Nitrogen recycling in the pea aphid ( Acyrthosiphon pisum ) symbiosis

Two lines of evidence suggest that the symbiotic bacteria in the pea aphid Acyrthosiphon pisum can assimilate ammonia. First, bacteria isolated from the aphids accumulated exogenous methylamine, an analogue of ammonia, and the bacterial transport system displayed saturation kinetics characteristic of carrier-mediated transport (apparent K$_{\text{m}}$ 50 $\mu $M; V$_{\max}$ 2 nmol (mg protein)$^{-1}$ min$^{-1}$). Second, the concentration of ammonia in the honeydew of aphids was significantly increased when the bacteria were disrupted by antibiotics (chloretetracycline and rifampicin). Taken with previously published results, these data suggest that the bacteria in aphids recycle aphid waste ammonia to essential amino acids, which are made available to the aphid tissues. This nutritional interaction between aphids and their bacteria is linked to the very high ratio of non-essential:essential amino acids in phloem sap, the usual diet of aphids.