Predator-prey mass ratio drives microbial activity under dry conditions in Sphagnum peatlands.

Monika K Reczuga,Michał Słowiński,Vincent E J Jassey,Alexandre Buttler,Sandra Słowińska,Daniel Gilbert,Philippe Binet,Edward A D Mitchell,Mariusz Lamentowicz,Bogdan Chojnicki,Geneviève Chiapusio,Matthieu Mulot

doi:10.1002/ece3.4114

Abstract

Mid‐ to high‐latitude peatlands are a major terrestrial carbon stock but become carbon sources during droughts, which are increasingly frequent as a result of climate warming. A critical question within this context is the sensitivity to drought of peatland microbial food webs. Microbiota drive key ecological and biogeochemical processes, but their response to drought is likely to impact these processes. Peatland food webs have, however, been little studied, especially the response of microbial predators. We studied the response of microbial predators (testate amoebae, ciliates, rotifers, and nematodes) living in Sphagnum moss carpet to droughts, and their influence on lower trophic levels and on related microbial enzyme activity. We assessed the impact of reduced water availability on microbial predators in two peatlands using experimental (Linje mire, Poland) and natural (Forbonnet mire, France) water level gradients, reflecting a sudden change in moisture regime (Linje), and a typically drier environment (Forbonnet). The sensitivity of different microbial groups to drought was size dependent; large sized microbiota such as testate amoebae declined most under dry conditions (−41% in Forbonnet and −80% in Linje). These shifts caused a decrease in the predator–prey mass ratio (PPMR). We related microbial enzymatic activity to PPMR; we found that a decrease in PPMR can have divergent effects on microbial enzymatic activity. In a community adapted to drier conditions, decreasing PPMR stimulated microbial enzyme activity, while in extreme drought experiment, it reduced microbial activity. These results suggest that microbial enzymatic activity resulting from food web structure is optimal only within a certain range of PPMR, and that different trophic mechanisms are involved in the response of peatlands to droughts. Our findings confirm the importance of large microbial consumers living at the surface of peatlands on the functioning of peatlands, and illustrate their value as early warning indicators of change.

Highlights

Over the past few decades, climate warming has caused numerous hydroclimate anomalies in northern latitudes resulting in dramatic hydrological shifts at regional and local scales (Ljungqvist et al, 2016)
In a community adapted to drier conditions, decreasing Predator-to-Prey Mass Ratio (PPMR) stimulated microbial enzyme activity, while in extreme drought experiment, it reduced microbial activity. These results suggest that microbial enzymatic activity resulting from food web structure is optimal only within a certain range of PPMR, and that different trophic mechanisms are involved in the response of peatlands to droughts
We found that phenoloxidase activity significantly decreased between W (0.023 nmolQ min−1 g−1 dry mass (DM)), NAT (0.01 nmolQ min−1 g−1 DM) and ED (0.006 nmolQ min−1 g−1 DM) treatments (Figure 5e)

Summary

Introduction

Over the past few decades, climate warming has caused numerous hydroclimate anomalies in northern latitudes resulting in dramatic hydrological shifts at regional and local scales (Ljungqvist et al, 2016). Model simulations predict further increases in the intensity and frequency of droughts along with warmer temperatures (Dai, 2013). Such changes may have devastating implications for northern ecosystems, many of which are especially dependent on water balance (Turetsky et al, 2017). The moist and acidic conditions prevailing in these ecosystems slow down microbial activities, leading to an imbalance between primary production and decomposition rates (Frolking et al, 2001). More frequent and intense droughts are likely to profoundly and durably alter peatland C balance due to the response of microbial communities. Drought may accelerate the rate of organic matter decomposition (Fenner & Freeman, 2011), thereby releasing previously locked C and turning peatlands from C sinks into C sources (Davidson & Janssens, 2006)

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