From high above, the Aquarius Reef Base looked something like an alien anthill. A small army of divers had descended on the structure to take care of some daily maintenance tasks, and all six aquanauts were shuttling samples and equipment back and forth from reef to station. Eventually, this would grow exhausting. “The pinnacle, where we sample, seems to get further away each day, particularly when the current gets ripping,” writes one aquanaut on the station’s blog. “But then we get to ride the current back in, Finding Nemo style!”

The coral reef Slattery was investigating offered a natural experiment on the effect of acidification on reef species. Some sponges lived in the crevices where pH was low; others lived out on the open reef, where pH was closer to normal. Slattery wanted to see whether the crevice dwellers had any special adaptations that their open-reef-dwelling cousins lacked. “It’s just like humans and the common cold,” he explains. “Some people go through the whole season without getting sick at all, and then others catch every little bug that’s been flying around. We want to see if there’s the same type of natural variation in sponges responding to acidification.”

The team’s main focus was the Xestospongia muta, a giant barrel sponge that thrives in both acidified and nonacidified microhabitats. By monitoring pH and CO₂ levels around each sponge, and taking tissue samples for later analysis in the lab, they hoped to determine whether the sponges that live in acidified waters have a different protein makeup that helps them adapt to, and even thrive in, these regions. To determine whether different species respond differently to acidification, the team also collected tissue samples from other species of sponges in various microhabitats, especially the acidified crevices. Likewise, these samples were assessed for protein expression to see if there were differences in the effect of exposure to lower pH levels in different species.

Finally, to complement the observational studies conducted on sponges living on the reef, the team members conducted an experimental study in which they transplanted paired sponges—from acidified and nonacidified habitats—to sites facing additional stress: temperature increases. The idea was to see what impact prior exposures to low pH had on the organisms’ health and also to see what effects these environmental changes had on physiology.

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Evaluation of the initial data did not reveal significant biochemical differences in wild Xestospongia populations living in acidified areas relative to those in nonacidified areas (possibly due to the rapidly fluctuating pH levels near the sponges as currents mixed waters of various pHs, making it hard to isolate areas of consistently lower pHs). The species census, however, did reveal biochemical differences in other sponge species collected from acidified crevice microhabitats compared to those from nonacidified habitats. The researchers found reduced growth in individuals of some sponge species, but the degree to which growth was reduced varied between species: Some species did better than others. There was also evidence that individuals living in acidified crevices produced more stress proteins than conspecifics (individuals of the same species) living in nonacidified environments, even in species whose growth did not appear to be negatively affected.

Unfortunately, funding cuts that canceled the second year of the mission prohibited completion of the transplant study on the reef. This did not, however, prevent Slattery from going back to the lab to conduct experimental studies on the effects of acidification on Xestospongia.

Slattery’s team conducted controlled laboratory experiments to examine the physiological effects of exposure to the warmer, more acidic conditions that are expected in the near future. Xestospongia individuals were exposed to one of four pH and temperature pairings: current pH and temperature, future pH (lower) and current temperature, current pH and future temperature (higher), and future pH and temperature. These experiments revealed that the health of sponges subjected to future pH and temperature was negatively impacted, compared to sponges grown at current pH and temperature; the decline was linked to a reduction in the ability of Xestospongia’s symbiotic partner to conduct photosynthesis under these conditions. Less photosynthesis means less food (carbohydrates) for Xestospongia, which translates into slower growth rates and reduced reproduction for the sponge.

So while the Xestospongia population and some other sponge species may decline as the ocean becomes warmer and more acidic, the natural variability in populations of other species on the reef may allow those species to survive the anticipated lower pH levels of the near future. Slattery summarizes this evidence by predicting that “the sponge community will survive on reefs of the future, but these communities will likely look different as the winners and losers take their respective places in the future reef communities.”

Unfortunately, CO₂-related effects are by no means the only threats.