Microb Ecol (1983) 9:355-362 MICROBI IL CCOLOGV @ 1983 Springer-Verlag Microbial Decomposition of Leaf Material at O~ T.-Y. Tam, C. I. Mayfield, and W. E. Inniss Department of Biology,Universityof Waterloo, Waterloo, Ontario N2L 3G1, Canada Abstract. The microbial decomposition of leaves (both fresh and autumn- shed) at 0~ using stream sediment-water was investigated. The maximum rates of loss of leaf carbohydrate and protein at 0~ were considerable, being about 40% of those at 20~ These rates were only slightly affected by the type of leaf material present being 1.3-fold higher with fresh leaves as compared with autumn-shed leaves. In addition, an epifluorescence microscopic counting technique was developed and utilized to enumerate the microbial populations colonizing the decomposing leaves. The average microbial densities on fresh and autumn-shed leaves after 35 days of in- cubation were 1.3 X 106 and 9.0 X l0 s microorganisms cm -2 at 0~ as compared with 5.5 X 106 and 3.3 X 106 microorganisms cm -2 at 20~ respectively. Antibacterial and antifungal antibiotics were used to estimate the comparative involvement of sediment bacteria and fungi in leaf deg- radation. Introduction Various studies have been carried out to investigate the decomposition of leaf detritus in streams [3, 10, 11, 20]. However, determination of the capacity to carry out such activity at 0~ the temperature at which growth must occur for such activity to be considered psychrophilic or psychrotrophic in nature, has rarely been examined. In addition, it was considered important to quantify the amount of leaf degradation that can occur at a relatively cold temperature (0~ and to compare it with that occurring at a relatively moderate temperature (20~ in order to estimate the leaf degradation that can occur during the winter months of a temperate zone climate. Consequently, the degree of psychrophilic or psychrotrophic activity in stream sediment-water was measured, with de- composition of leaf material being determined by direct measurement of the release of carbohydrate and protein. The ability of microorganisms to colonize leaf detritus at 0~ under similar conditions was examined also using a direct epifluorescence microscopic ob- servation and enumeration procedure. Microbial colonization of leaf detritus has been investigated but mostly by indirect means, such as plate counting techniques using an incubation temperature of 30~ [2, 10] which would pre- clude the growth of any psychrophilic microorganisms. Indeed, direct obser- vation and counting of epiphytic microorganisms appears to have rarely been reported [6].