Since the beginning of the industrial revolution in the mid-eighteenth century, the increase in carbon dioxide from humankind's combined industrial and agricultural activities has elevated atmospheric CO₂ concentration from approximately 280 μmol/mol to 390 μmol/mol. The ocean has absorbed about one third of anthropogenic carbon emission, thereby curtailing the growth of CO₂ level in the atmosphere. At the same time, however, the absorption of CO₂ has caused unprecedented changes to ocean chemistry, lowering seawater pH and carbonate ion concentration and increasing dissolved CO₂ and bicarbonate ion concentration. The changes in carbon chemistry are referred to as ocean acidification (OA). The oceanic uptake of CO₂ and the concomitant changes in seawater chemistry may benefit algal photosynthesis, but have adverse consequences for many calcifying organisms, and may result in changes in species composition and other ecosystem and biogeochemical processes. In this paper, we present the current status of ocean acidification, and review its effects on marine organisms and ecosystems, with the emphasis on two key biological processes-calcification and photosynthesis.
The ability of marine calcifying organisms, including corals, foraminifera and coccolithophores to produce calcareous skeletal structures is directly affected by decreased carbonate saturation state (Ω). When seawater p(CO₂) reaches two times of pre-industrial p(CO₂), biological calcification productivity will decrease by 20-40%. Most calcifying organisms investigated demonstrate reduced calcification in response to increased p(CO₂) and decreased CO₃^2- concentration and CaCO₃ saturation state (Ω_cal) as well as lowered pH. Coral reefs which are the most frequently investigated ecosystem showed reduced growth rate, lower calcification rate, and increased levels of bleaching or necrosis under ocean acidification. Pteropods, foraminifera and coccolithophores are major planktonic producers of CaCO₃ and account for nearly all the export flux of CaCO₃ from the upper ocean to the deep ocean. Their calcification rates will decrease with decreasing CO₃^2- concentration and Ω_cal. Benthic invertebrates such as echinoderms like sea urchins and brittlestars will mostly suffer from the deleterious carbonate conditions, especially in their early life stages. Enough evidence suggests that seawater acidification will damage the calcifying organisms, but to what extent calcifying organisms are affected and how they adapt to the changed chemical environment are not adequately addressed.
The increase in CO₂ availability facilitates photosynthetic carbon fixation of some phytoplankton groups, and the increase in photosynthesis will differ among different species due to the level of carbon concentration mechanism (CCM) and the efficiency of light utilization. The low pH conditions may also change taxonomic composition and uptake ratios of C to other nutrients. Elevated CO₂ concentration increases phytoplankton C∶N ratio which may take excess carbon, thereby buffering OA in the surface ocean. However, calcifying algae show contradictory evidence of either an increase or decrease in calcification and photosynthesis.
CO₂ in the atmosphere is projected to double in 50 years and to triple by the end of the century if the current CO₂ emission continues at the present level. The ocean will become more acidic as pH was projected to drop by 0.1 and 0.3 units, respectively. Ocean acidification may also alter ocean biogeochemical cycle, community composition and ecosystem stability directly or indirectly.