Plumes of fresh water carrying sediment gush out of the Amazon River into the Atlantic Ocean.

It had been assumed that low levels of light and oxygen, as well as the higher acidity levels of the river, caused a break in the coral reefs that fringe the Atlantic coast of the Americas.

But a team of American and Brazilian scientists has surprised the world by revealing a 1,000 kilometer long reef system at the mouth of the Amazon River.

"This is this first time a reef has been discovered in such conditions," said Fabiano Thompson, one of the scientists behind the discovery.

Rewriting the textbooks

"It is recognized in the text books that it's impossible to have reefs in such areas - reefs will not form at the mouth of big rivers like the Amazon and the Ganges because of the high sediment and acidic waters."

But the new research turns that assumption on its head.

Living on the reef, the team found 61 different sponges - including three new species - and 73 species of fish, as well as spiny lobsters and brittle stars.

With low light levels, the reefs contain few corals, but are dominated by sponges and rhodoliths - a type of marine algae that looks similar to coral.

Chemicals not light

Unlike tropical coral reefs, the Amazon reefs depend less on photosynthesis than chemosynthesis - biogeochemical and microbial processes that produce biomass from minerals rather than light.

"Photosynthesis is not a major player at the base of the food chain. It's paradigm-breaking to find a reef that's based on chemosynthesis," Thompson told DW, adding that similar reefs could be "hidden in many locations in the world."

The Amazon reefs lie on the continental shelf, around 80 kilometers off the coast, at a depth of up to120 meters - deeper than coral reefs are usually found.

Fish and sponges associated with reefs were first recorded in area in 1977, and in 1999, corals were found at the south end of the river's mouth. But this is the first time the reef - which Thompson described as "gigantic" - has been confirmed and mapped.

"There was some small evidence from the '77 and '99 papers," Thompson said. "But that did not guarantee that there were reefs - and more - these reefs are functional. These reefs are totally alive; they sustain large amounts of fish and lobsters."

Reefs under stress

Thompson and his team believe that studying the reef could also provide insights into how reef ecosystems cope in "suboptimal" conditions, with implications for other reefs around the world that are coming under increasing pressure from climate change and ocean acidification.

Last week, scientists in Australia reported that 93 percent of the Great Barrier Reef - the world's largest reef system - was affected by bleaching, which occurs as a result of rising sea temperatures.

Thompson said deep reefs in marginal conditions like the Amazon reef could be expected to be more resilient to acidification than tropical coral reefs.

Risk from oil drilling

But the Amazon reef itself could be at risk from an even more immediate threat than global warming - oil drilling.

In a paper released published in Science Advances the scientists note that, "remarkably", 125 blocks have been offered for oil drilling off the Amazon shelf. Of these, 20 "will soon be producing oil in close proximity to the coral reefs."

They call for a more comprehensive survey of the area's biodiversity.

"Such large scale industrial activities present a major environmental challenge, and companies should catalyze a more complete social-ecological assessment of the system," they write.

The scientists have so far mapped 1,000 square kilometers - or around one ninth - of the total area of the reef.