Ocean acidification weakens the structural integrity of coralline algae

Abstract

The uptake of anthropogenic emission of carbon dioxide is resulting in a lowering of the carbonate saturation state and a drop in ocean pH. Understanding how marine calcifying organisms such as coralline algae may acclimatize to ocean acidification is important to understand their survival over the coming century. We present the first long-term perturbation experiment on the cold-water coralline algae, which are important marine calcifiers in the benthic ecosystems particularly at the higher latitudes. Lithothamnion glaciale, after three months incubation, continued to calcify even in undersaturated conditions with a significant trend towards lower growth rates with increasing pCO2 . However, the major changes in the ultra-structure occur by 589 μatm (i.e. in saturated waters). Finite element models of the algae grown at these heightened levels show an increase in the total strain energy of nearly an order of magnitude and an uneven distribution of the stress inside the skeleton when subjected to similar loads as algae grown at ambient levels. This weakening of the structure is likely to reduce the ability of the alga to resist boring by predators and wave energy with severe consequences to the benthic community structure in the immediate future (50 years).

Potential and limitations of finite element modelling in assessing structural integrity of coralline algae under future global change

“…Coralline algae (Neogoniolithon sp.) and calcareous green algae (Halimeda incrassata) exposed to increased CO 2 concentrations (606 and 903 ppm for 60 days) showed an increase in calcification (Ries et al, 2009); whereas another study on Lithothamnion glaciale, a cold temperate coralline algae, displayed a decrease in growth rate at elevated CO 2 conditions (589 and 755 µatm for 3 months; Ragazzola et al, 2012), while also showing evidence for potential acclimation after 10 months (Ragazzola et al, 2013).…”

“…Under elevated CO 2 conditions the algal skeleton appeared to be weakened (Ragazzola et al, 2012), thereby favouring erosion and breakage Ragazzola et al, 2012).…”

“…2a-b) as observed in the 2-D FE model. These simple 2-D models represented the mechanical performance of a cross-section of uniform thickness (1 µm; Ragazzola et al, 2012) through the algal structure. However, these highly innovative models were simplistic in nature.…”

“…Finally, the loading and constraint taken from Ragazzola et al (2012) were a mixture of shear and compressive forces, which simulated boring forces by an organism exerted on the exposed corner of an attached thallus. As it was assumed that these organisms were more prone to shear forces than compressive ones, we also assessed the effect of sole compressive or sole shear forces on the compartmentalised and the biologically realistic model.…”

“…(1) Ragazzola et al (2012) were the first to use FEA in ocean acidification studies by creating simple 2-D FE models of coralline algae to quantify the effect of elevated CO 2 on the skeleton of Lithothamnion glaciale. Although the changes in growth rate were not significant, specimens grown under CO 2 conditions predicted for the year 2050 were found to have significantly larger cells and thinner cell walls.…”