NATURAL catastrophes such as asteroid impacts, massive volcanic eruptions or large-scale wildfires would have periodically plunged our planet into abnormal darkness. How did life survive without the sun’s life-giving rays during such episodes? With a little help from organisms that can switch to another source of energy while they wait for sunlight to pierce the darkness once more.
To figure out how organisms might have endured periods of so-called “catastrophic darkness”, Charles Cockell of the Open University’s Centre for Earth, Planetary, Space and Astronomical Research in Milton Keynes, UK, and his team placed samples of both freshwater and marine microorganisms in darkness for six months – a period similar to what might be expected following a catastrophic event. The samples included phototrophs, which convert sunlight into usable energy, and mixotrophs, which can use sunlight or consume dead organic matter.
The team found that the phototrophic species struggled to survive, with the majority of individuals dying off. The few that survived in a dormant state managed to repopulate when light returned.
Mixotrophs, however, seemed to thrive in the darkness. They even offered a helping hand to their light-dependent cousins: when the lights went out, the mixotrophs were able to switch to getting their energy from dead creatures and plants, and in doing so they kept the nutrients turning over. This improved the conditions for phototrophic recovery when the samples were returned to light
The results show that, contrary to common belief, catastrophic darkness does not completely destroy phototrophic organisms, says Cockell. “The photosynthetic biosphere is much more robust than generally assumed.”