The impact of different preservation methods on macroalgal tissue light absorptance values: A case study with Ulva australis

• We tested the influence of tissue preservation on light absorptance in Ulva australis.
• Light absorptance in Ulva australis changes under distinct preservation conditions.
• Wet preservation methods produced a significant drop in AL(PAR) values.
• Only herbarium presses generated light absorptance values close to those obtained from live material.
• Light absorption analysis in this study will allow more accurate ETR assessments.

Calculations of absolute ETR require accurate measurements of plant tissue Photosynthetic Active Radiation light absorption (AL(PAR)). The default leaf-specific AL(PAR) value of 0.84 estimated from and applicable to nearly all higher plants does not apply for marine macroalgae which are phylogenetically, structurally and chemically very different from higher plants and quite variable among themselves. Consequently, to date there is no default AL(PAR) value for all macroalgae, and hence AL(PAR) values always need to be recalculated from live specimens on every new study. This study compared AL(PAR) values from thalli of Ulva australis (Chlorophyta) preserved under different methods to test whether usable AL(PAR) data can be obtained from samples other than live ones. Light absorption measurements were made using an Integrating Sphere attached to a spectrophotometer. No statistically significant differences in AL(PAR) values were observed among live and pressed material whether recently collected (=4 days old) or stored more than seventy years but there were significant differences among live, frozen, formalin-preserved and bleached material. No significant differences in absorptance at wavelength of 675 nm were observed between fresh, frozen and formalin preserved material. AL(PAR) = 0.74 appeared to be an appropriate AL(PAR) default value for this species. This study demonstrated that accurate AL(PAR) can be obtained from some pressed herbarium preserved species while some commonly used methods of field tissue preservation in macroalgal photochemical studies fails to do so.