The effect of nutrient enrichment on three species of macroalgae as determined by photoacoustics

The efficiency of photorophs in storing light energy in products of photosynthesis, or the quantum yield of the process, determines the flux of energy into any ecosystem, and is readily affected by any stress factor, physiological or environmental. Hence it can serve as a sensitive reporter of imminent ecosystem perturbation. Current methods of estimating that parameter are either indirect, or tedious and labor consuming. The photoacoustic method is based on the direct sensing of the variable fraction of light energy degraded to heat, that while absorbed by the plant cells or tissue, failed to be stored as energy rich photosynthate. Algae are the main primary producers in all water bodies, marine, freshwater, natural and manmade, and as such they provide the energy basis for all aquatic ecosystems. The biomass and vitality of the algal population responds rapidly to seasonal changes in environmental factors such as temperature, light, vertical mixing, eutrophication, pollution and nutrient limitation. Typical processes resulting from human activities such as non-point sources of agrochemicals, sewage outflows, and effluents from food, animal and other chemical processing industries as well as from urban and rural runoff, contribute considerable amounts of nitrogen and phosphorous, in many cases in addition to heavy metals, to natural and artificial water bodies. In this study, we examined by the photoacoustic method the effects of enrichment by nitrogen and phosphorus on the photosynthetic efficiency of three common Mediterranean seaweed species which were collected from the field. The application developed by us, allows the rapid and direct determination of phytoplankton photosynthetic activity, by relating the energy stored photochemically to the total light energy absorbed by the plant material. The aim of this work was the examination of the applicability of the photoacoustic system to ecological work with macroalgae, since until now it has been successfully used only in homogenous phytoplankton cultures.