Loss of inorganic and organic carbon by 14C-labelled aquatic plants

Abstract

The loss of inorganic and extracellular organic carbon (EOC) from three 14C-labelled aquatic macrophytes was determined in a flow-through water system. The rates of inorganic 14C loss from Littorella uniflora (L.) Aschers. and Elodea canadensis Michx. during 2 h in the light decreased to 59 and 23% of the initial rates, respectively. The loss rate by Fontinalis antipyretica L. decreased only slightly during 2 h in the light. The rates of inorganic 14C loss in darkness were higher than those in the light. After the initial post-illumination CO 2 burst, the loss rates decreased only slightly in Littorella and Fontanalis`s, but very rapidly in Elodea. The loss of 14C-labelled EOC declined rapidly during the first hour after placing the plants in an unlabelled medium, and then approached almost constant rates. From 15 to 85% of the total EOC could be recovered on a 0.2 μm membrane filter. The particulate fraction was shown to be of direct macrophyte origin. The major portion of the dissolved EOC lost by Littorella had a molecular weight of about 200 and was probably an early photosynthetic product. Due to a decrease in EOC activity with time after placing labelled plants in an unlabelled medium, the use of the 14C labelling technique to measure quantitative loss of EOC must be used in a relative way.