CO2 currently accumulating in the atmosphere permeates into ocean surface layers,
where it may impact on marine animals in addition to effects caused by global warming.
At the same time, several countries are developing scenarios for the disposal of
anthropogenic CO2 in the worlds’ oceans, especially the deep sea. Elevated CO2 partial
pressures (hypercapnia) will affect the physiology of water breathing animals, a
phenomenon also considered in recent discussions of a role for CO2 in mass extinction
events in earth history. Our current knowledge of CO2 effects ranges from effects
of hypercapnia on acid-base regulation, calcification and growth to influences
on respiration, energy turnover and mode of metabolism. The present paper attempts
to evaluate critical processes and the thresholds beyond which these effects may become
detrimental. CO2 elicits acidosis not only in the water, but also in tissues and
body fluids. Despite compensatory accumulation of bicarbonate, acid-base parameters
(pH, bicarbonate and CO2 levels) and ion levels reach new steady-state values,
with specific, long-term effects on metabolic functions. Even though such processes
may not be detrimental, they are expected to affect long-term growth and reproduction
and may thus be harmful at population and species levels. Sensitivity is maximal
in ommastrephid squid, which are characterized by a high metabolic rate and extremely
pH-sensitive blood oxygen transport. Acute sensitivity is interpreted to be
less in fish with intracellular blood pigments and higher capacities to compensate for
CO2 induced acid-base disturbances than invertebrates. Virtually nothing is known
about the degree to which deep-sea fishes are affected by short or long term hypercapnia.
Sensitivity to CO2 is hypothesized to be related to the organizational level of
an animal, its energy requirements and mode of life. Long-term effects expected at
population and species levels are in line with recent considerations of a detrimental
role of CO2 during mass extinctions in the earth’s history. Future research is needed
in this area to evaluate critical effects of the various CO2 disposal scenarios.