Marine Life Can Resist Ocean Acidification by Modifying Shell-Building Process at the Nanoscale

Some of the most important and diverse animals in the ocean are shell-builders, which produce calcareous shells for both growth and protection. Therefore, the process of shell production (i.e. calcification) is fundamental to maintaining their populations and hence functioning of marine ecosystems. However, with the ocean becoming more acidic due to ever-increasing carbon dioxide (CO₂) emissions, the fitness and survival of shell-builders in the future have raised concerns. Based on the results of laboratory-based research, ocean acidification has been shown to reduce their shell-building capacity, weaken the mechanical strength of their shells, or hinder their population growth and survival. Nevertheless, growing evidence from the natural environments reveals that some shell-builders appear to be capable of resisting the “corrosive” effect of acidified seawater on their shells. Understanding how they can maintain their shell-building capacity in the future acidifying ocean becomes increasingly important in marine research.

Calcareous shells are primarily made up of calcium carbonate minerals and typically arranged in hierarchical structures. Unlike manufactured materials with known mechanical strength, the mechanical strength of calcareous shells can vary, subject possibly to the atomic arrangement of the calcium carbonate crystals – the building blocks. As such, the mechanical strength may be determined by the thickness and package of the carbonate crystals as well as how their nanoscale arrangements adjust to the changing environmental conditions. It is worth noting that the effects of ocean acidification also depend on the ecological and evolutionary processes that influence the adaptive mechanisms initiated by shell-builders to modify their shell-building process and leverage on the short-term negative effects. The adaptive mechanisms include adjusting the shell structural properties and packing of the calcium carbonate crystals to build durable shells.

Herein, the researchers in the University of Adelaide, Australia: Dr. Jonathan Leung, Dr. Yujie Chen, Prof. Ivan Nagelkerken, Prof. Zonghan Xie and Prof. Sean Connell, in collaboration with Prof. Sam Zhang from Southwest University, studied the response of the abundant calcifying marine snails (Eatoniella mortoni) to ocean acidification and whether they can adaptively modify their shell structural properties. This particular snail is ideal for the research objective because of its ability to inhabit natural CO₂-acidified environments for multiple generations. The work is published in the research journal, Small.

In their approach, the natural CO₂ vents at the western Pacific were utilized. To test for any limits of structural plasticity (i.e. adaptive modifications of shell structures) to acidified seawater, the marine snails were collected across the pH gradients that represent the contemporary conditions (pH 8.1), predicted future conditions in the year 2100 (pH 7.8) and extremely acidified environment (pH 6.6). The researchers correlated the shell properties, including porosity, nanotwin thickness and organic matter content, with mechanical performance to evaluate the adjustability of the marine snails to ocean acidification.

Results showed that the marine snails were able to adaptively modify the building block of their shells to build mechanically stronger and more resilient shells under the predicted future conditions, compared to contemporary conditions. The shells were characterized by increased resistance to fracture, which was associated with reduced porosity, reduced nanotwin thickness and increased organic matter content. However, the shells became more fragile with increased porosity in the extremely acidified environment, probably attributed to the excessive corrosion by the highly acidified seawater.

In summary, the authors are the first to report the nanoscale adjustments of shell structures as part of the adaptive mechanisms exhibited by shell-builders to maintain the mechanical performance of their shells under near-future ocean acidification. The results provided critical insights into why some shell-builders may thrive in natural CO₂-acidified environments. Nevertheless, the adaptive capacity of the snails had a limit as they failed to build durable and mechanically resilient shells under extreme levels of acidification. With continuous human-caused environmental changes, the information provided in this study would advance research on the adaptive responses among other shell-builders, such as corals, mussels, oysters and sea urchins. Particularly, authors explained their findings would pave the way for further understanding of the molecular adaptation of marine animals to the future acidifying ocean.