Weather makers: How microbes living in the clouds affect our lives

Clouds are more than just water vapor floating in the sky; they are bustling ecosystems filled with trillions of microscopic organisms such as bacteria, fungi, viruses, and single-celled life forms. These tiny creatures, collectively known as the aerobiome, travel vast distances high in the atmosphere and play crucial roles in weather patterns and even human health. Scientists have discovered that clouds act as floating islands of life, hosting thousands of species that originate from various parts of the Earth, including oceans, forests, and urban areas. For example, when ocean waves crash, they spray fine droplets of seawater into the air, carrying microbes with them. Similarly, winds can lift dust and microbes from the land, and forest fires can send microbes soaring into the atmosphere. Many plants and fungi have evolved special mechanisms to release spores or pollen into the air, allowing them to travel great distances.

Research into the aerobiome has advanced significantly thanks to modern technology. Scientists collect samples from high-altitude locations like the Puy de Dôme mountain in France, where clouds often cover the peak. By analyzing the DNA in cloud water, researchers have identified thousands of microbial species, many previously unknown. These microbes survive the harsh conditions of clouds, which are cold, wet, and nutrient-poor environments. Some bacteria, such as Methylobacterium, can even use sunlight to break down organic carbon within cloud droplets, effectively 'eating' the clouds. Studies comparing cloud samples to clear air samples show that some microbes are active and growing while suspended in clouds, indicating that clouds can be a habitat where life thrives despite the extreme conditions.

Beyond their survival, these microbes influence the weather itself. The formation of rain and snow depends on ice crystals forming in clouds, a process that requires tiny particles to act as seeds for ice. Biological particles like bacteria and fungal spores are particularly effective at triggering ice formation, a process called nucleation. One bacterium, Pseudomonas, is especially good at this and is commonly found on plants. When it causes ice to form on leaves, it helps the bacteria spread into the air and back into clouds, creating a cycle that links plant life to cloud formation and precipitation. This symbiotic relationship suggests that life on Earth and the weather are deeply interconnected. Scientists have even speculated that similar microbial life might exist in the clouds of other planets, such as Venus, where surface conditions are too harsh for life but the clouds might be more hospitable.

However, the presence of microbes in the atmosphere also raises concerns about the spread of antibiotic resistance. Humans have used antibiotics extensively, leading to the evolution of bacteria that can resist these drugs. These resistant bacteria and their genes can escape into the environment and become airborne, traveling long distances in clouds and wind. Studies have found high levels of antibiotic resistance genes in airborne bacteria collected from cities and clouds around the world. This means that clouds could be spreading antibiotic resistance far beyond where it originally developed, posing a serious challenge to public health. Each year, trillions of resistance genes travel through the atmosphere, highlighting the complex and sometimes troubling ways in which human activity affects the natural world. Next time you gaze at the clouds, remember they are not just weather phenomena but living ecosystems that connect the Earth’s surface, the atmosphere, and even human health in surprising ways.