Quirks & Quarks

Hippo poop provides a key mineral for vital algae's tiny skeletons

Silica from grass on land is vital to producing aquatic diatom's glassy shells

Silica from grass on land is vital to producing aquatic diatom's glassy shells

Hippo poop is rich in silica, a mineral that small unicellular algae called diatoms need to build their skeletons. (Chris Dutton)

Hippos contribute an enormous amount of "organic material" —dung, actually — to freshwater systems, which helps support small organisms in complex African ecosystems. But it turns out that they also move an important mineral from land to water that helps support the base of the food chain.

A new study has tracked how the silica they eat in grasses and other plants, and then defecate into the water, is harvested to build the skeletons of microscopic diatoms, a group of algae essential to the African lake ecosystems.

Living the life

Jonas Schoelynck, the biologist who led the research, looked at hippos living in the Masaai Mara Nature Reserve in southern Kenya, observing the way they move land resources into the water.

Hippo in the African Savannah (Amanda Subalusky)

They spend their evenings on land, feeding on kilograms of fresh grass, and their days in water, relaxing with other hippos.

"While they're lazying around, all the grass that was eaten during the night comes out after digestion and is pooped directly into the river," said Schoelynck.

Hippo pool (Chris Dutton)

The grass they eat is rich in silica, an essential building blocks of the glassy exoskeletons of microscopic aquatic algae. Silica is found in sand, rocks and glass, but also in biological organisms like grass.

Flush it all down

Once the silica leaves the hippo, it gets dissolved in the Mara River, and makes its way downstream to Lake Victoria for diatoms to use.

Diatoms found in the Senegal River are also present in Lake Victoria. (Bart Van de Vijver)

Diatoms are a group of microscopic single-celled algae. They dominate the base of the food web and use silica to build their cell walls. Globally, they contribute 25 per cent of the oxygen we breathe, and they are an important source of food for larger organisms.

"Because their skeleton is made of silica, they can outcompete other algae groups," said Schoelynck. But if silica is missing in the water, then other algae like blue green algae will take over, which can be toxic for animals and humans.

A different future

The study found that hippo dung in the Mara River contains tons of silica that make up 75 per cent of the total silica that gets flushed downstream.

The large herbivores are undoubtedly critical to the African ecosystem, but their numbers are going down as a result of poaching and habitat loss.

Without an adequate supply of silica, diatoms will die and it will devastate the lake ecosystem.

Schoelynck hopes his research will draw attention to the importance of silica in the ecosystem and the importance of hippos.

"[Hippos] are not only a cool tourist attraction, it's much more. It's an animal that's absolutely needed in the ecosystems of Africa."

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