The Mediterranean Is Now So Acidic That It Dissolves Plankton!

Nummulitid foraminiferans from the Eocene near Al Ain, United Arab Emirates.

The increase in CO2 levels in the atmosphere also affects the marine environment, leading to a rise in water acidity. This alteration in ocean chemistry has consequences for certain essential biogenic processes, particularly the calcification of some planktonic species.


The impact of the increase in atmospheric CO2 extends beyond climate warming. It is essential to recognize that the interactions between the air and the ocean are particularly significant. Consequently, any rise in atmospheric CO2 has repercussions for ocean chemistry. Since the industrial revolution, approximately 25% of the CO2 emitted by human activities has been absorbed by the world’s oceans, leading to a decrease in water pH. In other words, the ocean becomes acidic.

Increasingly Fine Foraminiferal Tests

water acidity on a calcareous shell.
This image shows the impact of water acidity on a calcareous shell. Image: NOAA

This development has had an impact on the fauna and flora of the oceans. Among the most affected organisms are foraminifera. These single-celled planktonic species have a thin envelope of calcium carbonate (known as the test). Acids attack the limestone and encourage it to dissolve.

Critical Situation in the Mediterranean

Sea surface temperature (SST) and surface pH at the 3 study locations in the western Mediterranean basin.
Sea surface temperature (SST) and surface pH at the 3 study locations in the western Mediterranean basin. Image: Nature.

A recent study, published in Communications Earth and Environment, highlights the critical situation unfolding in the Mediterranean, particularly concerning the escalating acidification. Over the past century, the pH in this region has decreased by 0.08 units. This decline is significant enough to impact the calcification process of foraminifera. Sediment analysis reveals that since the Industrial Revolution, the foraminifera tests have progressively become thinner. This evolution renders these microorganisms, foundational to the marine food chain, more vulnerable. Thus, this change in ocean chemistry poses a threat to not only planktonic populations but also the entire marine ecosystem.