Investigating the impacts of ocean acidification on marine benthic algae

Increasing atmospheric carbon dioxide (CO2) levels are lowering seawater pH and causing unprecedented changes to ocean chemistry. PhD student Vivienne Johnson (with Professor Jason Hall-Spencer and Professor Colin Brownlee at the Marine Biological Association) is researching the impacts of ocean acidification on benthic algae. Investigations along subtidal CO2 gradients created by volcanic seeps are revealing the in situ responses of a variety of benthic micro- and macroalgae to elevated levels of CO2.

Analysis of biofilms that have colonised artificial and natural substrata along a CO2 gradient off Vulcano Island, Italy has shown that benthic microalgae assemblages alter significantly as CO2 increases. They have found that elevated CO2 stimulates the growth of some benthic diatom species, promoting the primary productivity in shallow water habitats. This is likely to have significant ecological ramifications for coastal systems as atmospheric CO2 levels continue to increase.

A recent collaboration with Australian Researchers (University of Adelaide and the Australian Institute of Marine Science) compared ecological shifts along CO2 gradients in the Mediterranean and Papua New Guinea, focussing on an abundant calcified brown macroalgae (Padina spp.) and grazing sea urchins. In both vent systems Padina experienced significant reductions in calcification in the high CO2 waters but surprisingly these algae thrived in a decalcified, fleshy form. As Padina proliferated in high CO2 sea urchin numbers fell dramatically. Significant increases in photosynthetic rates and reduced sea urchin grazing pressure is thought to explain the unexpected success of decalcified Padina at elevated levels of CO2.

The similarities found in the responses of Padina and sea urchin abundance between tropical and temperate vent systems increases confidence in predictions of the ecological impacts of ocean acidification over a large geographical range.