A couple of years ago we heard about a study conducted at Britain's University of Exeter, which indicated that increasingly acidic oceans were harming fishes' sense of smell. New research now reinforces that finding, and it could mean that trouble is in store for the marine ecosystem.

Sea water becomes more acidic as it absorbs carbon dioxide (CO2) from the atmosphere. Therefore, as atmospheric CO2 levels rise, so does the acidification of the earth's oceans – they're already an estimated 43 percent more acidic than they were at the time of the Industrial Revolution, and by the end of this century are predicted to be 2.5 times more acidic than they are now.

Unfortunately, it appears that such elevated acidity compromises fishes' ability to detect odors. This in turn makes it harder for them to find food, evade predators, recognize one another, and locate suitable spawning grounds.

"First we compared the behaviour of juvenile sea bass at CO2 levels typical of today's ocean conditions, and those predicted for the end of the century," says lead scientist Dr. Cosima Porteus, describing Exeter's latest research. "Sea bass in acidic waters swam less and were less likely to respond when they encountered the smell of a predator."

Additionally, the scientists recorded the electrical activity of the animals' nervous systems, as their noses were subjected to sea water with varying levels of absorbed CO2.

"The sense of smell of sea bass was reduced by up to half in sea water that was acidified with a level of CO2 predicted for the end of the century," says Porteus. "Their ability to detect and respond to some odors associated with food and threatening situations was more strongly affected than for other odors. We think this is explained by acidified water affecting how odorant molecules bind to olfactory receptors in the fish's nose, reducing how well they can distinguish these important stimuli."

Adding to the problem is the fact that when exposed to water with increased CO2 and acidity, the fish experienced a reduction in the expression of smell-sensing/processing genes in their nose and brain. In other words, instead of compensating for the nose's decreased ability to detect odors by increasing the expression of these genes, the fishes' bodies were just making things worse.

A paper on the research was recently published in the journal Nature Climate Change.