Light absorption and fluorescence, and photoacclimation features in the marine macroalga Porphyra leucosticta (Rhodophyta)

Absorption and fluorescence spectra, photosynthetic rates, pigment concentrations, and cell size were investigated during photoacclimation of Porphyra leucosticta, a red marine macroalga that thrives during winter in the eastern Mediterranean. Optical properties were measured on intact thalli using image analysis or from crude extracts after disrupting algal tissue. Absorption spectra from crude extracts showed major peaks at 485, 620, and 664 nm for chlorophyll a (chl a) and 500, 564 and 615 nm for phycoerythrin (PE) and phycocyanin (PC). Absorption peaks from intact thalli were less conspicuous than those from crude extracts, possibly due to PE contributing to the bulk absorption of light by chl a in intact cells. Fluorescence emission of intact thalli was about threefold higher than that of crude extracts at 420 and 560 nm. Image analysis revealed differences in maximal fluorescence emissions as related to cell size, cell age, and thallus functional areas, with female cells having nearly a twofold higher emission than vegetative cells or asexual spores. Changes in the fluorescence emissions observed during acclimation to irradiance could reflect the extent of energy transfer from PE and PC to chl a, variable photosynthetic efficiency, or optical properties of the samples. This study suggests that image analysis can provide insights on seaweed photoacclimation that seem to be lost when disrupting their tissues to crude extracts. For example, the high PE to chl a ratios observed with image analysis in fresh tissues may confer a unique photoprotective role of PE when P. leucosticta becomes exposed to high irradiances, even during winter time.