Syracuse Sun

The atmosphere, the ocean, and life on Earth have interacted over the past 500-plus million years in ways that improved conditions for early organisms to thrive. An interdisciplinary team of scientists has now produced a perspective article on this co-evolutionary history, published in the multidisciplinary open-access journal National Science Review (Oxford University Press, Impact Factor 20.7).

“One of our tasks was to summarize the most important discoveries about carbon dioxide and oxygen in the atmosphere and ocean over the past 500 million years,” says Zunli Lu, Thonis Family Professor: Low-Temperature Geochemistry and Earth System Evolution in the College of Arts and Sciences and lead author on the paper. “We reviewed how those physical changes affected the evolution of life in the ocean. But it’s a two-way street. The evolution of life also impacted the chemical environment. It is not a trivial task to understand how to build a habitable Earth over long time scales.”

The team from Syracuse University, Oxford University, and Stanford University explored intricate feedbacks among ancient life forms, including plants and animals, and their chemical environment during the current Phanerozoic Eon, which began approximately 540 million years ago.

At the start of the Phanerozoic, carbon dioxide levels in the atmosphere were high while oxygen levels were low—a condition challenging for many modern organisms. Ocean algae played a crucial role by absorbing atmospheric carbon dioxide, locking it into organic matter, and producing oxygen through photosynthesis.

The ability of animals to live in an ocean environment was influenced by oxygen levels. Lu is studying where and when ocean oxygen levels may have risen or fallen during the Phanerozoic using geochemical proxies and model simulations. Co-author Jonathan Payne, professor of Earth and planetary sciences at Stanford University, compares ancient animals' estimated metabolic requirements with places where they survived or disappeared in fossil records.

As photosynthetic algae removed atmospheric carbon into sedimentary rocks to lower carbon dioxide levels while raising oxygen levels, their enzymes became less efficient at fixing carbon. Algae adapted by creating internal compartments for photosynthesis with controlled chemistry.

“For algae, it is changes in the environmental ratio of O2/CO2 that seems to be key to driving improved photosynthetic efficiency,” says co-author Rosalind Rickaby, professor of geology at Oxford. “What is really intriguing is that these improvements in photosynthetic efficiency may have expanded the chemical envelope of habitability for many forms of life.”

Ancient photosynthesizers had to adapt to changes in their self-created physical environment notes Lu: “The first part of the history of the Phanerozoic is increasing habitability for life; then the second part is adaptation.”

To further understand this interplay between life and its physical environment as well as drivers and limits on habitability, mapping out spatial patterns of ocean oxygen along with biomarkers for photosynthesis and metabolic tolerance shown in fossil records will be key future research directions.

This story was written by John H. Tibbetts