP does not seem relevant in our area of study, as no significant correlations were found between BP and inorganic nutrient concentrations or temperature (data not shown). A recent study by Fouilland et al (2014) demonstrated that a strong direct C coupling between bacteria and phytoplankton only occurs under early phytoplankton bloom conditions, when nutrient availability is very high and grazing pressure on microorganisms is very low. In contrast, an uncoupling between phytoplankton exudates and BP can be observed under strong grazing pressure.…”

“…A recent review by Fouilland & Mostajir (2010) clearly questions the immediate and direct dependency of bacteria on phytoplankton exudates for carbon acquisition in aquatic systems, based on the relatively small fraction of bacterial carbon demand (BCD) met by concurrent dissolved primary production. According to Fouilland et al (2014) such a lack of direct coupling would occur under strong grazing pressure conditions, as bacteria would primarily consume indirectly released DOC.…”

Virus-mediated transfer of nitrogen from heterotrophic bacteria to phytoplankton

“…When the C : N of DOM is low relative to bacterial nutritional requirements, bacteria deaminate DOM and release ammonium (Hollibaugh, 1978;Goldman et al, 1987) to acquire carbon for energy and growth (e.g. Brussaard et al, 1996;Fouilland et al, 2014); the release of ammonium can support phytoplankton growth (Haaber and Middelboe, 2009;Weinbauer et al, 2011;Shelford et al, 2012).…”