The influence of bacteria‐dominated diets on D aphnia magna somatic growth, reproduction, and lipid composition

We explored how dietary bacteria affect the life history traits and biochemical composition of D aphnia magna , using three bacteria taxa with very different lipid composition. Our objectives were to (1) examine whether and how bacteria‐dominated diets affect D aphnia survival, growth, and fecundity, (2) see whether bacteria‐specific fatty acid ( FA ) biomarkers accrued in D aphnia lipids, and (3) explore the quantitative relationship between bacteria availability in D aphnia diets and the amounts of bacterial FA in their lipids. D aphnia were fed monospecific and mixed diets of heterotrophic ( M icrococcus luteus ) or methanotrophic bacteria ( M ethylomonas methanica and M ethylosinus trichosporium ) and two phytoplankton species ( C ryptomonas ozolinii and S cenedesmus obliquus ). D aphnia neonates fed pure bacteria diets died after 6–12 days and produced no viable offspring, whereas those fed pure phytoplankton diets had high survival, growth, and reproduction success. D aphnia fed a mixed diet with 80% M . luteus and 20% of either phytoplankton had high somatic growth, but low reproduction. Conversely, D aphnia fed mixed diets including 80% of either methane‐oxidizing bacteria and 20% C ryptomonas had high reproduction rates, but low somatic growth. All D aphnia fed mixed bacteria and phytoplankton diets had strong evidence of both bacteria‐ and phytoplankton‐specific FA biomarkers in their lipids. FA mixing model calculations indicated that D aphnia that received 80% of their carbon from bacteria assimilated 46 ± 25% of their FA from this source. A bacteria–phytoplankton gradient experiment showed a strong positive correlation between the proportions of the bacterial FA in the D aphnia and their diet, indicating that bacterial utilization can be traced in this keystone consumer using FA biomarkers.