Microbial production of auxin indole-3-acetic acid in marine sediments

Auxin indole-3-acetic acid (IAA) was detected in marine sediments for the first time using HPLC and immunological identification methods. The IAA content of the interstitial water in sediments varied, in the order of 10⁻¹⁰ to 10⁻⁶ mol l⁻¹, with sampling season and location. The content in seawater was much lower than that of sediments, ca 10⁻¹¹ to 10⁻¹⁰ mol l⁻¹. Considerable amounts of IAA were produced by various marine bacteria in vitro, especially in the presence of tryptophan. IAA production and accumulation was also enhanced when glucose, tryptophan and natural seaweed substrates were added to sediments and incubated under both aerobic and anaerobic conditions in the dark, which suggests that the natural microbial community contributes to IAA production in sediments. The amount of IAA that accumulated in the water surrounding sediments reached 10⁻⁷ mol l⁻¹ in the presence of seaweed substrate, and corresponded to the level in glucose plus tryptophan. Auxin-active compounds produced during microbial decay of natural organic materials may affect algal growth in natural environments.