Testate amoebae are microbial predators and hydrological proxies in peatlands. Their functionality and community dynamics are assessed through functional traits (e.g., test size, aperture size, mixotrophy/ heterotrophy, test material). Photoautotrophic microbes have been recently recognized as potentially important contributors to the peatland carbon sink. The responses of these microbes to the drying and associated succession towards arboreal vegetation, which are threatening peatlands as the climate warms and evaporation increases, remain poorly known.
This thesis comprises three studies examining the responses of testate amoebae and photoautotrophic microbes to long-term drying in three boreal peatland types (rich fen, poor fen, bog). The research utilised Lakkasuo water level drawdown (WLD) experiment, where water level has been lowered for two decades to simulate the impacts of climate change. Samples were collected in summer 2022. Testate amoebae were identified and measured microscopically, photoautotrophic communities were analysed using metabarcoding, and photosynthetic capacity was measured with PhytoPAM. The results were compared between control and WLD areas and among sites.
WLD affected photoautotrophic assemblages regardless of the site fertility, whereas testate amoebae responded the most strongly in the rich fen and the least in the bog. In both ends of the fertility gradient, taxonomic changes were decoupled from functional changes. In the poor fen, the functional structure of testate amoeba assemblages differed between control and WLD, and photosynthetic capacity was higher in the WLD area. The sites differed in taxonomic and functional compositions, WLD responses, and functional stability.
Evidence for functional changes was observed only in the poor fen, likely due to a greater rise in nutrient availability and shading compared to the other sites, and, for testate amoebae, because of adaptation to wet and open conditions and low functional redundancy. In contrast, testate amoebae were resistant to the moderate drying in the bog, whereas functional redundancy buffered against changes in testate amoeba communities’ functional structure in the rich fen. Overall, the results highlight that microbial responses to drying cannot be generalised across peatland types or microbial groups.