Algae slow the rate of leaf decomposition in aquatic ecosystems

Halvor M. Halvorson, Jacob R. Barry, Matthew B. Lodato, Robert H. Findlay, Steven N. Francoeur, Kevin A. Kuehn

Many freshwater ecosystems receive large inputs of plant detritus, such as leaves and wood, from the surrounding terrestrial landscape. Upon submersion, this plant detritus is colonized by a diversity of aquatic microbes which interact to influence its fate. For example, microbes influence whether detritus remains stored in the ecosystem, decomposes quickly, or is consumed by animals. In recent years, scientists have found that algae play an overlooked role determining this fate, because algae are widespread and produce high-quality exudates which stimulate decomposer fungi and bacteria, enhancing decomposition of low-quality detritus. This algal-induced “priming effect” can speed leaf decomposition by greater than 20%, but the priming effect remains poorly tested in many aquatic settings.

In this study, we investigated the priming effect of algae during decomposition of two leaf species, Tulip Poplar and Water Oak, in experimental streams under shading or ambient sunlight. We found that algae colonized leaves decomposing under light, and consistent with the priming effect, algae stimulated both fungal growth rates and the accrual of bacterial biomass on both species of leaf. Although algae increased fungal growth rates, we surprisingly found that algae reduced the accrual of fungal biomass, and also reduced the rate of leaf decomposition. For example, algae nearly tripled the days required for complete Tulip Poplar leaf decomposition, from 50 days to 143 days. These trends are consistent with a negative priming effect, in which algae stimulate fungi and bacteria, but these decomposer microbes switch to preferentially use high-quality algal exudates instead of the low-quality decomposing plant detritus as a resource.

Our study reveals an overlooked interaction of algae with the fate of plant detritus, and suggests that algae may slow the decomposition of plant detritus in many well-lit aquatic environments including streams, wetlands, and lakes. This is important, because it suggests algae may influence freshwater ecosystems’ roles in storing versus releasing carbon from terrestrial plant detritus around the globe.

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