Low recycling of the key nutrient phosphorus may have limited the amount of biomass—life—in Earth’s ancient oceans, new research suggests.
The research also comments on the role of volcanism in supporting Earth’s early biosphere—and may even apply to the search for life on other worlds.
The researchers’ aim was to use theoretical modeling to study how ocean phosphorus levels have changed throughout Earth’s history, says lead author Michael Kipp, a doctoral student in earth and space sciences at the University of Washington.
“We were interested in phosphorus because it is thought to be the nutrient that limits the amount of life there is in the ocean, along with carbon and nitrogen,” says Kipp. “You change the relative amount of those and you change, basically, the amount of biological productivity.”
Kipp says their model shows the ability of phosphorus to be recycled in the ancient ocean “was much lower than today, maybe on the order of 10 times less.”
Life requires food
All life needs abundant food to thrive, and the chemical element phosphorus—which washes into the ocean from rivers as phosphate—is a key nutrient. Once in the ocean, phosphorus gets recycled several times as organisms such as plankton or eukaryotic algae that “eat” it are in turn consumed by other organisms.
“Every gardener knows that their plants grow only small and scraggly without phosphate fertilizer…”
“As these organisms use the phosphorus, they in turn get grazed upon, or they die and other bacteria decompose their organic matter,” says Kipp, “and they release some of that phosphorus back into the ocean. It actually cycles through several times,” allowing the liberated phosphorus to build up in the ocean. The amount of recycling is a key control on the amount of total phosphorus in the ocean, which in turn supports life.
“Every gardener knows that their plants grow only small and scraggly without phosphate fertilizer. The same applies for photosynthetic life in the oceans, where the phosphate ‘fertilizer’ comes largely from phosphorus liberated by the degradation of dead plankton,” explains Roger Buick, a professor of earth and space sciences who advised the researchers.
But all of this requires oxygen. In today’s oxygen-rich oceans, nearly all phosphorus gets recycled in this way and little falls to the ocean floor. Several billion years ago, in the Precambrian era, however, there was little or no oxygen in the environment.
A ‘canned environment’
“There are some alternatives to oxygen that certain bacteria could use, says coauthor Eva Stüeken, a research fellow at the University of St. Andrews. “Some bacteria can digest food using sulfate. Others use iron oxides.” Sulfate, she says, was the most important control on phosphorus recycling in the Precambrian era.
“Our analysis shows that these alternative pathways were the dominant route of phosphorus recycling in the Precambrian, when oxygen was very low,” Stüeken says. “However, they are much less effective than digestion with oxygen, meaning that only a smaller amount of biomass could be digested. As a consequence, much less phosphorus would have been recycled, and therefore total biological productivity would have been suppressed relative to today.”
Kipp likens early Earth’s low-oxygen ocean to a kind of “canned” environment, with oxygen sealed out: “It’s a closed system. If you go back to the early Precambrian oceans, there’s not very much going on in terms of biological activity.”
Stüeken notes that volcanoes were the biggest source of sulfate in the Precambrian, unlike now, and so they were necessary for sustaining a significant biosphere by enabling phosphorus recycling.
In fact, minus such volcanic sulfate, Stüeken says, Earth’s biosphere would have been very small, and may not have survived over billions of years. The findings, then, illustrate “how strongly life is tied to fundamental geological processes such as volcanism on the early Earth,” she says.
The search for alien life
Kipp and Stüeken’s modeling may have implications as well for the search for life beyond Earth.
Astronomers will use upcoming ground- and space-based telescopes such as the James Webb Space Telescope, set for launch in 2019, to look for the impact of a marine biosphere, as Earth has, on a planet’s atmosphere. But low phosphorus, the researchers say, could cause an inhabited world to appear uninhabited—making a sort of “false negative.”
Kipp says, “If there is less life—basically, less photosynthetic output—it’s harder to accumulate atmospheric oxygen than if you had modern phosphorus levels and production rates. This could mean that some planets might appear to be uninhabited due to their lack of oxygen, but in reality they have biospheres that are limited in extent due to low phosphorus availability.
“These ‘false negatives’ are one of the biggest challenges facing us in the search for life elsewhere,” says Victoria Meadows, astronomy professor and principal investigator for the NASA Astrobiology Institute’s Virtual Planetary Laboratory, based at the University of Washington.
“But research on early Earth’s environments increases our chance of success by revealing processes and planetary properties that guide our search for life on nearby exoplanets.”
The researchers report their findings in the journal Science Advances. Grants from NASA and the National Science Foundation funded the work.
Source: University of Washington