Life’s essentials: sponges packed a lot of punch (Image: Todd Winner/Corbis)

If ever there was proof of the power of small things, surely this is it. Last year, came the suggestion that sponges transformed Earth’s deep oceans 750 million years ago, turning them into an oxygen-rich haven for life. Now it seems tiny bacteria living inside the sponges also played a part in the drama.

The discovery was an accident, says Fan Zhang of the University of Maryland in Baltimore. He and his colleagues happened to notice microscopic phosphate granules spread throughout the tissue of three different species of Caribbean sponges they had in the lab.

Living inside the sponges were bacteria that suck phosphorus out of seawater and precipitate it into granules, which Zhang’s team suggests may be used by sponges. Phosphorus is an essential but rare nutrient for ocean life. Like all marine organisms, sponges need it to survive but can’t extract it from the water – but the bacteria can help them out. Meanwhile, bacteria need the sponges to pump vast amounts of seawater over them so they can extract more phosphorus. So, Zhang says, the sponges and bacteria live in symbiosis.


Working together in this way, the two have evolved a hyper-efficient system for extracting phosphorus – the ultimate limiting factor for life in today’s oceans, given that it is one of essential building blocks of life’s molecules.

Early life link

Sponges and many of their symbiotic bacteria have been around for hundreds of millions of years, since the very early days of animal life on Earth.

Researchers suggested last year that sponges could have triggered key events in Earth’s history, some 750 million years ago, when life was evolving rapidly in the oceans.

Around that time it seems the deep oceans became rich in oxygen – a transformation traditionally credited with driving the Cambrian explosion, the sudden flourishing of all the different types of animal shapes we know today.

Nicholas Butterfield of the University of Cambridge and his colleagues proposed last year that early animals, specifically, sponges, led to a surge in deep ocean oxygen, not the other way around.

They point out that sponges can survive on very low levels of oxygen, so would have been kings in the early ocean. “What do sponges do? They are absolutely fantastic pumpers and filterers,” says Butterfield. “Invent something like a sponge [750 million years ago] and the world is their oyster. They have no competition. And now, for first time water is being pumped and filtered.”

Sudden slug

As the sponges pumped, they aerated the deep oceans with oxygen produced by photosynthesising organism floating in the upper layers, says Butterfield. Phosphorus in the water would have reacted with all that oxygen to form a hard phosphate precipitate that is not much use to living things. Thus the sponges and their microbiome may have been responsible for making phosphorus the ultimate limiting factor, by converting it into phosphate and so restricting the amount of the element available to oxygen-consuming life-forms, and hence further boosting oxygen levels in the deep.

Evidence for a sudden surge in phosphate is found in fossil deposits from around this time, many of which are preserved in the stuff. “There has never been a good explanation for why, in the first time in millions of years, we get a huge slug of phosphate,” says Butterfield.

He says Zhang’s study offers a compelling explanation for how the sponges and their symbiotic bacteria might have sequestered and regulated phosphate in the early ocean.

“It has largely been thought that early animals evolved rather passively in response to physical and chemical changes on the early Earth,” says Daniel Brady Mills of the University of Southern Denmark in Odense. “Studies like this suggest that early animals could have actually exerted significant controls over Earth’s ancient chemistry.”

Journal reference: PNAS, DOI: 10.1073/pnas.1423768112