Diversity of coralline red algae is greater than expected and could increase further

By André Julião  |  Agência FAPESP – A group of researchers supported by FAPESP has proposed a number of new species of coralline red algae found on the coast of Brazil. They look more like rocks than plants and are important reef builders, providing a habitat for a wide variety of marine life and helping to mitigate climate change by accumulating carbon in their structure.

Articles on the group’s research are published in Coral Reefs and the European Journal of Phycology.

Genetic sequencing of specimens collected on the coast of Brazil and around its oceanic islands shows that at least 79 species occur in Brazil. The research included mapping the distribution of these algae and identifying the most important areas for conservation. The group has also described a new genus and seven new species, named in honor of indigenous nations in the areas where they were found.

The researchers expect to describe more new species in the near future. So far most of their specimens were collected in shallow waters and a vast unknown diversity awaits at greater depths. “These algae are the main reef builders in the southwestern Atlantic. The only atoll in Brazil, for example, Rocas Atoll [in Rio Grande do Norte], consists mainly of algae, in contrast with so many other reefs built by coral polyps,” said Marina Nasri Sissini, first author of the Coral Reefs article. She conducted part of her PhD research in the Program of Graduate Studies in Ecology at the Federal University of Santa Catarina (UFSC).

Long rhodolith beds are found in much of Brazil’s continental shelf, occupying some 230,000 square kilometers, she added. Rhodoliths are unattached nodules of coralline algae that live on the seabed.

“These algae are calcareous and deposit calcium carbonate, a kind of lime, on their cell walls as they grow. That makes them hard. They take different forms, including articulated species that have branches, like those of better-known algae but hard. Crustose species encrust rocks, turtle shells, whales and even other macroalgae,” said Mariana Cabral de Oliveira, a professor at the University of São Paulo’s Institute of Biosciences (IB-USP) and one of the coordinators of the studies.

Rhodoliths form when the algae bind to something like a piece of shell, and this nodule grows to become spherical and resemble a stone, albeit alive.

The studies were part of the project Diversity and phylogeny of economically and ecologically important red algae (Rhodophyta), supported by FAPESP under its Research Program on Biodiversity Characterization, Conservation, Restoration and Sustainable Use (BIOTA-FAPESP) and led by Oliveira.

Hard plants

Analyzing structures and distinguishing between specimens of an organism as hard as stone, with highly similar shapes, is no easy task. This may explain why many inventories of algae on the Brazilian coast conducted in recent decades have left out its coralline red algae.

“Many surveys of the algae present in certain regions have been performed, but they typically exclude coralline red algae,” said Oliveira, who has studied algae for more than 30 years.

The techniques used to study these marine plants include decalcifying each specimen with a special solvent and cutting it carefully to view its reproductive and other internal structures, which normally differentiate one species from another.

Studies using such techniques, alongside others that involve molecular tools, enabled the researchers to establish 73 species for the Brazilian coast, 30 of which have been described in the last ten years alone. Much uncertainty remains, however.

“Specimens that were visually identical actually were 10% different when their genetic sequences were compared. That’s more than sufficient to separate them into more than one species,” Sissini said.

One of the latest studies brought together the efforts of research groups that collected specimens on long stretches of the Brazilian coast to discover which species are in fact present in Brazil. The authors of the articles are from the states of Bahia and Rio de Janeiro, as well as São Paulo and Santa Catarina.

The sequences revealed 79 taxonomic units, equivalent to species, but they each have to be described in the usual manner, entailing a search for comparisons of morphology, any former descriptions, and names that have already been used.

The study also showed that of the four ecoregions in South America, the eastern one from southern Bahia to northern Rio de Janeiro has the most biodiversity, with 50 species, 21 of which are found only in that ecoregion. Throughout the American coast of the Atlantic, the marine diversity in this ecoregion is superseded only by that of the Caribbean.

Native algae

Seven new species are described in the European Journal of Phycology article, all of them in the new genus Roseolithon, which is also described there. The researchers concluded that the genus also contains at least 18 other species previously placed in the genus Lithothamnion. Molecular analysis showed that some species in the latter genus had a different ancestor, justifying the naming of a new genus for them. Description of Roseolithon was made possible by an analysis of specimens collected in the northwestern and southwestern Atlantic. Integration of the molecular and morphological data enabled the group to propose seven new species while noting that more new species have yet to be described.

“The description of the genus Roseolithon was initially based on different molecular markers. We then embarked on a morphological analysis with the aim of finding morpho-anatomical details that could help us describe the new genus and the seven species covered by the article. Discovery of the new genus shows how insufficient our existing knowledge of the diversity of coralline red algae in Brazil still is, despite the importance of their ecological role,” said Luana Miranda Coutinho, first author of the article. The research was conducted while she was a PhD candidate at the National Museum in Rio de Janeiro.

“The next step is to describe and confirm the existence of other species in this genus, and to describe other taxa of coralline algae that have already been defined by molecular analysis for the coast of Brazil. All this research has only been possible thanks to collaboration by researchers and students,” said Maria Beatriz Barbosa de Barros Barreto, a professor at the Federal University of Rio de Janeiro (UFRJ) and last author of the article.

The names given to the new species were chosen with the assistance of an anthropologist. Paying tribute to indigenous nations in the areas where the specimens were found, they are: Roseolithon tupiiR. tamoioi, R. tremembei, R. potiguarae, R. karaiborum, R. purii and R. goytacae.

“These algae are also indigenous. To be precise, they originated in the seas of the Cretaceous [the geological period lasting from about 145 million to 65 million years ago]. They survived mass extinctions and are among the oldest living beings in the ocean,” Sissini said.

For the researchers, the discovery of so much diversity is a reminder that conservation strategies must take into account the peculiarities of each region and the connectivity between regions.

Intensely harvested in the past for extraction of lime to lower soil acidity levels in agriculture, these algae are now valued as large carbon stores. Their cell walls immobilize the carbon gas diluted in seawater. As such, they help mitigate global warming, and if they are removed the carbon returns to the atmosphere.

The article “Diversity, distribution, and environmental drivers of coralline red algae: the major reef builders in the Southwestern Atlantic” is at: https://link.springer.com/article/10.1007%2Fs00338-021-02171-1.

The article “Cryptic diversity in non-geniculate coralline algae: a new genus Roseolithon (Hapalidiales, Rhodophyta) and seven new species from the Western Atlantic” is at: www.tandfonline.com/doi/full/10.1080/09670262.2021.1950839.