Assessing the susceptibility of microalgae to space conditions using stratospheric balloons

Biological exposure experiments under near-space conditions remain limited by the high cost and low accessibility of spaceflight platforms, motivating low-cost analog approaches for initial screening. Stratospheric balloon flights offer a practical route to near-space exposure experiments relevant to space biology and astrobiology. Here we report an exploratory assessment of four green microalgae ( Chlamydomonas reinhardtii , Chlorella vulgaris , Scenedesmus obliquus , and Parachlorella kessleri ) flown in a custom-built ~ 2 kg payload equipped with heating, LED illumination, UV sensing, and GPS. Thirty-six microalgal samples (4 mL each) were flown during a ~ 2.5 h stratospheric mission, including ~ 30–60 min at 25–35 km altitude, within a total mission cost of < $4000. In total, 54 microalgal cuvettes were flown in two stratospheric missions to evaluate post-flight integrity, and recovered samples were recultivated under standardized conditions and analyzed two weeks post-flight for pigment composition and antioxidant-related parameters. After flight, 29/54 samples (53.7%) were classified as visually undamaged after landing, while damage was associated with freezing (13/54; 24.1%), thermal damage (13/54; 24.1%, partly overlapping), loss due to containment failure (5/54; 9.3%), or non-recoverable material (3/54; 5.6%). In recultivated biomass, Near-space-exposed cultures showed significant shifts in pigment profiles (reduced chlorophyll a and/or b in multiple species) and increased total antioxidant capacity, indicating persistent physiological differences following the mission workflow. These results provide quantitative benchmarks for microalgal near-space exposure using a low-cost balloon platform and motivate follow-up studies with higher replication and direct functional stress readouts.