Effects of Simulated Microgravity on Limnospira platensis Growth and Transcript Expression Related to Photosystem and Open Reading Frames

Limnospira platensis (formerly Arthrospira platensis) was cultured for 14 days under simulated microgravity (SMG) generated by a three-dimensional clinostat and compared with motion-matched controls using optical density, live/dead staining and real-time RT-PCR. Control cultures appeared to proliferate faster initially but declined after day 8 and died by day 14, whereas SMG cultures maintained a prolonged exponential phase and reached a final OD750 of 1.43 ± 0.05, approximately 29% above the control maximum (1.11 ± 0.12; p = 0.040). Specific growth rates did not differ over days 0–6 (p = 0.13), but between days 8 and 12, the control declined (−0.111 ± 0.008 day−1) while SMG cultures continued to grow (0.025 ± 0.011 day−1; p < 0.000001). Live/dead staining confirmed that SMG-cultured cells retained normal pigmentation and viability throughout, whereas control cells exhibited progressive cell death from day 10 onward. ftsZ was significantly lower under SMG on day 2 and higher on day 12. Eleven of 19 photosystem and open reading frame genes were significantly higher in the control on day 2, in SMG on day 10 and in the control again on day 12. At matched cell density (control day 6 versus SMG day 10; OD750 p = 0.52), 19 of the 20 genes remained significantly higher under SMG, so this divergence is not an artefact of the growth stage. Reducing mechanical shear stress through SMG thus maintains physiological integrity, prolongs the productive growth phase and enhances biomass accumulation, supporting the use of L. platensis in bioregenerative life-support systems.