(Photo : krugerpark)

Hippos are considered one of the most dangerous animals in Africa and have been feared by most. However, they are in danger of extinction because of poachers who are after their ivory teeth.

Recently, a team led by biologist Jonas Schoelynck of the University of Antwerp in Belgium discovered that "water horses", as they're also called, transport nearly half a metric ton of silicon, an important nutrient for both plants and animals, from land to water (as published online in Science Advocates). The group discovered that each day, a hippo graze grasses in the savanna and consumes more than 800 kilograms of silicon via plants, half of which is, in turn, excreted as it stays lounging in the water.

One might ask, why this is noteworthy. 'Having more silicon available is particularly important for tiny floating alga called diatoms to build their silica shells' (SN: 7/17/04, p. 42), the researchers note. Diatoms are a type of plankton that carries out photosynthesis in ocean and freshwater ecosystems. Along with terrestrial plants, they reduce the amount of carbon dioxide in the atmosphere why converting them into sugar and oxygen. Almost a quarter of all photosynthesis on our planet is carried out by diatoms.

These single-celled algae are a type of plankton and turn sunlight into chemical energy by way of photosynthesis, thus they're a very important part of ocean and freshwater ecosystems as well. By fixing carbon or converting it from carbon dioxide into sugar, diatoms also reduce the amount of carbon dioxide in the atmosphere just as terrestrial plants do. Somewhere between a fifth and a quarter of all photosynthesis on our planet is carried out by diatoms which means that more than a quarter of Earth's oxygen comes from diatoms. Since humans and all other animals need oxygen to breathe, we all rely indirectly on diatoms to sustain us.

Furthermore, in oceans, diatoms are eaten by tiny animals called zooplankton which in turn sustain larger animals, without them, the ocean would be unable to support the amount of life that it does.

One of the most unusual features of diatoms is their silica-based shells. When diatoms die, their shells fall to the bottom of the water body they inhabit and accumulate as sediment. When mined, these deposits can be used as a filter and abrasive, or for pest control. Another very important use of diatoms is that when they are compacted beneath sediment they may also, over time, become compressed to form oil that may be used for fuel.

The "Hippo poop" discovery was done by analyzing ratios of two silicon isotopes - versions of the element with different masses - in grasses, hippo feces, soil, and waters. Those ratios are modified by different biological and chemical processes, so can act as fingerprints for the different sources of silicon. According to the team the animals "pumped" 0.4 metric tons of silicon from the grasslands into the Mara River daily (where they lounge), increasing the total amount of silicon in the water by more than 76 percent.

Given their threatened status, if Hippos were to decrease significantly or vanish altogether, diatom growth in the water could decrease dramatically which could, in turn, cause a cascade of food shortages across the ecosystem.