Recent Changes in the Use of Phototrophy by a Mixotrophic Testate Amoeba Inferred from δ13C Measurements from an Arctic Peat Core

High-latitude ecosystems are undergoing rapid ecological changes in response to climate warming. While some changes are well studied, the responses of microbial communities remain less understood. Testate amoebae, shell-producing protists well preserved in peat, provide a means to reconstruct past microbial dynamics. Mixotrophic taxa such as Archerella flavum host algal endosymbionts (zoochlorellae), allowing both heterotrophic and phototrophic energy acquisition. Previous work has demonstrated that these pathways result in different δ¹³C values. We applied a novel stable isotope approach to a peat core from the North Slope of Alaska to reconstruct changes in phototrophy by Archerella flavum. δ¹³C values were measured on Archerella flavum tests (i.e. shells) and Sphagnum, and a two-endmember mixing model was used to estimate relative usage of phototrophy through time. δ¹³C values were compared with testate amoeba community composition, test size, vegetation, and historical climate. Archerella flavum δ¹³C values were consistently more positive than Sphagnum δ¹³C values in the peat core, and patterns indicated greater phototrophy use after the late 1980s CE. This shift was followed by expansion of Archerella flavum populations and a trend of decreasing test length in several testate amoeba taxa. Increased phototrophy was associated with higher peat C:N ratios, indicating more oligotrophic conditions. From 2007 to 2019 CE, the length of the snow-free growing season was correlated with estimates of phototrophy usage, with more phototrophy during longer growing seasons. δ¹³C analyses of mixotrophic testate amoebae are a powerful tool for reconstructing microbial nutritional strategies and responses to past environmental change.