Microbiota-dependent and independent production of L-dopa in the gut of Daphnia magna

The host-microbiome interactions are essential for the physiological and ecological performance of the host, yet these interactions are challenging to identify. Neurotransmitters are commonly implicated in these interactions, but we know very little about the mechanisms of their involvement, especially in invertebrates. Here, we report a peripheral Catecholamine (CA) pathway involving the gut microbiome of the model species Daphnia magna . We demonstrate that: (1) tyrosine hydroxylase and dopa decarboxylase enzymes are present in the gut wall; (2) DOPA decarboxylase gene is expressed in the gut by the host, and its expression follows the molt cycle peaking after ecdysis; (3) biologically active L-Dopa, but not Dopamine, is present in the gut lumen; and (4) gut bacteria produce L-Dopa in a concentration-dependent manner when provided L-Tyrosine as a substrate. Impinging on gut bacteria involvement in host physiology and ecologically relevant traits, we suggest L-Dopa as a communication agent in the host-microbiome interactions in daphnids and, possibly, other crustaceans.