Calcification acidifies the microenvironment of a benthic foraminifer (Ammonia sp.)

Abstract

Calcareous foraminifera are well known for their CaCO 3 shells. Yet, CaCO 3 precipitation acidifies the calcifying fluid. Calcification without pH regulation would therefore rapidly create a negative feedback for CaCO 3 precipitation. In unicellular organisms, like foraminifera, an effective mechanism to counteract this acidification could be the externalization of H + from the site of calcification. In this study we show that a benthic symbiont-free foraminifer Ammonia sp. actively decreases pH within its extracellular microenvironment only while precipitating calcite. During chamber formation events the strongest pH decreases occurred in the vicinity of a newly forming chamber (range of gradient~100 μm) with a recorded minimum of 6.31 (b 10 μm from the shell) and a maximum duration of 7 h. The acidification was actively regulated by the foraminifera and correlated with shell diameters, indicating that the amount of protons removed during calcification is directly related to the volume of calcite precipitated. The here presented findings imply that H + expulsion as a result of calcification may be a wider strategy for maintaining pH homeostasis in unicellular calcifying organisms.

Biomineralization in perforate foraminifera

“…Also in the planktonic species G. hirsuta and G. truncatulinoides, calcification proceeds adjacent to a cytoplasmatic envelope (or outer protective envelope) that may play a role in maintaining SOC integrity and shape, and promoting initial calcification (Bé et al, 1979). In the benthic Ammonia sp., a pH gradient of N2 pH units is observed across several μm distance and is maintained for hours between the site of calcification (De Nooijer et al, 2009a) and the specimen's microenvironment (Glas et al, 2012). These observations suggest that in Ammonia sp., the SOC is separated from the outside environment.…”

“…Transmembrane channels simply allow facilitated diffusion along electrochemical gradients by creating a selective pore through the cell membrane. For the uptake of inorganic carbon by foraminifera during calcification, a strong pH gradient (high inside; Bentov et al, 2009;De Nooijer et al, 2009a; low outside; Glas et al, 2012) may promote the influx of CO 2 and thus circumvent the need for specialized transmembrane proteins.…”

“…The efflux of H + during calcification (Glas et al, 2012) may therefore help to constrain estimates of calcium pumping rates during calcification. Carbon dioxide uptake and proton efflux are also directly related in cyanobacteria (Ogawa and Kaplan, 1987).…”

“…The concentration of CO 2 at the site of calcification or in internal reservoirs is determined by pH. Since foraminifera can control the pH in these compartments (Erez, 2003;Bentov et al, 2009;De Nooijer et al, 2009a;Glas et al, 2012), they can produce large CO 2 concentration gradients and hence promote the influx of DIC to the sites of calcification. The flux of ions can also be calculated from calcification rates, which is discussed in Section 4.…”

Reconsidering the role of carbonate ion concentration in calcification by marine organisms

“…Here, organisms do not exclusively rely on direct inorganic carbon utilization from seawater but supplement calcification to a variable degree with CO 2 gained intracellularly from respired biomass. This CO 2 utilization may be further strengthened (1) when metabolic CO 2 is "trapped" inside the organisms through the establishment of pH gradients, which limit the diffusive loss of CO 2 (Bentov et al, 2009, Glas et al, 2012b, or (2) when CO 2 is transported actively towards the site of calcification (de Nooijer et al, 2014). Thus, CO 2 reacting with H 2 O to form HCO − 3 and H + (catalyzed by the ubiquitous enzyme carbonic anhydrase) could be an alternative inorganic carbon source for calcification in particular taxa.…”