PATTERNS OF AMINO-ACID UTILIZATION BY AQUATIC HYPHOMYCETES

The utilization of amino acids in leaf protein and leaf leachate by aquatic hyphomycetes was studied during decomposition in a combined field and laboratory experiment. Leaves were sampled from a stream which exhibited a seasonal variation in free amino acid concentration in surface water, reaching peaks in autumn and winter. In the leaf drift environment the concentration of amino acids was .apprx. 2 orders of magnitude higher than in surface water. Protein amino acid content, which was higher in alder [Alnus glutinosa] leaves than in beech [Fagus sylvatica] leaves, decreased exponentially and faster in alder leaves, so that protein amino acid content was similar in the 2 leaf types after 9-10 wk decomposition. From 55 to 75% of leaf amino acids were used instantaneously by attached fungi, which grew well, especially on alder leaves, regardless of the presence of a grazing amphipod. If N was a limiting nutrient source for fungi, it appeared to be more advantageous to colonize alder leaves. Four times more fungal species were present on alder leaves than on beech leaves. The change in concentration of amino acids in leaves and water was described by a set of differential equations. Rate constants for the transfer of amino acids from leaves and water were estimated from experimental data and the preference in fungi for protein-bound and free amino acids evaluated. The amounts of free amino acids in water absorbed by fungi varied between leaf types and leaves at different stages of decay. There was a switching behavior in fungal absorption of dissolved amino acids so that absorption became superproportional at a certain proportion of free amino acids available in the water.