Ciliate dependent production of microbial anthranilic acid occurring within aspen litter

Terrestrial protozoa and their role in the soil microbial loop are intricately linked to the functioning of forest soils. Yet, in spite of their recognition as vital components of soil ecosystems, protozoa remain understudied when compared to other soil microorganisms. In addition to directly stimulating soil nutrient levels by releasing bacterial nutrients, soil protozoa may affect plant growth indirectly by promoting bacteria that produce plant auxins. We conducted a four week incubation study using defaunated trembling aspen leaves that were selectively inoculated with ciliates. Ciliates modified microbial community structure, as assessed with phospholipid fatty acid (PLFA) analysis. Using High Performance Liquid Chromatography (HPLC), we found that the presence of ciliates did not favor bacteria that produce the plant auxin, indole-3-acetic acid (IAA). Instead, ciliates were associated with the production of anthranilic acid, which indicates that they were responsible for maintaining populations of r-selected bacteria within a relatively stable and nutrient poor environment. Additionally, while ciliates were observed using phase contrast microscopy, the commonly used PLFA indicator for soil protozoa (20:4) was not detected therefore it was shown to be a poor and potentially invalid biomarker.