C3 Bacterial degradation of lignin RAFAEL VICUNA Unidad de Microbiologia y GenOtica Molecular, Pontificia Universidad CatOlica de Chile, Casilla I14-D. Santiago, Chile Summary. During the year 1983, there was a breakthrough in the field of lignin biodegradation when fungal ligninases and their hydrogen peroxide requirement were described. A comparable progression has not yet occurred with lignino- lytic bacteria, although it is expected to take place in the near future once depolymerizing enzymes are isolated. Sev- eral bacterial strains have been found to mineralize aerobi- cally [14C-lignin]lignocellulose as well as 14C-labelled syn- thetic lignins, even though the most efficient are still far from reaching the rates exhibited by ligninolytic fungi. Acti- nomycetes follow a characteristic pattern of lignocellulose decomposition, with the release of lignin-rich, water-soluble fragments that are slowly metabolized thereafter. Research is being carried out to find the key enzymes involved in both lignin solubilization and mineralization by bacteria and un- cover their mechanism of action. The ability of bacteria to grow on low-molecular-weight lignin oligomers as the sole source of carbon and energy indicates that bacteria produce enzymes catalysing cleavage of intermonomeric linkages. Various strains metabolize cyclic lignans, biphenyl struc- tures, and other "dimeric" compounds, including those that possess the arylglycerol -fl-aryl ether (fl-O-4) linkage. Clea- vage of the latter apparently is reductive, fl-O-4 dimers be- ing metabolized by some bacteria through Cc~-Cfl cleavage. In contrast, no one has isolated a bacterium capable of decomposing a dimeric structure of the 1,2-diarylpropane- 1,3-diol (fl-1) type, although strains metabolizing 1,2-diaryl- ethane compounds have been found. In the absence of oxy- gen, only low-molecular-weight oligomers or chemically modified lignins are significantly degraded. The contribution of bacteria to the complete biodegradation of lignin in natu- ral environments where fungi are also present is not known. However, bacteria seem to play a leading role in decom- posing lignin in aquatic ecosystems. Keywords: Lignin;lignocellulose; lignin-related compounds;biode- gradation; ligninolytic bacteria Introduction Bacteria are known to display an ample metabolic ver- satility towards aromatic substrates. This characteris- tic applies for naturally occurring compounds, as well as for those of xenobiotic origin. (i.e., polycyclic aro- matic hydrocarbons, halogenated biocides, azo dyes, etc.). The high rate of genetic adaptation exhibited by these microorganisms has contributed to enable them to develop this versatility through evolution. 1,2 Lignin is by far the most abundant aromatic sub- stance present in the biosphere. It is a polymer com- posed of phenylpropanoid units linked through a vari- ety of nonhydrolysable C--C and C--O--C bonds. 3 Bacteria of several genera, including Pseudomonas, Alcaligenes, Arthrobacter, Nocardia, and Strepto- myces, readily degrade the single-ring aromatic com- pounds that build up the lignin macromolecule. 4-s However, the extent to which bacteria are able to bring about the decay of the lignin polymer itself has not been properly assessed. In part this is because lignin biodegradation studies have concentrated on filamentous fungi. For example, in recent meetings in this field,9'1° presentations on ligninolytic fungi out- numbered those on bacteria by a factor of seven. This is understandable, since some species of the former can efficiently mineralize lignin. In contrast, informa- tion available to date indicates that lignin is fairly re- calcitrant to bacterial attack, at least under laboratory conditions. For this reason, some authors believe that bacteria play a secondary role in lignin biodegradation in natural environments.~l-14 Lignin breakdown involves multiple biochemical reactions that have to take place more or less simulta- neously: cleavage of intermonomeric linkages, de- methylations, hydroxylations, side chain modifica- tions, and aromatic ring fission followed by dissimilation of the aliphatic metabolites produced. In- solubility of lignin and its lack of stereoregularity con- tribute to making it a substrate that is difficult for the microflora to degrade. Fungi overcome these con- straints by virtue of a family of extracellular iso- enzymes collectively called "ligninases, ''15-19 which are peroxidases that act through a mechanism involv- ing free radical formationfl ° Bacterial counterparts of ligninases have not yet been found, although distinct- ive intracellular enzymes catalysing all of the above reactions except cleavage of intermonomeric connec- tions have been described for low-molecular-weight substrates. An additional challenge for potential ligninolytic mi- croorganisms is the need to gain access to the sub- stfate by penetration of plant tissues. Fungi, as well as 646 Enzyme Microb. Technol., 1988, vol. 10, November © 1988 Butterworth Publishers