The ocean's most mysterious bacteria are finally emerging from the dark

Gardening · Published 3 October 2026 Swipe more like this →

In short: For years, the SAR202 clade remained an enigma, dominating deep-sea microbial populations while refusing to grow in laboratory settings. Now, researchers have successfully cultivated these elusive bacteria, revealing how they evolved to harvest energy from complex organic molecules in the ocean's depths as the Earth's carbon cycle expanded.

The SAR202 clade, a group within the phylum Chloroflexota, represents a significant portion of planktonic prokaryotes in the deep ocean, sometimes accounting for 10–30% of the total population. Despite their ubiquity, they were long considered one of the most wanted targets for cultivation because their unique metabolic properties could not be accurately predicted from genomic data alone. A recent study successfully retrieved twenty-four isolates of SAR202 subclade I from surface seawater samples near the Yellow Sea, marking the first time these microorganisms have been cultured.

The cultivation process involved dilution-to-extinction experiments conducted in continuous darkness. These isolates revealed a slow growth rate and a distinct sensitivity to light, which explains why they are more abundant in the dark, meso-, bathy-, hadal-, and abyssopelagic zones. While some SAR202 groups possess rhodopsin genes that allow them to thrive in epipelagic environments, the group III isolates lack these genes and are primarily responsible for the clade's high relative abundance in the deep ocean.

Metabolic analysis suggests that SAR202 bacteria evolved alongside the rise of oxygenic phototrophy. They developed large expansions of paralogous genes—such as flavin-dependent monooxygenases and enolase protein superfamilies—to metabolize diverse and complex organic compounds. These evolutionary adaptations allowed them to occupy a specific niche, harvesting dilute carbohydrate-related molecules that resist biological oxidation. By successfully isolating these strains, scientists can now move beyond genome-based inference to experimentally confirm how these enigmatic cells function within the modern ocean's carbon cycle.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• Life in the deep sea thrives in darkness, fueled by chemicals rather than sunlight
• Microbes in the deep sea are essential to the health of our planet
• Deep beneath the Pacific, scientists have revived microbes dormant for one hundred million years.
• The ancient, microscopic engine driving our planet's oxygen and life
• The midnight zone is the largest habitat on Earth, yet remains largely a mystery.
• The spiral-shaped algae found on every continent, including the frozen wastes of Antarctica
• The water mold that triggered a famine and still threatens global potato crops
• The parasite that shaped African history by turning the tsetse fly into a gatekeeper.
• Why your garden mushrooms are actually closer relatives to you than to plants
• The silent parasite: how a tiny bug bite can alter your heart for decades
• These shapeshifting organisms can grow to one metre across and weigh twenty kilograms.
• Why yellow-green algae are a warning sign for our planet's aging aquatic ecosystems

More in Other eukaryotes · More in Gardening · Latest · A–Z · All topics