Monitoring microbial predator-prey interactions: an experimental study using fatty acid biomarker and compound-specific stable isotope techniques

Naturally occurring microbial communities are complex, with autotrophs and heterotrophs often similarly sized and impossible to separate by conventional size fractionation approaches. However, if it was possible to identify specific compounds that are characteristic of particular groups of microbes and determine the stable isotope composition of these biomarkers, the requirement for size fractionation could potentially be negated. This work considered the usefulness of such an approach by analysis of a simple laboratory predator-prey system comprising Nanochloropsis oculata, an autotrophic flagellate prey and Oxyrrhis marina, a heterotrophic flagellate predator. In growth-grazing experiments the fatty acids 20:5(n-3) and 22:6(n-3) were used as biomarkers for N. oculata and O. marina respectively. Interpretation of delta C-13 values of these predator and prey fatty acid biomarkers was not straightforward since although isotopic signature of the O. marina biomarker was consistently enriched compared to that of its N. oculata prey, the magnitude of enrichment in C-13 increased with age of culture (1.0-5.4 %). Given the variability we observed in our experimental cultures, it will be difficult to apply this approach to complex field situations without a comprehensive understanding of the factors determining the delta C-13 values of specific biomarker molecules.