The Ocean’s Hidden Engine: Urea Powers the Open Sea

For decades, oceanographers viewed the open ocean as a biological desert—vast, nutrient-poor, and struggling to sustain life compared to the bustling, nutrient-dense coasts. We assumed the microscopic engines driving these waters, specifically ammonia-oxidizing archaea (AOA), ran solely on ammonium. But that view is outdated. We have discovered a hidden fuel source that powers the vast blue expanse: urea.

The Great Divide

This research identifies a fundamental split in how marine microorganisms survive. In the nutrient-rich coastal waters, the genus Nitrosopumilus dominates, feasting on the abundant ammonium found near the shore. However, as we venture into the ammonium-depleted open ocean, the strategy shifts. Here, a different player, Nitrosopelagicus, takes the stage. Unlike its coastal cousin, Nitrosopelagicus possesses the distinct ability to utilize urea—an organic nitrogen compound—to fuel its growth and activity.

Fueling the Blue Desert

This metabolic flexibility changes our understanding of marine resilience. In the deep, open waters where ammonium is scarce, Nitrosopelagicus does not starve; it adapts. By unlocking the energy stored in urea, these archaea maintain high population densities even in 'desert' conditions. This mechanism explains how the open ocean sustains its microbial workforce without the constant ammonium supply found in coastal zones.

Planetary Life Support

The implications extend far beyond microbial taxonomy. These archaea are critical drivers of the marine nitrogen cycle, which in turn supports primary productivity—the creation of organic matter at the base of the food web. By recycling urea, Nitrosopelagicus effectively fertilises the open ocean, ensuring that these vast regions continue to function as a vital component of the Earth's biosphere. We are witnessing the ingenuity of life: where one fuel runs out, another is harnessed to keep the planetary engine running.