The warm waters that bathe the region of Abrolhos, in the south of Bahia, guard the largest bank of calcareous algae in the world. In an area of some 21,000 km2, similar to the state of Alagoas, the ocean bed is rocky. It is covered by hard spheres of varying size – the biggest have the diameter of a 5-a-side football – and color, which range from brown to pink. The spheres are lime nodules deposited by red millimeter-long algae that live on their surface. Also known as rhodoliths, these structures create an environment with recesses and protrusions that serve as shelter for fish, crustaceans and invertebrates. Mapped out now by Brazilian researchers, the rhodolith bank of Abrolhos extends from the north of Espírito Santo to the South of Bahia and produces 25 million tons of limestone a year, or 5% of the global production of this mineral, which is used in agriculture, in the cosmetics industry and even in medicine.
“Rhodoliths are commonly called living rocks because of the algae that form its exterior,” says Gilberto Menezes Amado Filho, a biologist from the Botanical Garden Research Institute in Rio de Janeiro, one of the authors of the map published in April in PLoS ONE. Along with the limestone produced by coral and mollusks with shells, they contribute to the formation of the ocean bed. “Part of the Brazilian continental shelf is the result of the calcareous growth that has occurred over the last 18,000 years,” he explains.
Looking like pebbles as they roll along being dragged by the marine currents, rhodoliths are formed by the accumulation of small algae that grow one on top of each other, or are encrusted with shell fragments or grains of sand. They increase in size as their skeleton, rich in calcium carbonate (CaCO3), mineralizes. The rhodoliths from Abrolhos measure, on average, 5.9 centimeters in diameter – the biggest reach 14 centimeters – and grow a little more than a millimeter per year. They have been found at depths that varied from 20 meters to 110 meters, with almost half their surface covered by algae of one or more species – six have been identified in Abrolhos. Along this stretch of the coast, rhodoliths occupy 70% of the sea bottom (the rest is sediment) and, according to dating, the oldest are around 8,000 years old.
Rhodoliths along Brazil’s coastline have been known about since the 1970s, but no one imagined that they covered such a large area. In projects coordinated by the Oceanographic Institute of the University of São Paulo (IO-USP), by Conservação Internacional do Brasil [Conservation International] and by the Federal University of Espírito Santo, researchers mapped out the sea bed in that region between 2007 and 2011, when they realized that they were looking at something important. “When we realized we were looking at a huge rhodolith bank, we started focusing our efforts on understanding the diversity associated with them and the functional role of this ecosystem,” Amado Filho reports.
After sonar mapping, the researchers used two underwater robots to assess the distribution, extension, composition and structure of the bank. “We used the robots to analyze the deepest areas and detail the relevant points,” says Paulo Sumida, from IO-USP. In the third stage, they dived to collect samples and carry out experiments to estimate the production of calcium carbonate.
Gilberto M. Amado Filho / IPJBRJThere are rhodolith banks in all oceans. The most extensive, in addition to those in Brazil, are along the coasts of Mexico and Australia. They are important for the life of other organisms because they serve as shelter and provide an environment that is richer biologically than a sandy bottom. “They function as corridors between coral reefs, facilitating the migration of lobsters and fish,” says Amado Filho.
From the environmental viewpoint, rhodoliths also have another important function: they help remove carbon from the atmosphere and thus have an influence on regulating the planet’s climate. They absorb the carbon dioxide (CO2) that is diluted in the water and transform it into limestone, but they are threatened by human activities. The biggest threat comes from the increase in the sea’s acidity, which is the consequence of the rise in CO2 levels in the atmosphere – largely due to the burning of fossil fuels. “A third of the carbon emitted by human activities and added to the atmosphere is absorbed by the oceans,” says Amado Filho. “It’s estimated that by the end of the century the pH of sea water will drop by 0.4 units, making it more acidic. Carbonatic reef structures, atolls and rhodolith banks will be dissolved.” This change is also likely to reduce the calcification of marine organisms by up to 40%.
“In general it’s the coral reefs that draw attention, but now it’s known that Brazil has these other calcium carbonate plants that are of vital importance for marine biodiversity,” comments biologist Jason Hall-Spencer, from the University of Plymouth, England. “These coralline algae are among the calcifying organisms that seem to be most sensitive to the acidification of the oceans.”
Another threat to the rhodoliths of Abrolhos is the economic exploitation of limestone. As they are easy to collect, there are companies that use them as a source of the mineral. In addition to limestone, they contain variable quantities of other chemical elements (iron, magnesium, bromine, nickel, copper, zinc and molybdenum) that are used in agriculture, in dietary and cosmetic industries, for animal food and in water treatment.
“Rhodoliths are found in shallow waters, at 20 down to 110 meters, and they’re in a format that makes it easy to extract them on a large scale,” says Rodrigo Leão de Moura, from the Federal University of Rio de Janeiro, who took part in the survey. “Additionally, they’re sensitive to the quality of the sea water, which has been affected by the poor state of conservation of the catchment basins,” he adds. Although one part is alive, rhodoliths are not renewable resources. “Thousands of years are necessary for rhodoliths to form and create a significant bank, like the one that’s been recently discovered,” explains Moura. Given these threats, the researchers say that it is necessary to increase protection with the creation of conservation areas, the recovery of river banks and the control of effluents. “Of the 46,000 square kilometer Abrolhos bank,” Sumida warns, “only 2% is protected by conservation units.”
AMADO-FILHO, G.M.; MOURA, L. R. et al. Rhodolith beds are major CaCO3 bio-factories in the tropical south west Atlantic. PLoS ONE. v. 7(4). Apr. 2012.