Algae as the coal mine-canary of the Pacific NW

Above is a photo of University of Chicago graduate student Sophie McCoy, studying algal biodiversity in Washington. It's not the sodden, green-brown macroalgae blanketing the rocks McCoy is concerned with – she's looking at crustose coralline algae, species of algae that actually put down calcium carbonate skeletons:

Pseudolithophyllum muricatum, the species McCoy is studying as a bellwether of the health of coastal ecosystems in Tatoosh Island, Washington. Note the genus name: Pseudolitho ("false rock") + phyllum ("plant", or "leaf").

Calcareous coralline algae are red algae (Rhodophytes) in the order Corallinales. As a creative Wikipedia author puts it, coralline algae with rocks and coral skeletons as substrata look like patches of paint "splashed as though by a mad painter over rock surfaces". Some coralline algae in tropical seas is epizoic (grows on animals) or epiphytic (grows on other plants). Occasionally, other (non-calcified) algae grows in and on coralline algae, particularly in warm waters; many coralline algae shed their epithallus (the outer layer of their "body", or thallus) to shake off these unwelcome guests. There are two types (not phylogenetically distinct) of coralline algae: branching, geniculate algae and crustose, flat algae like the ones McCoy is studying.

Coralline algae have an important hermatypic (reef-building) purpose in coral reef communities, but perhaps the most important ecological role coralline algae fill is providing a microhabitat for the larvae of molluscs, echinoderms, and other herbivorous marine invertebrates. On the rocky coasts in the northwest Atlantic, for example, the larvae of sea urchins, chitons and limpets rely heavily on the calcareous microstructures of coralline algae as a refuge from predators and suffer 100% mortality without their presence. Coralline algae also serves as an important food source for some of those invertebrates once they reach adulthood.

The lives of coralline algae (and the lives of their invertebrate larvae-partners) revolve around even the most minute changes in marine chemistry, and in communities with several similar sympatric, competing species of coralline algae, changes in diversity and relative abundance can be indicators that something greater is amiss. McCoy's research has revealed that Pseudolithophyllum muricatum, once the dominant species of coralline algae in Tatoosh Island, now has only a 25% success rate over competitors. McCoy explains that P. muricatum is losing out because it produces more calcium carbonate than its competitors:

“Some species are more affected than others. So the ones that need to make more calcium carbonate tissue, like P. muricatum, are under more stress than the ones that don’t.”

Bottom line: from the waters off Tatoosh Island to the Caribbean Sea, ocean acidification is a universal problem. It's increasingly becoming a fact of life for ocean species, and it will only accelerate as humans continue increasing atmospheric carbon concentrations. 

Learn more about Sophie McCoy's research here.

Author: Matthew