Prochlorococcus

Prochlorococcus is the smallest known photosynthetic organism—a cyanobacterium streamlined to the absolute minimum needed for survival in the nutrient-poor open ocean. At just 0.5-0.7 micrometers in diameter, it's barely visible under a light microscope. Yet Prochlorococcus may be the most abundant photosynthetic organism on Earth, with populations exceeding 10^27 cells globally. It produces an estimated 20% of the oxygen in Earth's atmosphere—every fifth breath you take comes from this invisible organism.

The secret to Prochlorococcus's success is extreme efficiency. Its genome is among the smallest of any free-living organism, having jettisoned every gene not essential for photosynthetic life in stable ocean waters. It doesn't waste resources on motility, elaborate defenses, or metabolic versatility—it just converts light into biomass with minimal overhead. This minimalist strategy works brilliantly in the stable, nutrient-poor open ocean where specialization beats flexibility.

Prochlorococcus populations stratify by light intensity. High-light-adapted strains dominate surface waters; low-light-adapted strains dominate deeper zones. Different strains coexist by partitioning the water column, each optimized for its specific depth. This niche partitioning enables astronomical total population sizes without direct competition between strains. The organism demonstrates how extreme specialization for specific conditions enables dominance within those conditions, even if it means inability to survive elsewhere.