The Earth is constantly struck by radiation coming from the interstellar medium. The very low energy end of the spectrum is shielded by the geomagnetic field but charged particles with energies higher than the geomagnetic cutoff will penetrate the atmosphere and are likely to interact, giving rise to secondary particles. Some astrophysical events, such as γ-ray bursts and supernovae, when happening at short distances, may affect the planet's biosphere, due to the temporary enhanced radiation flux. Muons are abundantly produced by high-energy cosmic rays in the Earth's atmosphere. These particles, due to their low cross-section, are able to penetrate deep both underground and underwater, with the possibility of affecting biological niches normally considered shielded from radiation. We investigate the interaction of muons produced by high-energy cosmic rays on the Earth's atmosphere using the Geant4 toolkit. We analyse its penetration power in water and crust and also the interaction effects within bacteria-like material according to the particle type and energy, and noticed the possibility of off-track damage due to secondary particles.

We modify a mathematical model of photosynthesis to quantify the perturbations that high energy muons could make on aquatic primary productivity. Then, we apply this in the context of the extragalactic shock model, according to which Earth receives an enhanced dose of high-energy cosmic rays when it is at the galactic north. We obtain considerable reduction in the photosynthesis rates, consistent with potential drops in biodiversity.