Nearly 500 million people depend on healthy coral reefs for sustenance, coastal protection, renewable resources, and tourism, with an estimated 30 million of the world's poorest people depending entirely on the reefs for food (3).

Coral reefs face two challenges from increasing atmospheric CO2. First, higher CO2 concentrations in the atmosphere are linked to warmer global temperatures, which in turn lead to warmer water temperatures. Corals are very sensitive to temperature change: a 1–2º C change in local temperature above their normal summer maximum can lead to a phenomenon called ‘bleaching’, whereby the corals expel their vital algal symbionts (algae which live in the cells of the coral), leaving the coral tissues translucent.

In 1998, a single bleaching event led to the loss of almost 20% of the world’s living coral. Corals can recover from these events but repeated episodes are likely to weaken the coral ecosystem, making them more susceptible to disease and causing a loss of biodiversity.

The second challenge faced by corals is the increasing acidity of the water caused by higher CO2 concentrations (4). Lowered calcification rates affect the reef ’s ability to grow its carbonate skeleton, leading to slower growth of the reef and a more fragile structural support, which makes it more vulnerable to erosion.

By the middle of this century, the estimated reduction in calcification rates may lead to more reef area erosion than can be rebuilt through new calcification (5). See "How is ocean acidity changing? What can we expect in the future?".

Photo credits: Coral Reef Alliance (Top); James Oliver/ ReefBase (Middle); ReefBase (bottom).

Changes in the carbonate chemistry of the ocean may have a strong negative impact on many plankton and zooplankton species that form the base of the marine food chain.

In almost all calcifying organisms tested, ranging from single-celled organisms up to reef building corals, there is a decrease in the ability of the organism to produce calcium carbonate in more acidic waters (6). One study has documented the changes in two species of coccolithophores grown under conditions expected by the end of this century, where both species show significant decreases (25 - 45%) in calcification rates and clear signs of structural damage in their shells, which may affect their physical functioning and reproduction. However, not all species of calcifying organisms are negatively affected by increased acidity, and more research is needed to understand these mechanisms and possible adaptation pathways(7).

The first two photos on the left show scanning electron microscopy photographs of the calcifying phytoplankton Gephyrocapsa oceanica under CO2 conditions of today (top) and under the high CO2 conditions expected by the end of this century (middle).

Pteropods are small "winged snails" that form the basis of the food chain for many commercial fish species. A recent study has observed shell dissolution in living pteropods when exposed to the carbonate content of the ocean expected in the next 50 years in the high latitudes (6).

If ocean acidification leads to disturbances in the populations of these organisms, other non-calcifying organisms may out-compete them for food and nutrients, leading to a change in the ecosystem composition of the system. Although some of the affected organisms are important prey for higher organisms, it is not yet clear how such changes would affect fisheries.

Photo courtesy of Ulf Riebesell, IFM-GEOMAR. Adapted and reprinted with permission of Nature (www.nature.com): Riebesell et al. (2000) Reduced calcification of marine phytoplankton in response to increased atmospheric CO2, Nature 407, 364-367. (Top and Middle); Photo credit: Russ Hopcroft, California State University San Marcos (Bottom)

The rate at which mussels and oysters build their shells is also lower in a high CO2 (low pH) environment, which could have dire consequences on the aquaculture of molluscs, an industry of 12 million tonnes per year and a market value of over $10.5 billion US dollars (8).

Higher marine life forms, animals such as invertebrates and even fish may be affected by lower pH environments through acidosis (an increase in carbonic acid in the body fluids causing lower pH values in blood) leading to lowered resistance, metabolic depression, behavioural depression affecting physical activity and reproduction, and asphyxiation (9).

Cephalopods such as squid seem to be particularly sensitive to CO2 increases because their energy-demanding way of swimming requires a good supply of oxygen to the blood, which is impaired by lowered blood pH values.

Depending on the effect of other stressors like warmer temperatures, sensitivities may differ between life stages of a species, with larvae possibly being more sensitive. The full range of physiological mechanisms eliciting whole animal responses to elevated CO2 levels is still insufficiently explored. Current experiments address the extent to which genomic information defines sensitivity and if animals can physiologically adapt to elevated CO2 levels.

Direct effect of CO2 on marine mammals (seals and whales) or birds are not expected because they breathe air, and thus will not be directly affected by acidification of the surrounding seawater.

Changing food webs will, however, affect these animals and their well-being in ways that are not fully understood.

Photo credits: Scuba Diving Magazine On-Line (Top), Peter Parks / imagequest3d.com (Middle), Paul Duxfield / Scuba.co.uk (Bottom)