The Sun gives life to our planet through its rays, and yet some fascinating lifeforms don't need its light to live.

Instead of using photosynthesis to store energy in their chemical bonds, some microbes rely purely on the oxidation of inorganic molecules like hydrogen to do the trick.

Chemosynthesis, as it's known, was speculated as a potential source of energy for microbes in the 19th century, though it wasn't confirmed until ecosystems surrounding deep ocean hydrothermal vents were discovered in the 1970s.

Since then, the means of harvesting energy through the oxidation of inorganic compounds has been considered rare, confined to extreme habitats.

But emerging research from the sea suggests this strategy for survival is actually abundant, from pole to pole.

In fact, as sunlight fades to darkness, a team led by researchers from Monash University in Australia has found evidence that chemosynthesis becomes the primary way of life for invisible marine microbes.

"Hydrogen and carbon monoxide in fact 'fed' microbes in all regions we've looked at: from urban bays to around tropical islands to hundreds of meters below the surface," says Monash University microbiologist Chris Greening.

"Some can even be found beneath Antarctica's ice shelves."

Unlike sunlight, molecular hydrogen is a convenient source of energy that is present in at least trace amounts across a wide variety of ecosystems, from the atmosphere to the surface and even below.

In previous studies, Greening and his colleagues have shown that in a lot of the world's soil, bacterial cultures that can consume hydrogen are "abundant, diverse and active" – in many ways, the base of the entire food chain.

Now, he and some of the same researchers have shown this applies to the deep ocean as well.

Their study is the first to investigate whether bacteria in the open ocean can use hydrogen for fuel. The findings are based on 14 seawater samples, gathered from the Atlantic, the Indian, the Pacific, and the Southern Oceans.

In all but one of these samples, the team detected microbes that had the genetic machinery required to both chemosynthesize using hydrogen and photosynthesize.

Based on the activity of microbes in the lab, models suggest their rate of chemosynthesis is enough to sustain the community's growth and survival.

Hydrogen, the authors conclude, must be an important energy source for bacteria in seawater, especially for those that live in the deepest, darkest depths.

The oxidation of hydrogen is useful when sunlight is not readily available, but it's not without its costs. It requires an investment of iron in a context where iron is already a precious commodity. This means marine bacteria probably only use hydrogen as fuel when absolutely necessary.

At the surface of the ocean, it's probably much more valuable for microbes to rely on sunlight. In the darkness, however, a switch could be flipped. Nearer the seafloor, iron is more available while sunlight becomes scarce.

Marine bacteria that can switch between chemosynthesis and photosynthesis would probably have a major competitive advantage when populating all the various levels of ocean habitat.

That's probably why these flexible lifeforms are still so abundant to this day.

"The first life probably emerged in deep-sea vents using hydrogen, not sunlight, as the energy source," speculates Greening.

"It's incredible that, 3.7 billion years later, so many microbes in the oceans are still using this high-energy gas and we've completely overlooked this until now."

The study was published in Nature Microbiology.