Identification and characterization of humic substances-degrading bacterial isolates from an estuarine environment

Bacterial isolates were obtained from enrichment cultures containing humic substances extracted from estuarine water using an XAD-8 resin. Eighteen is olates were chosen for phylogenetic and physiological characterization base d on numerical importance in serial dilutions of the enrichment culture and unique colony morphology. Partial sequences of the 16S rRNA genes indicate d that six of the isolates were associated with the a subclass of Proteobac teria, three with the gamma -Proteobacteria, and nine with the Gram-positiv e bacteria. Ten isolates degraded at least one (and up to six) selected aro matic single-ring compounds. Six isolates showed ability to degrade [C-14]h umic substances derived from the dominant salt marsh grass in the estuary f rom which they were isolated (Spartina alterniflora), mineralizing 0.4-1.1% of the humic substances over 4 weeks. A mixture of all Is isolates did not degrade humic substances significantly faster than any of the individual s trains, however, and no isolate degraded humic substances to the same exten t as the natural marine bacterial community (3.0%). Similar studies with a radiolabeled synthetic lignin ([beta-C-14]dehydropolymerisate) showed measu rable levels of degradation by all 18 bacteria (3.0-8.8% in 4 weeks), but m ineralization levels were again lower than that observed for the natural ma rine bacterial community (28.2%). Metabolic capabilities of the Is isolates were highly Variable and generally did not map to phylogenetic affiliation . (C) 2000 Federation of European Microbiological Societies. Published by E lsevier Science B.V. All rights reserved.