In headwater streams, allochthonous litter inputs are an important source of energy that drive nutrient cycling. Litter decomposition is a microbially-mediated process that progresses in stages as first fungi – particularly aquatic hyphomycetes – and then bacteria colonize and degrade litter by secreting extracellular enzymes. However, initial microbial colonization is affected by the original litter chemistry, which can be altered to contain more recalcitrant compounds by the presence of endophytes. Rhytisma punctatum is a fungal endophyte of Acer macrophyllum that causes black stromata patches that persist in senesced litter. We hypothesized that endophyte-infected tissues would not only decompose more slowly, but have decreased sporulation rates of aquatic hyphomycetes compared to uninfected tissues. We also hypothesized that microbial communities would vary by litter treatment (leaf discs infected by Rhytisma punctum, infected by an unknown bullseye lesion, or without symptomatic infection) and over time. Litter bags were deployed in a small forested stream and harvested after 1, 2, 4, and 6 weeks of incubation. Upon harvest, discs were either incubated in sterile stream water to induce sporulation of aquatic hyphomycetes, combusted for ash-free dry mass to determine decomposition rates, or processed for amplicon-based sequencing to characterize epiphytic microbial decomposer communities.
Results/Conclusions
Symptomatic endophyte infection did not significantly affect decomposition rate (ANCOVA treatment × time interaction: F2,49=2.936, p=0.0625). However, significantly lower sporulation rates were observed for discs with symptomatic endophyte infections (Kruskal-Wallis, χ2=6.1682, df=2, p=0.0458) than for uninfected tissue. Additionally, both fungal and bacterial colonizer communities differed by litter treatment and harvest (perMANOVA, Ffungal=4.2029 and pfungal=0.0002; Fbacterial=9.4800 and pbacterial=0.0002), although there was no significant treatment × harvest interaction. These results suggest that factors other than symptomatic endophyte presence may mitigate overall decomposition effects, but that endophyte presence alters microbial decomposer communities. Given that decomposition is a microbially-mediated process, our results have broad implications for global carbon cycling models thought to be mainly driven by temperature. Endophyte presence in litter may alter decomposition processes and our ability to predict litter breakdown based on temperature-dependent decomposition.