Newswise — UNIVERSITY PARK, Pa. —Faced with climate change and other ecological shifts, coral populations in the Atlantic Ocean have sharply decreased in recent times, whereas coral populations in the Pacific and Indian Oceans are performing comparatively better. By characterizing various types of symbiotic algae essential for the growth of these corals, a global group headed by Penn State has discovered that these mutually beneficial associations from the Indo-Pacific may exhibit greater adaptability and durability to warmer ocean temperatures than those found in the Atlantic.
Coral reefs are extensive geological formations composed of calcium carbonate created by coral creatures, whose colonies contain concentrated populations of photosynthetic algae from the family Symbiodiniaceae, referred to as "symbionts," within their tissues. Coral bleaching happens when environmental factors, including increasing ocean temperatures, disrupt the symbiotic relationship between the algae and the coral animals, causing the colony to turn white, or bleach. Although corals can bounce back, the degree and duration of stress caused by bleaching may lead to coral death.
Todd LaJeunesse, a professor of biology at Penn State, stated that "Coral bleaching has an impact not only on the coral colonies but also on entire ecosystems of creatures, including invertebrates such as sea urchins and spiny lobsters, as well as vertebrates such as fish and sea turtles." He emphasized the significance of studying the biology of corals and their symbionts to forecast their reaction to forthcoming environmental changes, particularly ocean warming.
However, as per LaJeunesse, the reaction of corals and their symbionts will not be uniform. This is because the Earth's oceans contain numerous coral species, each with its own distinct characteristics. He added that, until recently, the wide range of symbiont species and their significance for coral survival had not been fully understood.
LaJeunesse noted that "Scientists used to classify all the symbionts into a few general categories." He further stated that his laboratory's research in recent years has focused on characterizing individual symbiont species so that their nature is well understood. Without such information, he added, the study of the ecology, physiology, and biogeography of corals cannot be adequately conducted.
LaJeunesse and his colleagues discovered that some symbiont species are specialists, meaning they can only associate with one or a few coral host species, while others are generalists, meaning they can associate with many coral host species. Furthermore, they observed that certain corals, particularly those from the Caribbean, depend on specialist symbionts, while corals from the Indo-Pacific region associate with generalists. As a result, Caribbean corals' inflexibility may make them more vulnerable to environmental changes, whereas Indo-Pacific corals with more adaptable partnerships may better endure significant environmental changes.
LaJeunesse emphasized that the symbiont species described by his team are crucial to reef ecosystems due to their ecological dominance and their significance to numerous coral species across vast geographical regions.
LaJeunesse suggested that as the Earth's oceans continue to warm, it is feasible for these symbiont species to become the primary inhabitants of coral communities, as more sensitive symbionts are likely to perish.
The team's recent findings, which were published in the Journal of Phycology on May 1, present official descriptions of multiple host-generalist symbiont species found in the Indo-Pacific region. To conduct their research, the team gathered coral samples from several locations across the Indo-Pacific, including the Palau reefs, Thailand, Zanzibar of Tanzania, the Phoenix Islands, the Great Barrier Reef of Australia, and New Caledonia. They subsequently extracted the symbiotic algae from these samples and sequenced their DNA. Following this, they identified and characterized five symbiont species that can establish associations with a range of host coral species.
Caleb Butler, a graduate student in biology at Penn State and the first author of the paper, stated that "it's challenging to communicate about topics we are unfamiliar with, or even lack a name for." He further explained that formal species descriptions aid in establishing an identity for these organisms, enabling scientists to connect past and future research and discuss them more effectively. The organisms that the team characterized are extensively distributed, and as ocean temperatures rise, these generalist symbionts with higher thermal tolerance are expected to spread to new coral communities. Acknowledging these unique species leads to informed research into their ecology and the ability to precisely communicate the implications of their findings.
Specifically, the symbionts that the team described are in the genus Cladocopium.
Matthew Nitschke, a research scientist from the Australian Institute of Marine Science (AIMS), noted that the Cladocopium genus is highly diverse compared to other coral symbionts, yet only a few of its species have been successfully cultured. He mentioned that one of the species described by the team, C. proliferum, can be cultured in a test tube, which allows researchers to understand the mechanisms underlying coral-algal symbiosis. C. proliferum has become a model species for such research in Australia, where it is being used in reef restoration research and development to investigate how these algal symbionts contribute to the heat-tolerance of corals under the leadership of Professor Madeleine van Oppen.
The paper has also been authored by a team of scientists, including Kira Turnham from Penn State, Allison Lewis from Lawrence Berkeley National Laboratory, Mark Warner from the University of Delaware, Dustin Kemp from the University of Alabama at Birmingham, Ove Hoegh-Guldberg from the University of Queensland, Bill Fitt from the University of Georgia, and Madeleine van Oppen from the Australian Institute of Marine Science and the University of Melbourne.
The National Science Foundation, IOC-UNESCO-World Bank and Eberly College at Penn State supported this research.