Light acclimation of the charophyte Lamprothamnium papulosum

Two experimental designs were carried out to study how Lamprothamnium papulosum (Wallr.) J. Gr. is able to acclimate to underwater irradiance. Field measurements showed that L. papulosum can acclimate quickly to daily changes in solar irradiance. Quantum yield of Photosystem II (PSII) photochemistry showed a fast decrease with increasing irradiances in the morning and an increase with decreasing irradiances in the afternoon. L. papulosum recovered its PSII photochemical yield completely within 1 h after transfer into darkness. Pigmentation analysis supported these results showing a distinct acclimation of pigment composition and pigment ratios in regard to both acclimation irradiance and daily irradiance changes. Growth experiments in the laboratory with daily irradiance doses up to 21.6 mol photons m-2 per day showed increasing growth rates with increasing irradiances. The calculated daily light dose needed for zero net growth was about 1 mol photons m-2 per day. The determination of relative electron transport rates exhibited a pronounced light acclimation in terms of the initial slope of the photosynthesis versus irradiance curve at limiting irradiances (), the maximum photosynthesis rate at saturating irradiances (Pmax) and the irradiance saturation point for photosynthesis (Ek=Pmax/) in dependency of the acclimation conditions. The correlation of Ek versus irradiance showed that light acclimation mechanisms of L. papulosum decreased at irradiances below 5 mol photons m-2 per day. Thus, an irradiance of more than 5 mol photons m-2 per day is necessary for growth. The growth limits corresponded well to the fact that L. papulosum is often referred to as a species which prefers to grow in shallow water areas.