Skeletonema Costatum Extract

Skeletonema Costatum Extract is a marine microalgae (diatom) extract typically used at low concentrations for antioxidant/soothing or conditioning claims, but it is a complex botanical mixture rather than a single inert molecule. While broad clinical irritation data are limited, algae extracts can trigger stinging or dermatitis in reactive/eczema-prone users due to variable proteins/polysaccharides and potential trace contaminants from cultivation, making occasional sensitivity plausible. Given the uncertainty and patient-safety priority, I rate it as mild irritancy rather than assuming it is universally gentle. Safety Notes: In commercial skincare, Skeletonema costatum (a marine microalgae/diatom) extract is most often used as a low-dose “active” within blends, with many leave-on serums/creams listing it at trace-to-low levels around 0.001–0.05% to support antioxidant/soothing claims while maintaining color/odor and stability. Higher-strength consumer products (typically leave-on ampoules, concentrates, or “microalgae” booster serums) can reach ~0.5–2.0% when the extract is supplied as a usable solids content active; above this is uncommon in OTC due to cost, sensory impact, and formulation stability constraints. Rinse-off products generally sit toward the low end because of short contact time and foam/surfactant compatibility.