by Hayley Dunning
Most modern bacteria descended from ancestors who could convert the Sun's energy to fuel more than 3.5 billion years ago.
Photosynthesis is the process by which plants, algae and cyanobacteria use the energy from the Sun to make sugar from water and carbon dioxide, releasing oxygen as a waste product. But a few groups of bacteria carry out a simpler form of photosynthesis that does not produce oxygen, which evolved first.
The picture that is starting to emerge is that during the first half of Earth’s history the majority of life forms were probably capable of photosynthesis.
– Dr Tanai Cardona
A new study by an Imperial researcher suggests that this more primitive form of photosynthesis evolved in much more ancient bacteria than scientists had imagined, more than 3.5 billion years ago.
Photosynthesis sustains life on Earth today by releasing oxygen into the atmosphere and providing energy for food chains. The rise of oxygen-producing photosynthesis allowed the evolution of complex life forms like animals and land plants around 2.4 billion years ago.
However, the first type of photosynthesis that evolved did not produce oxygen. It was known to have first evolved around 3.5-3.8 billion years ago, but until now, scientists thought that one of the groups of bacteria alive today that still uses this more primitive photosynthesis was the first to evolve the ability.
But the new research reveals that a more ancient bacteria, that probably no longer exists today, was actually the first to evolve the simpler form of photosynthesis, and that this bacteria was an ancestor to most bacteria alive today.
“The picture that is starting to emerge is that during the first half of Earth’s history the majority of life forms were probably capable of photosynthesis,” said study author Dr Tanai Cardona, from the Department of Life Sciences at Imperial College London.
Earlier ancestor
The more primitive form of photosynthesis is known as anoxygenic photosynthesis, which uses molecules such as hydrogen, hydrogen sulfide, or iron as fuel - instead of water.
Traditionally, scientists had assumed that one of the groups of bacteria that still use anoxygenic photosynthesis today evolved the ability and then passed it on to other bacteria using horizontal gene transfer – the process of donating an entire set of genes, in this case those required for photosynthesis, to unrelated organisms.
However, Dr Cardona created an evolutionary tree for the bacteria by analyzing the history of a protein essential for anoxygenic photosynthesis. Through this, he was able to uncover a much more ancient origin for photosynthesis.
Instead of one group of bacteria evolving the ability and transferring it to others, Dr Cardona’s analysis reveals that anoxygenic photosynthesis evolved before most of the groups of bacteria alive today branched off and diversified. The results are published in the journal PLOS ONE.
Triggering a change
“Pretty much every group of photosynthetic bacteria we know of has been suggested, at some point or another, to be the first innovators of photosynthesis,” said Dr Cardona. “But this means that all these groups of bacteria would have to have branched off from each other before anoxygenic photosynthesis evolved, around 3.5 billion years ago.
“My analysis has instead shown that anoxygenic photosynthesis predates the diversification of bacteria into modern groups, so that they all should have been able to do it. In fact, the evolution of oxygneic photosynthesis probably led to the extinction of many groups of bacteria capable of anoxygenic photosynthesis, triggering the diversification of modern groups.”
To find the origin of anoxygenic photosynthesis, Dr Cardona traced the evolution of BchF, a protein that is key in the biosynthesis of bacteriochlorophyll a, the main pigment employed in anoxygenic photosynthesis. The special characteristic of this protein is that it is exclusively found in anoxygenic photosynthetic bacteria and without it bacteriochlorophyll a cannot be made.
By comparing sequences of proteins and reconstructing an evolutionary tree for BchF, he discovered that it originated before most described groups of bacteria alive today.
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'Origin of Bacteriochlorophyll a and the Early Diversification of Photosynthesis' by Tanai Cardona is published in PLoS One.
Today's climate change doesn't hold a candle to the chemical warfare waged on Earth more than 2 billion years ago.
Before plants discovered the power of photosynthesis, single-celled life survived on chemicals, not sunlight, burning through hydrogen, methane and sulfur, among other yummy compounds. These "anaerobes" that live without oxygen were poisoned when blue-green algae called cyanobacteria evolved photosynthesis and started exhaling oxygen. The highly reactive gas combines with metals and proteins in anaerobic cells, killing them. But cyanobacteria thrived, turning sunlight into sugar and excreting oxygen as waste.
Oxygen levels in rocks suddenly rise starting 2.5 billion years ago — a spike called the "Great Oxidation Event." The jump was long held up as evidence for when cyanobacteria evolved photosynthesis. But a study published today (March 23) in the journal Nature Geoscience joins a growing body of data that suggests the earliest sun-lovers appeared long before this oxygen spike. [7 Theories on the Origin of Life]
Many researchers now think the first photosynthetic organisms lived on Earth 3 billion years ago. And like art restorers who find a hidden image under an Old Master painting, these scientists are discovering a new picture of Earth's first breath.
Heavy metals
In the new study, Yale University geochemist Noah Planavsky and his colleagues analyzed levels of molybdenum and iron in 2.95-billion-year-old rocks from South Africa. The rocks were laid down in water, in a shallow ocean setting near the shore. The metals serve as markers of photosynthesis. Molybdenum isotopes, or elements with the same number of protons but a different number of neutrons, track manganese oxidization, a process that requires high levels of oxygen, Planavsky said.
The chemical traces in the rocks, from the Pongola Supergroup, indicate cyanobacteria were producing oxygen in ocean surface water, Planavsky said. "Our study is telling you that there was localized cyanobacteria production in the oceans," he told Live Science's Our Amazing Planet.
In another recent study, also on South Africa's Pongola rocks, scientists looked at chromium isotopes to estimate atmospheric oxygen levels 3 billion years ago. The results suggest atmospheric oxygen was about 100,000 times higher than could be explained by non-biological chemical reactions, according to the research, published Sept. 26, 2013, in the journal Nature.
"The two studies are quite complementary," Planavsky said. "We're providing independent evidence of the presence of cyanobacteria. We're tracking surface ocean processes and they're tracking terrestrial processes."
However, Woodward Fischer, a geobiologist at Caltech in Pasadena, Calif., cautions that the trace metal techniques need further validation. Both analytic methods are just about a decade old and are being tested in extremely old rocks. "The quality of our interpretations derived from them remains a little bit uncertain," said Fischer, who was not involved in either study. "In all fairness, we don't understand the molybdenum and the chromium cycle today."
Which came first?
As more sensitive techniques emerge for peering into deep time, a new debate has surfaced: Did microbes pump our planet's first breath, or did environmental changes push the planet into oxygen richness?
Emerging evidence suggests oxygen levels took a roller coaster ride in the 500 million years between when the first cyanobacteria evolved photosynthesis and the Great Oxidation Event. That's a long time for life — it's about the same as the time between Earth's first trilobites and humans.
Some researchers think Earth itself played a role in boosting oxygen levels as continents grew in size. Erosion of the crust and the changing nature of volcanoes — bigger continents mean more land-based eruptions spewing out gas into the atmosphere, rather than underwater blasts. These geologic shifts could have pushed Earth's atmosphere toward oxygen in concert with the cyanobacteria.
"What's really exciting about this is the relative role of biological evolution versus geological evolution in the major turning points in Earth's history," Planavsky said. "That's what's driving our research."
Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook and Google+. Original article at Live Science's Our Amazing Planet.