A Crucial Ingredient in Early Life May Have Gushed Out of Deep-Sea Vents
(Inside Science) -- Phosphorus, the 15th element, courses through the tree of life. It is a building block in DNA, RNA, cell membranes and bones. But because it is so scarce on the Earth's surface, scientists have long puzzled over why phosphorus is so common in biology. Now, a team of researchers believes the truth may be that it wasn't all that uncommon, because at the time life was beginning phosphorus-rich fluids may have spewed from deep-sea vents.
In today's oceans, iron and oxygen team up to quickly scavenge phosphorus and trap it inside minerals. This has been the case for the last 2.5 billion years -- ever since photosynthetic bacteria filled the Earth's atmosphere with oxygen, during what geologists call the Great Oxygenation Event.
Most geologists have thought that prior to this huge change, little phosphorus was entering the oceans. Most of the phosphorus that scientists are aware of comes from the weathering of continental rocks. Back during the Archean Eon, which spanned 4 billion to 2.5 billion years ago, large landmasses began rising out of the oceans. At first, there were fewer continents, meaning less phosphorus would have flowed into the sea.
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During the Archean, "we assumed there wasn't a lot of phosphorus in the sea," said Kurt Konhauser, a geologist at the University of Alberta. "We didn't know a mechanism by which it could get to the open ocean." According to Konhauser, many geologists had believed low levels of phosphorus prevented photosynthetic life from proliferating after it had evolved.
But now a team of scientists, led by Birger Rasmussen, a geologist from the University of Western Australia, has discovered evidence that phosphorus-rich fluids may have spewed from vents in the seafloor. The team's findings, recently published in the journal Geology, indicate that during life's earliest days, phosphorus may have been more plentiful in the ocean than previously believed.
"What we've discovered is that phosphorus wasn't rare," said Woodward Fischer, a geologist at the California Institute of Technology and a co-author on the study. As this is around the time life is believed to have originated, Fischer said the findings "help explain why it is that phosphorus is so common throughout cell biology."
Rasmussen, Fischer, and their colleagues examined ancient rocks from South Africa and Australia that were 3.46 billion to 2.46 billion years old.
Upon close inspection of these Archean samples, Rasmussen and his team found a mineral called chert in thin layers, which Fischer nicknamed "windows to the past."
That's because the chert had crystallized around grains of sediment deposited during the Archean and preserved them like insects trapped in amber.
To the team's surprise, the chert had trapped tiny particles of a phosphorus-bearing mineral called apatite. "We did not expect to find that at all," said Fischer.
For apatite to crystallize, and for the team to have found the mineral present in rocks from both South Africa and Australia, ancient seawater must have held much more phosphorus than today's oceans, said Rasmussen.
The apatite's presence alongside tiny grains of a mineral called greenalite, along with the chemistry of the chert, suggested to the researchers the phosphorus had flushed out of vents on the seafloor. These vents would have acted as a second source of phosphorus in the deep ocean, complementing what was carried there by erosion, said Rasmussen.
But the findings do more than just address what researchers call the phosphorus problem -- they also challenge widely accepted ideas about the evolution of cyanobacteria. These microscopic life forms were the first to use water, carbon dioxide and sunlight to produce sugar and oxygen, via a process known as "oxygenic photosynthesis," or just "photosynthesis" to most of us.
"Most [geologists] look at the rock record and think [cyanobacteria] already evolved by 3 billion years ago," said Konhauser, but these organisms didn't flourish until 2.5 billion years ago, when they caused the Great Oxygenation Event. The argument tends to be their expansion was restricted by phosphorus, so they existed for some time before the Great Oxygenation Event but weren't abundant enough to fill the atmosphere with oxygen.
But what changed 2.5 billion years ago to suddenly allow for the boom in cyanobacteria is a contested issue. "There is no consensus," said Eva Stüeken, a geologist from the University of St. Andrews not involved in the study.
Rasmussen believes his team's work resolves this problem by showing that phosphorus didn't stall the rise of cyanobacteria and photosynthesis. Instead, it's possible cyanobacteria simply evolved later than most geologists think. "The simplest interpretation is that the Great Oxygenation Event marks the evolution of oxygenic photosynthesis," said Rasmussen.
Konhauser, however, believes the study will need to be supported by more experimental work. "They're looking at specific rocks and telling a global story," said Konhauser. But "if there is an additional phosphorus source, it's something we haven't really been thinking about."
Stüeken would similarly like to see more work done to test the team's conclusions. "These ideas are really tantalizing," she said. "If you think about how life could have originated on other planets, [deep-sea vents] could be important for delivering key nutrients into the biosphere."