Physiology, biochemistry and taxonomy of deep-sea nitrile metabolising Rhodococcus strains

A collection of nitrile-hydrolysing rhodococci was isolated from sediments sampled from a range of deep coastal, and abyssal and hadal trench sites in the NW Pacific Ocean, as part of our programme on the diversity of marine actinomycetes. Nitrile-hydrolysing strains were obtained by batch enrichments on nitrile substrates with or without dispersion and differential centrifugation pre-treatment of sediments, and were recovered from all of the depths sampled (approximately 1100-6500 m). Two isolates obtained from the Ryukyu (5425 m) and Japan (6475 m) Trenches, and identified as strains of Rhodococcus erythropolis, were chosen for detailed study. Both of the deep-sea isolates grew at in situ temperature (4 degrees C), salinities (0-4% NaCl) and pressures (40-60 MPa), results that suggest, but do not prove, that they may be indigenous marine bacteria. However, the absence of culturable Thermoactinomyces points to little or no run off of terrestrial microbiota into these particular trench sediments. Nitrile-hydrolysis by these rhodococci was catalysed by a nitrile hydratase-amidase system. The hydratase accommodated aliphatic, aromatic and dinitrile substrates, and enabled growth to occur on a much wider range of nitriles than the only other reported marine nitrile-hydrolysing R. erythropolis which was isolated from coastal sediments. Also unlike the latter strain, the nitrile hydratases of the deep-sea rhodococci were constitutive. The possession of novel growth and enzyme activities on nitriles by these deep-sea R. erythropolis strains recommends their further development as industrial biocatalysts.