Benthic algae stimulate leaf litter decomposition in detritus‐based headwater streams: a case of aquatic priming effect?

Abstract

In detritus-based ecosystems, autochthonous primary production contributes very little to the detritus pool. Yet primary producers may still influence the functioning of these ecosystems through complex interactions with decomposers and detritivores. Recent studies have suggested that, in aquatic systems, small amounts of labile carbon (C) (e.g., producer exudates), could increase the mineralization of more recalcitrant organic-matter pools (e.g., leaf litter). This process, called priming effect, should be exacerbated under low-nutrient conditions and may alter the nature of interactions among microbial groups, from competition under low-nutrient conditions to indirect mutualism under high-nutrient conditions. Theoretical models further predict that primary producers may be competitively excluded when allochthonous C sources enter an ecosystem. In this study, the effects of a benthic diatom on aquatic hyphomycetes, bacteria, and leaf litter decomposition were investigated under two nutrient levels in a factorial microcosm experiment simulating detritus-based, headwater stream ecosystems. Contrary to theoretical expectations, diatoms and decomposers were able to coexist under both nutrient conditions. Under low-nutrient conditions, diatoms increased leaf litter decomposition rate by 20% compared to treatments where they were absent. No effect was observed under high-nutrient conditions. The increase in leaf litter mineralization rate induced a positive feedback on diatom densities. We attribute these results to the priming effect of labile C exudates from primary producers. The presence of diatoms in combination with fungal decomposers also promoted decomposer diversity and, under low-nutrient conditions, led to a significant decrease in leaf litter C:P ratio that could improve secondary production. Results from our microcosm experiment suggest new mechanisms by which primary producers may influence organic matter dynamics even in ecosystems where autochthonous primary production is low.

Ecological stoichiometry of aquatic fungi: current knowledge and perspectives

“…This observation is congruent with the fact that fungi often dominate microbial biomass in the early stages of leaf decomposition Baldy et al, 1995), when the C:nutrient ratio of decomposing leaf litter is high (Findlay et al, 2002). Aquatic fungi have also been found to outcompete bacteria on refractory litter in microcosm experiments unless labile carbon provided by benthic algae permitted the two groups of microbes to co-exist (Danger et al, 2013b). However, measured C:nutrient ratios in organisms do not necessarily reflect actual requirements (Frost et al, 2006).…”

Selective consumption and metabolic allocation of terrestrial and algal carbon determine allochthony in lake bacteria

“…It appears that as lakes become more productive, bacteria consume increasing proportions of algal C (Figure 4), yet the efficiency of incorporation of terrestrial C also increases along the same productivity gradient (Figure 2), such that the actual proportion of terrestrial C in bacterial biomass remains relatively constant across lakes. It has recently been hypothesized that an increase in the availability of labile compounds of algal origin may enhance the degradation of recalcitrant terrestrial DOC by aquatic bacterial communities (Danger et al, 2013;Guenet et al, 2014;Hotchkiss et al, 2014), although the actual occurrence and significance of this priming mechanism is still debated (Bengtsson et al, 2014;Catalán et al, 2015). Our experimental set up did not allow to directly test this hypothesis in terms of the enhancement of the consumption of terrestrial C, yet our results do suggest a significant interplay between the preferential allocation of algal-derived C to respiration and the incorporation of terrestrial DOC into bacteria biomass.…”

The ecology and biogeochemistry of stream biofilms

“…However, evidence for the priming effect in stream biofilms remains equivocal at present. There is suggestive evidence that the presence of algae stimulates bacterial and fungal growth that is associated with leaf litter 72,73 , but no evidence for the priming effect could be found in hyporheic biofilms 74 .…”

“…However, evidence for the priming effect in stream biofilms remains equivocal at present. There is suggestive evidence that the presence of algae stimulates bacterial and fungal growth that is associated with leaf litter 72,73 , but no evidence for the priming effect could be found in hyporheic biofilms 74 .The extensive diversity of bacteria in stream biofilms makes it extremely difficult to establish relationships between bacterial diversity and biofilm function. For example, complementarity among algal species has been shown to increase the uptake of nitrate by stream biofilms 75 but, although it is plausible that this may also occur among bacteria in biofilms, we are not aware of any such study.…”