Size-fractionated phytoplankton carboxylase activities in the Indian sector of the Southern Ocean

Abstract

During the ANTARES 3 cruise in the Indian sector of the Southern Ocean in October-November 1995, the surface waters of Kerguelen Islands plume, and the surface and deeper waters (30-60 m) along a transect on 62°E from 48°36ЈS to the ice edge (58°50ЈS), were sampled. The phytoplankton community was size-fractionated (2 µm) and cell numbers, chlorophyll biomass and carbon assimilation, through Rubisco and ␤-carboxylase activities, were characterized. The highest contribution of <2 µm cells to total biomass and total Rubisco activity was reported in the waters of the Permanent Open Ocean Zone (POOZ) located between 52°S and 55°S along 62°E. In this zone, the picophytoplankton contributed from 26 to 50% of the total chlorophyll (a + b + c) with an average of 0.09 ± 0.02 µg Chl l -1 for <2 µm cells. Picophytoplankton also contributed 36 to 64% of the total Rubisco activity, with an average of 0.80 ± 0.30 mg C mg Chl a -1 h -1 for <2 µm cells. The picophytoplankton cells had a higher ␤-carboxylase activity than larger cells >2 µm. The mixotrophic capacity of these small cells is proposed. From sampling stations of the Kerguelen plume, a relationship was observed between the Rubisco activity per picophytoplankton cell and apparent cell size, which varied with the sampled water masses. Moreover, a depth-dependent photoperiodicity of Rubisco activity per cell for <2 µm phytoplankton was observed during the day/night cycle in the POOZ. In the near ice zone, a physiological change in picophytoplankton cells favouring phosphoenolpyruvate carboxykinase (PEPCK) activity was reported. A species succession, or an adaptation to unfavourable environmental conditions such as low temperature and/or available irradiance levels, may have provoked this change. The high contribution of picophytoplankton to the total biomass, and its high CO 2 fixation capacity via autotrophy and mixotrophy, emphasize the strong regeneration of organic materials in the euphotic layer in the Southern Ocean.

Inorganic carbon uptake and intracellular assimilation by subarctic Pacific phytoplankton assemblages

“…While our measured PEPC/Rubisco activity ratios are consistent with previous field studies (Bentaleb et al 1998;Fouilland et al 2000), the CO 2 dependence of PEPC activity and PEPC/Rubisco ratios we observed (Fig. 3) is opposite that reported by Reinfelder et al (2000).…”

“…To our knowledge, our study is the first to explicitly examine the relationship between CO 2 concentrations and PEPC activity in natural phytoplankton populations. While our measured PEPC/Rubisco activity ratios are consistent with previous field studies (Bentaleb et al 1998;Fouilland et al 2000), the CO 2 dependence of PEPC activity and PEPC/Rubisco ratios we observed (Fig. 3) is opposite that reported by Reinfelder et al (2000).…”

“…The C 4 mechanism does not require CA activity if HCO 2 3 is supplied to the cytoplasm via a direct transport system. Previous laboratory and field studies have demonstrated significant PEPC activity in a number of phytoplankton species and natural assemblages (Descolas-Gros and Oriol 1992; Bentaleb et al 1998;Fouilland et al 2000). This b carboxylation activity was attributed, in part, to the anaplerotic role of PEPC, which replenishes intermediates for the Krebs cycle, and/or to the presence of heterotrophic (dark) C fixation (see Morris 1980).…”

Combined pigment and metatranscriptomic analysis reveals highly synchronized diel patterns of phenotypic light response across domains in the open oligotrophic ocean

“…Other studies, measuring extracted Chl, also showed clear and reproducible nighttime maxima for both near surface waters (e.g., [9]) and culture experiments at high light intensities (e.g., [26][27][28][29]). Additionally, Fouilland et al [30] found a depth-dependence of diel variations in Rubisco activity and Chl concentration per cell further supporting a light regime-dependence of Chl profiles.…”

Assessment of bacterial dependence on marine primary production along a northern latitudinal gradient

“…Radioactivity was measured in a Beckman Coulter LS6500 liquid scintillation counter. No correction for dark 14 C uptake was performed, in order to assess the uptake of all autotrophically fixed CO 2 , as high anaplerotic C fixation by phytoplankton may occur in marine surface waters (Fouilland et al 2000;Fouilland et al 2001), and can be enhanced under sudden darkness (Mortain-Bertrand et al 1987). The dissolved inorganic carbon DIC concentrations measured in the samples (Charalampopoulou et al 2008; Tynan Eithne pers comm) were used to calculate the carbon production rates using an isotopic discrimination factor of 1.05.…”