Certain algae help Kaneohe Bay corals weather warmer seas

A unique algae may be helping corals at Kaneohe Bay become more resilient to climate change, but the algae might also cause the corals to eat less, according to a study by University of Hawaii researchers.

The study was published this month in The ISME Journal, a peer-reviewed science publication, by lead author Chris Wall of UH Manoa’s Pacific Biosciences Research Center and others.

Wall said the coral reef at Kaneohe Bay is doing well with an abundance of Hawaiian rice coral, a species considered an “ecological winner” because of its survival ability. The reef has survived decades of human impacts and coral bleaching events in 2014, 2015 and 2019 related to climate change.

“The coral at Kaneohe Bay are actually quite resilient to environmental stress,” Wall said by phone Saturday.

One reason may be an algae in the tissue of the rice coral, Montipora capitata, that can survive in warmer temperatures and brighter light.

Coral feeds two ways: on small zooplankton and algae in its tissue. While there are multiple types of algae, the study found the bay’s rice coral had more of the algae that can handle warmer water in its tissue. The concern about this algae is it does not provide as much energy to the coral as other types of algae, Wall said.

Wall and the other researchers used chemical traces in the coral to monitor the amount of food that came from the zooplankton and the algae. They found the coral was not eating more zooplankton to make up for the decreased food supply from the algae.

Wall said it was hard to say how much less food the coral was getting, but the coral appeared healthy even with more of the heat-tolerant algae in the tissue.

It was unclear what the long-term impact of less food would be on the coral, such as smaller tissue growth or decreased reproduction, but the coral may be receiving an overall benefit from the algae helping it to survive in stressful times, such as during ocean warming, Wall said.

In bleaching events an environmental stressor, such as warmer water, will cause coral to expel its algae, leaving the coral uncovered and causing it to appear white. Corals that survive bleaching events may be left in a weakened state.

Wall said the heat-tolerant algae lives in brighter areas of the bay, up to about 5 feet underwater. After that point the algae in the coral suddenly changes to a species that prefers darker waters.

“It kind of tells us how this coral is surviving change,” Wall said.