Extra‐ and intra‐cellular carbonic anhydrase in relation to culture age in a high‐calcifying strain of Emiliania huxleyi Lohmann

Abstract

SL'MMARYThe relationships between extra-and intra-cellular carbonic anhydrase, calcification rate, utilization of dissolved inorganic carbon (DIC) and culture age were investigated in a high-calcifying strain oi Emiliania huxleyi. The detection of carbonic anhydrase was dependent on culture age; neither the extra-ceilular acti\ ity of intact cells nor activity in crude homogenates was detected until the stationary phase. By the stationary phase DIC in the medium was totally depleted and the calcification rate had decreased by 60 "o. Extra-and intra-cellular carbonic anhydrase was stimulated by Na* but not C] ions. The isolation of intact organelles by isopycnic gradient centrifugation from exponential and stationary phase cuUures of high calcifying cells showed high carbonic anhydrase activity in the chloroplast fraction but carbonic anhydrase was not detected in low-calcifying cells even after gradient centrifugation. Although 50 //,M ethoxyzolamide was an effective inhibitor of carbonic anhydrase activity in vitro, photosynthetic ^*C()^ fixation was only inhibited 30"^, calcification rate lO";, and tbe internal inorganic carbon pool unaffected in intact cells. It is proposed that chloroplast carbonic anhydrase maintains the steady state flux of CO.J from the chloroplast envelope to Rubisco, much of the CO^ arising by reWase from HCOj" in the cytosol, required to maintain cytoplasmic pH near neutrality in high-calcifying cells.

Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels

“…We thus hypothesize that this stimulation of carbonic anhydrase activity may have been an attempt by the algae to compensate for decreased calciWcation/increased dissolution under high CO 2 concentrations. Carbonic anhydrase has been shown to play a role in the calciWcation process of many organisms, particularly corals, by regulating the internal and external cellular speciation of dissolved inorganic carbon (Kingsley and Watabe 1987;Nimer et al 1994;Al-Horani et al 2003;Rahman et al 2007;Tambutté et al 2007). Under high CO 2 concentrations, the following mechanism of external calciWcation in C. oYcinalis could be possible (originally proposed by Tambutté et al 2007 In the Wrst reaction, carbonic anhydrase catalyzes the conversion of CO 2 into HCO 3 ¡ and then CO 3…”

Light‐dependent carbon isotope fractionation in the coccolithophorid Emiliania huxleyi

“…Since our data show significant deviations from these predictions, an exclusive CO 2 uptake by diffusion is not indicated for E. huxleyi. In fact, increasing isotope fractionation with increasing growth rates requires active carbon uptake, suggesting the operation of a carbon concentrating mechanism (CCM) in E. huxleyi (Nimer and Merret 1996;Laws et al 1998). Light has been suggested to influence the CCM of microalgae (e.g., Badger and Price 1992;Sültemeyer et al 1993).…”

Zinc‐bicarbonate colimitation of Emiliania huxleyi

“…1, in which Zn 2ϩ acts as the cofactor of carbonic anhydrase. In this case Zn 2ϩ increases the efficiency with which HCO is converted to CO 2 , which toplasmatic pH, from which it was concluded that the intracellular speciation of DIC acts as a pH buffer (Nimer et al 1994a). Carbonic anhydrase would be needed to keep this pH buffer functioning at low HCO con-…”

“…The pH in the chloroplast was measured as 7.9 Ϯ 0.6 (Anning et al 1996). At present it is unclear whether E. huxleyi has a carbon concentrating mechanism (Sekino and Shiraiwa 1994) or not (Nimer et al 1994b) Filled circles represent experimental data points and are identical in the three panels. The data points of Fig.…”

“…was found in the cytoplasm (Nimer et al 1994b), it has been found that a carbon concentrating mechanism can have carbonic anhydrase-like properties, and a putative new type of carbonic anhydrase in Synechococcus that is directly linked to a HCO transporter has been iden-…”