Interactions between stream fungi and bacteria associated with decomposing leaf litter at different levels of nutrient availability

We examined the potential for interactions between aquatic hyphomycetes and bacteria isolated from leaves decaying in a headwater stream. In agar plate assays, culture filtrates of each of 28 aquatic hyphomycete isolates tested (5 species) inhibited bacterial growth (16 Gram-negative bacterial isolates belonging to 6 colony morphotypes were tested). Inhibition of bacterial growth occurred in 20% of the combinations. To determine whether such interactions could occur on decomposing leaves, Articulospora tetracladia (isolate 24-4) and bacterial isolate B2NPM3-1 (tentatively placed in Comamonadaceae) were grown in axenic and dual cultures on leaf litter in microcosms. Performance of both microorganisms was estimated by measuring leaf mass loss, fungal and bacterial biomass, conidia production, respiration and calculating carbon flow through different microbial compartments in 2 treatments that differed with respect to inorganic nutrient (N and P) concentrations. High fungal antagonistic activity demonstrated in plate assays was not corroborated in microcosms. Cumulative Articulospora tetracladia production decreased 21 to 24% in 2-membered microcosms regardless of nutrient level, whereas the bacterial isolate exhibited a differential response (1.7 times lower cumulative production in the low nutrient and 52% increase in the high nutrient treatment in 2-membered cultures compared with axenic cultures), suggesting that nutrient availability may modify microbial interactions. Fungal performance (yield coefficient and production efficiency) was not affected in 2-membered microcosms, whereas the bacterial yield coefficient was 1.7 to 2.2 times lower when grown with the fungus in comparison to axenic cultures. We observed only antagonistic or competitive interactions and no signs of synergistic relationships causing faster leaf litter decomposition or resulting in enhanced microbial production. Overall, the interactions were relatively mild and did not affect fungal dominance in the transformation of leaf organic matter.