A recent study with Unisense microsensors has highlighted the importance of micro-refuges for enhancing the resilience of organisms and ecosystems to ocean acidification.
Increasing atmospheric carbon dioxide (CO2) will lead to more CO2 being absorbed by oceans. This, in turn, will make the oceans more acidic. This process, known as ocean acidification, will have widespread impacts on marine organisms particularly those with shells and carbonate skeletons (e.g. corals).
However, a study by researchers from the University of Tasmania, Australia, demonstrated that photosynthetic organisms in the ocean, such as seaweed, kelp and seagrass, can provide refuges for vulnerable organisms. These refuges are extremely small and only extend between 0.1 to 1 mm distance from the photosynthetic blade. Nevertheless, even in this small spatial area, a refuge is created for a vast variety of marine organisms.
The small zone extending from the photosynthetic blade is known as the diffusive boundary layer. This zone, which is present on the surface of all primary producers in the ocean, is a discrete region where ions and molecules pass between the organism and the environment by gradients of diffusion.
The metabolic activity of photosynthetic organisms can change the chemical composition of the diffusive boundary layer. That is, during daytime, when photosynthesis is taking place, CO2 declines and oxygen (O2) increases in the diffusive boundary layer. During the night, when respiration is higher, CO2 increases and O2 decreases.
This process can have a direct effect on the pH, or the acidity, of seawater in the diffusive boundary layer. There is a strong correlation between CO2 and pH where an increase in CO2 leads directly to a decrease in pH. In other words, increasing CO2 in seawater increases acidity. But, as photosynthetic organisms decrease CO2 in seawater during the daytime, then acidity also decreases in the diffusive boundary layer.
The study by Noisette and Hurd demonstrated this process using Ecklonia radiata (kelp) blades, collected near Hobart, Tasmania, Australia. It also showed that as the seawater became more acidic (that is, as ocean acidification intensified) the greater the change in pH values (see Figure 3A in their paper). Therefore, in an ocean that is expected to be more acidic, the changes in the diffusive boundary layer will provide zones of refuge for micro-organisms enabling them to cope with greater changes to the environment.
The Unisense microsensors enabled the scientists to observed changes in pH and oxygen in the diffusive boundary layer. The researchers used the 50 µm diameter tip sized pH micro-electrode and oxygen microsensor connected to a motorized micromanipulator. pH and oxygen were measured at approximately 0.05 mm steps in a transect from the photosynthetic blade to the bulk seawater. With the fast responding sensors, measuring at extremely fine spatial resolution, it was possible to detail the diffusive boundary layer and micro-environment.
Noisette & Hurd, 2018, Abiotic and biotic interactions in the diffusive boundary layer of kelp blades create a potential refuge from ocean acidification, Functional Ecology, 32, 1329-1342. Weblink.