Evidence of structural and functional adaptation in epilithon exposed to zinc

Abstract

Artificial streams were used to study the effect of chronic zinc dosing (0.00,0.05 or 1.00 mg Zn 1- ') on epilithic communities during summer and early fall, 1984 and 1985. Following zinc addition, epilithon was periodically measured for '4C-glucose and 14C-glutamate respiration, ash-free dry weight (AFDW), protein, carbohydrate, chlorophyll a (Chl a), pheophytin a (pheo a), and colony forming units (CFU) on zinc amended and unamended media. Transient peaks in glucose respiration rates mg- ' protein and AFDW occurred within 5 to 10 days in streams dosed with 1.0 mg Zn 1- ' and coincided with decreases in AFDW, protein, carbohydrate, Chl a, and pheo a. Respiration of glutamate did not show a significant response to dosing (a = 0.05). Epilithic plate counts demonstrated more total and zinc-tolerant CFU in dosed than in control epilithon after 20 days. After 30 days, greater biomass and lower protein: carbohydrate ratios were evident in epilithon dosed with 1.0 mg Zn 1- ' compared to unamended treatments. Peaks in glucose respiration suggested the presence of zinc-tolerant heterotrophs transiently capable of increased respiration of glucose but not glutamate. Concurrently, minimal but measurable biomass inferred the persistence of zinc-tolerant microorganisms in zinc-dosed epilithon. By the end of the studies, results indicated that metal-tolerant populations replaced metal-sensitive organisms under conditions of elevated zinc concentration. These zinc adapted communities had lower potential value as a food source to grazers (low protein: carbohydrate ratio) and were no longer distinctive with respect to glucose respiration.