Ocean acidification weighing heavily upon marine algae

Even a small loss of skeletal calcification caused by exposure to corrosive waters can have a significant impact and leave algae at risk of losing access to light and nutrients.

Scientists at Plymouth University, the University of Washington, and the University of Palermo, made the discovery during fieldwork at volcanic vents in the Mediterranean and subsequent laboratory testing. Their paper, Ocean acidification bends the Mermaid's wineglass, is the cover story of the new edition of the Royal Society's Biology Letters.

One of the authors, Professor Jason Hall-Spencer, from Plymouth's School of Marine Science and Engineering, and its Marine Institute, said: "Based upon current forecasts, many calcified organisms will be corroded by acidified waters by the end of the century. What this study shows is that a dramatic weakening of algal skeletal strength can have implications for performance, which in turn could transform an entire ecosystem."

The team of scientists conducted the research off the Sicilian island of Vulcano, last year, as part of the EU-funded MedSeA project. Three sites of high, medium and low CO₂ were surveyed by snorkel for evidence of the presence and appearance of the green algae Acetabularia acetabulum, also known as 'mermaid's wineglass'.

All surveys revealed that the algae ranged in appearance from those with bright white cups at the low CO₂ site to green cups at the high site, and no calcified algae were present in the region nearest the volcanic seeps. When specimens were scanned under electron microscope, they found that those from areas of low CO₂ had an intact sheath of aragonite that supported the stem, while those in acidic waters had had theirs eroded and pitted and were up to 32% less calcified.

Further tests were conducted on samples using static cantilever beam theory, with the base of the algae clamped between two horizontal glass slides, suspending the hydrated stem and cup in air. A weight was then hung from the stem to exert force and test its flexural stiffness and ability to resist the load.

It revealed that the stems from those specimens collected from the high CO₂ site were up to 40% less stiff and 40% droopier -- raising important questions about the impact upon their performance as a result. And the scientists say the relationship between calcification and material stiffness was exponential, not linear, so even relatively small reductions in calcification led to a disproportionate drop in the ability of the material to resist a load.

"Although calcifying organisms can tolerate high CO₂ conditions, even subtle changes in calcification can cause dramatic changes in skeletal performance, which may in turn affect key biotic and abiotic interactions," said Professor Hall Spencer.

"A less rigid stem droops towards the seafloor likely reducing the distance spores can travel away from the cup. The cup is also photosynthetic, so bending may reorient it away from light and increase shading by neighbours, thereby reducing the scope for growth. But there are also potential benefits to being less stiff. A more flexible stem allows the algae to reorient in flow, reducing drag and the likelihood of dislodgement, and may aid in gas exchange as the stem moves back and forth like a pendulum."

Professor Laura Newcomb, of the University of Washington, added: "This study underscores the fact that some organisms may survive ongoing ocean acidification despite reduced calcification; this facultative calcification may explain why certain calcified organisms reappear in the fossil record after mass extinctions associated with periods of high atmospheric CO₂.

"Many ocean acidification studies show reduced calcification at high CO₂, but do not examine the consequences for organismal performance. Our ecomaterial approach establishes these linkages between calcification and performance (and ultimately fitness) which are vital for long-term predictions of how organisms will fare in a high CO₂ world."