Experimental analysis of rhodophyte photoacclimation to PAR and UV-radiation using in vivo absorbance spectroscopy

Analysis of absorbance spectra of the red alga, Ahnfeltiopsis concinna, revealed the capacity of tissues from the canopy and understory to respond to changes in the intensity of photosynthetically active radiation. No differences in photoacclimation responses were observed in the experiments performed with A. concinna under PAR-only, PAR+UV-A, and PAR+UV-A+UV-B treatments. In situ, photoacclimation capacities were manifested as marked alterations in pigmentation for tissues from canopy to understory microsites in individual thalli of this dense tropical turf. This phenotypic adjustment is dynamic and recovery from irradiance stress induced by canopy removal simulating herbivory or bleaching events is complete within 29 days. In vivo absorbance spectra allowed simultaneous observation of multiple pigment pools and monitoring photoacclimation changes over the course of days in this alga. Insight into the cascade of photoacclimation events was revealed by concurrent measurement of pigments and photosynthetic performance. Overall, acclimation from a sun to shade or shade to sun states proceeded faster for degradative than for synthetic processes. Specifically, phycobilin degradation in tissues from understory microsites exposed to high photon flux density (PFD) was faster than phycobilin synthesis in tissues from canopy microsites exposed to low PFD. Additionally, levels of phycoerythrin changed to a greater relative extent than did shifts of compensation or saturation irradiance thresholds for photosynthesis in acclimation from shade to sun conditions. Compensation and saturation irradiances changed to a greater relative extent than did levels of phycoerythrin in acclimation from sun to shade conditions. Lastly, carotenoid and putative mycosporine amino acid absorbances adjusted in response to changes in quantity of incident photon flux regardless of light quality (PAR, UV-B, UV-A). This suggests a similar signal transduction mechanism or other metabolic linkages in this photoacclimation mechanism.