A cosmic gamma-ray burst striking the Earth could be harmful to ocean plankton at depths of up to 75 m, according to a team of Cuban researchers. These organisms account for up to 40% of the ocean's photosynthesis, so such an event could have a serious impact on Earth's carbon dioxide levels.

Gamma-ray bursts (GRBs) are the most luminous electromagnetic events known to occur in the universe, releasing up to 1044 J of gamma-ray energy in a narrow beam over several seconds. They come in two types, long and short, with the former the most common and thought to be caused by the core-collapse of a supernova. To date the GRBs observed have been in distant galaxies and not our own Milky Way. However, some researchers believe that a GRB was responsible for the Ordovician mass extinction approximately 450 million years ago.

With this in mind, a team of biologists and physicists at the Central University of Las Villas, in Santa Clara, Cuba have modelled what might happen should a nearby GRB – about 6000 light-years away – strike Earth today. "Our wish was to link astrophysics with environmental science, which is quite an unexplored area. We wanted to know how stellar explosions might affect the evolution of life on Earth," physicist Rolando Cárdenas told physicsworld.com.

Ripping electrons

The danger for plankton living in the oceans would not be the gamma-rays themselves but the flashes of ultraviolet (UV) radiation caused by the interaction of the gamma-rays with the atmosphere. The initial arrival of gamma-rays from the GRB would rip electrons from gas molecules. These electrons would then excite other molecules creating an emission of UV energy. According to Cárdenas about 1–10% of the incident gamma-ray energy reaches the ground in the form of UV light and has the potential to damage plankton. The reminder comes in the form of visible or infra-red light which is less dangerous to life.

In order to model the effect of this UV radiation, the team examined the typical albedo for the Earth's oceans in order to calculate the UV spectrum at different depths. They also took into account the optical quality of the water because not all oceans are as clear as others. Combining these with several other factors they found that a UV flash could penetrate up to 75 m in clear water, damaging a crucial enzyme required for photosynthesis and well as causing the plankton to divert energy from photosynthesis to repairing damaged DNA.

This suppression of the plankton's ability to photosynthesize could have a profound effect on the Earth's climate. Carbon dioxide is consumed in great quantity by ocean-based plankton with just one species, Prochlorococcus marinus, accounting for 20% of the entire biosphere's photosynthesis. The plankton are also the first link in many of the ocean's food chains and their demise at the hands of a GRB would have a knock-on effect all the way up the food chain.

Rare in metal-rich galaxies

However, GRBs are rare in galaxies like the Milky Way. "The most likely explanation for this is that the Milky Way is more metal rich – with many elements heavier than helium – and GRBs occur less in metal rich environments," explains Andrew Levan, a GRB researcher at the University of Warwick, UK. Despite this rarity, a GRB strike on Earth isn't that far fetched. "GRBs are likely to happen in our galaxy around every 10 million years or so. To affect the Earth it would have to be lined up with us and not too far away. However, it is plausible that over the Earth's 4.5 billion year history we could be affected by a GRB," Levan added.

The findings have been accepted for publication in Astrophysics and Space Science and a pre-print is available on arXiv.