dissociation constants favor bicarbonate, both raising the [H+] (lowering pH) and
reducing the concentration of free carbonate. The second reaction is both temperature
and pressure dependent: more carbonate remains at warmer temperatures, less at cold,
and greater pressure pushes the reaction toward bicarbonate. Thus, descending in the
ocean (colder, more pressure) carbonate concentration goes down, so carbonate
minerals, particularly calcium carbonate, tend to dissolve. As the ocean absorbs more
CO 2 , the depths at which this effect is pronounced will rise.
(^) Greater acidity will have significant effects on the rather open physiology of marine
organisms with exchange membranes exposed to seawater, from phytoplankton and
protozoa to fish. At approximately three-fold pre-industrial CO 2 levels, pH will drop
to ∼7.8 (Feely et al. 2009). The present level is already down by about 0.05 pH units
globally, but more in some coastal areas, and the general effects are not well
characterized or mostly even identified.
(^) There is potential that increased bicarbonate concentration will moderately
stimulate phytoplankton photosynthesis and growth, while also shifting carbon
physiology. For example, Wu et al. (2010) report culture studies of the diatom
Phaeodactylum tricornutum adapted through many generations to atmospheric CO 2 in
culture headspace (pH 8.12) and to three-fold pre-industrial CO 2 (pH 7.8). Carbon
uptake rate of the high-CO 2 culture was 12% greater, but dark respiration was 34%
greater as well, so growth only increased by 5%. There was a marked effect on the
carbon-uptake system at high CO2; its Ks value increased 20% (reduced affinity).
Similar effects have been found for prymnesiophytes, Prochlorococcus, and others,
but not all species show these modest increases, and the likely impacts of persistently
high DIC and low pH remain obscure.
(^) Carbonate biominerals have received the most attention and concern. Important
phytoplankton (coccolithophores), some coralline algae, and animals (foraminifera,
pteropods, clams, corals, echinoderms, ...) secrete their shells, support skeletons,
spines, and spicules from CaCO 3 . There are two mineral forms of CaCO 3 , calcite
(foraminifera, clams, echinoderms) and aragonite (pteropods, corals), and they have
different temperature and pressure responses. Aragonite becomes soluble at greater
carbonate concentrations than calcite and is susceptible to dissolution at somewhat
warmer temperatures and shoaler depths. The levels of DIC accumulated at depth, and
deep layer [H+] are less in the Atlantic, more in the Pacific, and thus shell dissolution
occurs at lesser depths in the Pacific. That largely determines the difference in
sediment composition: lots of CaCO 3 in the Atlantic, even pteropod oozes, mostly
opal (diatoms, radiolarians) in the Pacific (the Pacific is also mostly deeper). The
levels at which aragonite and calcite readily dissolve are termed their compensation
depths.