Turquoise Sea

Launched in February 2024, NASA's PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) satellite carries the OCI (Ocean Color Instrument), a hyperspectral radiometer covering 285 spectral bands from ultraviolet to shortwave infrared. Unlike previous ocean-color sensors, OCI can fingerprint specific phytoplankton species by their pigment signatures alone — turning the world ocean into a living map of microscopic life. On June 22, 2026 OCI resolved the Black Sea bloom in unprecedented detail, capturing the reflectance peak of calcium carbonate at 443–490 nm.

Emiliania huxleyi and its relatives are single-celled algae roughly 5 μm in diameter — forty would fit across a human hair. Each cell surrounds itself with 10–30 disc-shaped calcium carbonate (CaCO₃) scales called coccoliths arranged in a protective sphere. The intense white reflectance of billions of these calcium plates collectively turns the sea a milky turquoise-cyan visible from orbit, sometimes spanning areas larger than the British Isles. An astronaut aboard the ISS also photographed the 2026 bloom swirling through the Bosphorus into the Sea of Marmara.

Every June–July, the Black Sea thermally stratifies: warm surface water traps nutrients from the cold layer below, triggering massive Emiliania huxleyi blooms. The blooms spread across 50,000–100,000 km² — comparable to Iceland — and are so regular that scientists use them as an annual calibration target for satellite ocean-color sensors. The 2026 bloom extended into the Bosphorus, the narrow waterway running through Istanbul, linking the Black Sea with the Sea of Marmara.

Coccolithophore blooms run two opposing carbon processes simultaneously. Photosynthesis pulls CO₂ from seawater and locks it as organic carbon. But CaCO₃ plate formation actually releases CO₂ (2HCO₃⁻ → CaCO₃ + CO₂ + H₂O). The net result is a carbon sink: when cells die, their heavy plate-coated bodies sink rapidly — the 'carbonate pump' — carrying organic carbon to depths where it can remain stored for centuries. Estimates for a major Black Sea bloom run ~1.6 Tg of carbon sequestered per season.

In July 2026, the journal Astrobiology published a paper exploring whether the distinctive turquoise reflectance of a coccolithophore bloom could serve as a detectable ocean biosignature on an exoplanet. The calcium carbonate reflectance peak at 443–490 nm is spectrally distinct from mineral scatter or ice and would be distinguishable by a future large space telescope using reflected-light spectroscopy. PACE's Black Sea data provides the ground-truth calibration dataset for that detection methodology.