First oceans may have been acidic

Acidity and alkalinity are measured on the pH scale of 0-14. On this scale, 7 is neutral, higher is alkaline, lower is acidic. At around 8.2, today’s oceans are mildly alkaline, and we know that rising CO2 levels are currently increasing the oceans’ acidity (decreasing pH).

Halevy, of the Weizmann Institute’s Earth and Planetary Sciences Department, explains that billions of years ago “the early Sun was dimmer, even though we don’t have evidence for a much colder climate. We think that this is because the early atmosphere had more of the greenhouse gas CO2 than at present, and that as the Sun got brighter, CO2 levels decreased,” says Halevy.

CO2, and water produce carbonic acid, so it stands to reason that the early oceans would have been more acidic. But higher early CO2 levels would also have resulted in acidic rainwater and this, in turn, could have led to higher rates of chemical weathering of Earth’s rocky crust, washing down ions that would partly neutralize the acidity of CO2. Which effect is the stronger? This has been unclear; thus previous models of the history of seawater pH have come up with everything from high values to low.

The model that Halevy and Bachan developed accounts for these processes and the way in which they influence the fluxes of ions into and out of ocean water. According to their model, the acidifying effect of higher CO2 levels dominated, and the early oceans had a lower-than-present pH.

“On a very fundamental level,” says Bachan, “we show that the pH of the ocean has been controlled by a few simple processes for all of geologic time.”

Putting numbers to the proposed pH, Halevy says that three to four billion years ago, the pH of ocean water was somewhere between 6.0 and 7.5 — between that of milk and human blood. Halevy: “This gives us some clues as to the conditions under which life emerged in the early oceans.”

“We had an early ocean more acidic than today in which primitive life thrived and chemical cycles were balanced; but if we want to apply this insight to today, we have to remember that this balance of acids and bases was maintained over geological timescales — millions of years,” he adds. “Today’s acidification from CO2 is much more rapid, so this model does not apply to the short-term problem. Hundreds of thousands of years from now, the oceans will have found a new balance, but between now and then, marine organisms and environments may suffer.”

African lake provides new clues about ancient marine life

UBC researchers travelled to Lake Kivu in the Democratic Republic of Congo, because of its similar chemistry to the oceans of the Proterozoic eon, some 2.3 to 0.5 billion years ago. The deep waters of part of the lake have no oxygen and are one of the few places on Earth where dissolved iron is present at high concentrations.

“This is the first time that we have observed microbes recycling nitrogen by reacting it with iron in such a body of water,” said Céline Michiels, lead author of the study and PhD student at UBC. “While these reactions have been observed in the lab, their activity in Lake Kivu gives us confidence that they can play an important role in natural ecosystems and allows us to build math models that can describe these reactions in oceans of the past.”

Michiels and her colleagues found that when microorganisms from Lake Kivu react iron with nitrogen in the form of nitrate, some of this nitrogen is converted to gas, which is lost to the atmosphere, but the rest of the nitrogen is recycled from nitrate to ammonium, which remains dissolved and available for diverse microorganisms to use as a nutrient.

The research team used math models, informed by data collected from lake Kivu, to learn more about how this recycling could have affected life in the oceans during the Proterozoic eon. They learned that biological activity was not limited by the availability of nitrogen, as previously thought, but rather was likely limited by another key nutrient, phosphorus. Nutrient availability would have played an important role in shaping the nature and activity of life in the oceans at this time, thus setting the stage for the evolution of multicellular life and Eukaryotes.

“It’s really exciting that we can use information recovered from modern environments like Lake Kivu to create and calibrate math models that reconstruct chemistry and biology from almost two billion years ago,” said Sean Crowe, senior author of the study and Assistant Professor and Canada Research Chair in Geomicrobiology at UBC. “With these models and clues from rocks, we’re learning more and more about how evolving life in the ancient oceans shaped Earth’s surface chemistry over long stretches of early history.”

This research was part of the East African Great Lakes Ecosystem Sensitivity to changes project, a broader initiative to study microbial ecology in African Great Lakes, led by Belgium researchers François Darchambeau, of the Université de Liège, and Jean-Pierre Descy, of the Université of Namur.

Vision, not limbs, led fish onto land 385 million years ago

Neuroscientist and engineer Malcolm A. MacIver of Northwestern and evolutionary biologist and paleontologist Lars Schmitz of Claremont McKenna, Scripps and Pitzer colleges studied the fossil record and discovered that eyes nearly tripled in size before — not after — the water-to-land transition. The tripling coincided with a shift in location of the eyes from the side of the head to the top. The expanded visual range of seeing through air may have eventually led to larger brains in early terrestrial vertebrates and the ability to plan and not merely react, as fish do.

“Why did we come up onto land 385 million years ago? We are the first to think that vision might have something to do with it,” said MacIver, professor of biomedical engineering and of mechanical engineering in the McCormick School of Engineering.

“We found a huge increase in visual capability in vertebrates just before the transition from water to land. Our hypothesis is that maybe it was seeing an unexploited cornucopia of food on land — millipedes, centipedes, spiders and more — that drove evolution to come up with limbs from fins,” MacIver said. (Invertebrates came onto land 50 million years before our vertebrate ancestors made that transition.)

The enlargement of eyes is significant. By just popping those eyes above the water line, the fish could see 70 times farther in air than in water. With the tripling of eye size, the animal’s visually monitored space increased a millionfold. This happened millions of years before fully terrestrial animals existed.