Abstract
Ocean acidification (OA) can have adverse effects on marine calcifiers. Yet, phototrophic marine calcifiers elevate their external oxygen and pH microenvironment in daylight, through the uptake of dissolved inorganic carbon (DIC) by photosynthesis. We studied to which extent pH elevation within their microenvironments in daylight can counteract ambient seawater pH reductions, i.e. OA conditions. We measured the O2 and pH microenvironment of four photosymbiotic and two symbiont-free benthic tropical foraminiferal species at three different OA treatments (∼432, 1141 and 2151 µatm pCO2). The O2 concentration difference between the seawater and the test surface (ΔO2) was taken as a measure for the photosynthetic rate. Our results showed that O2 and pH levels were significantly higher on photosymbiotic foraminiferal surfaces in light than in dark conditions, and than on surfaces of symbiont-free foraminifera. Rates of photosynthesis at saturated light conditions did not change significantly between OA treatments (except in individuals that exhibited symbiont loss, i.e. bleaching, at elevated pCO2). The pH at the cell surface decreased during incubations at elevated pCO2, also during light incubations. Photosynthesis increased the surface pH but this increase was insufficient to compensate for ambient seawater pH decreases. We thus conclude that photosynthesis does only partly protect symbiont bearing foraminifera against OA.
Highlights
Ocean acidification has become a major threat to our world’s oceans [1]
The pH diffusive boundary layer (DBL) might form a shield around the organism protecting it from ocean acidification’ (OA). We studied whether this pH elevation within their microenvironment can protect photosymbiotic calcifiers from the effects of ocean acidification in daylight and lend additional resistance compared to non photosymbiotic calcifiers
DO2, DH+ and DCa2+ Dynamics To test whether OA induced increases of seawater dissolved inorganic carbon (DIC) enhance photosynthesis of photosymbiotic foraminifera and result in increased pH levels within their microenvironments, we conducted microenvironmental O2 and pH measurements of photosymbiotic and symbiont-free foraminifera
Summary
Ocean acidification has become a major threat to our world’s oceans [1]. From preindustrial times until today, atmospheric carbon dioxide (pCO2) concentrations increased from ,280 ppm to .390 ppm, and are predicated to rise to ,800 ppm by the end of this century under the IPCC business-as-usual emission scenario (WG 1, A2, [2]), which is likely to be exceeded [1,3]. Is CO2 a potent greenhouse gas in the atmosphere resulting in global warming, but about one third of the anthropogenic CO2 increase is taken up by the oceans [1,7]. This uptake reduces pH and consequent carbonate saturation state (V) of the ocean surface waters, a process generally termed as ‘ocean acidification’ (OA). By changes in ocean chemistry ocean acidification poses a direct threat to most calcifying organisms and the biological pumps [1,11,12]
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