Twenty meters beneath the frosty surface of Antarctica lies Lake Vida, a thin pool of salty slush that, against all expectations, teems with life.
The presence of life under such extreme conditions is an assemblage of amazings. The temperature of the pool is -13°C. The water is 19% salt, over five times the concentration of the ocean.
And most incredibly, an ice cap has sealed off the lake from the rest of the earth’s ecosystem for almost 3000 years. All light is blocked by 20 meters of dirty snow. Organic matter – otherwise known as “food” – can neither enter nor leave.
Without food or energy, how do its resident microbes survive?
Lake Vida, Antarctica. Image NASA Ames/Chris McKay
The answer is that the Earth is a giant battery that powers life. And if that sounds weird, it helps to first dwell a bit on the impossible story of the lake itself.
Roll back the clock around 5000 years: At the time an unknown human hero invents the first wheel, a minor change in the Earth’s orbit sets off a slow cooling. Over the next two thousand years water in Antarctica’s lakes gradually crystallizes into ice, leaving a salty brine behind. Snow falls, and the brine – now too salty to freeze – is sequestered from the rest of the planet under a shell of ice.
This story is not fiction – we know the broad scope of the tale from global climate records. And carbon 14 – which is formed in the atmosphere when a cosmic ray transmutes nitrogen atoms to carbon – stopped collecting in the lake when it sealed. Living, respiring bacteria look to a carbon 14 test as though they have been dead for 2800 years. But they haven’t been dead, just hiding.
It is almost certain that not all bacteria survived the changes – once the oxygen supply was cut off, the aerobic bacteria quickly perished. But according to water samples, at least 32 different bacteria made the transition and started a new life “off the grid”.
But what did they eat for the last 2800 years of isolation? Protein bars?
The bacterial equivalent of protein bars is other, dead bacteria and plants. But even accounting for the slow metabolism of life at -13°C, there would not be enough food to keep this population thriving for millennia.
Instead, the most likely scenario is that the bacteria are hydrogen-powered. Exactly where the hydrogen originates we can’t pinpoint precisely, but it permeates the lake, and it does not appear to be generated by the bacteria living there. Since entry from the atmosphere above is blocked, the hydrogen is probably coming from the rocks below. This hydrogen becomes food for several bacterial strains, who in turn become food for the rest. The wheel of life is kept in motion, powered by a constant supply of hydrogen.
Microbes look just like this when they are hungry.
The geologic production of hydrogen is a well-known – geologists use the horrifyingly ungainly term “serpentinization”, in reference to the green colors of the most common rocks involved. The hydrogen (and some related chemicals like methane) produced by serpentinization feed small ocean ecosystems across the globe.
And there is a good chance that this reaction provided the food for the first life on this planet. That is where the battery metaphor comes in.
The Earth is not uniform stuff – the heavy elements, like iron and lead and gold, have sunk to the hot interior, while the lighter elements like oxygen and hydrogen have bobbed to the cold surface. When the heavy and light come together they react, especially when it is hot.
And each group of elements reacts their own particular way – the heavy elements tend to give up electrons, while the lighter elements receive them (it’s a chemistry thing). This forms our battery: Rocks in the mantle acts like one electrode, and the water a second. When there is a fissure in the earth’s crust, hot rock touches water, and electrons hop from one to the other. When electrons are added to water, hydrogen forms. And becomes food for life.
The Earth-battery metaphor isn’t perfect – the earth doesn’t have giant wires stuck into it to collect electrons as electrical current. Instead, the electrons react with water molecules to generate hydrogen. The process usefully transports the energy from the planet’s core – where it is hidden from life – to its surface, where it can be consumed.
Food is, after all, just stored energy. On its own, the heat of the Earth’s interior makes lousy food — it spreads out quickly, and can’t be stored. Hydrogen is nature’s D cell, a tight container that carries energy from the planet’s hot interior back to the cold, wet world of the surface. Microbes such as the ones in Lake Vida have evolved mechanisms to release this energy in a controlled way, pouring it just into the right parts of the cell to sustain life.
When scientists talk about water being special and necessary for life, this is one of the many reasons why. The serpentine rocks that provide food for the microbes in Lake Vida are likely to be quite common in the universe – we have found them on asteroids and on Mars, and expect them to be on the Jovian moon Europa and any other place that rock is.
And a large enough supply of cold water and hot rock provides a big enough battery to power life for thousands, millions, or even billions of years. No light needed.