Biocatalytic pretreatment of rice straw by ligninolytic enzymes produced by newly isolated Micrococcus unnanensis strain B4 for downstream cellulolytic saccharification

Abstract

Rice (Oryza sativa L.) straw is an agricultural byproduct of high yield, and its disposal by burning has detrimental effect on ecosystem. It has potential as source of fermentable sugars for industrial use; however, it requires effective pretreatment to remove lignin. Bacterial enzymes based pretreatment is advantageous due to their extracellular nature, and tolerance to higher temperature, pH and oxygen limitation. We herein report screening of lignocellulose degradation environment of vermicompost for ligninolytic bacteria, and studying role of Micrococcus unnanensis strain B4 in delignification of rice straw. The bacterium was capable to degrade acid soluble and insoluble lignin; and produced lignin degrading laccase and peroxidase having maximum activity at pH 6.5 and 72 h incubation. Both enzymes exhibited alkaline pH stability, and thermal stability with retention of 100 % activity on pre-incubation at 60 ℃ for 1 h. The enzymes were used for pretreatment of rice straw using chemicals (acetic acid:hydrogen peroxide) pretreatment as reference. Scanning electron microscopy of pretreated rice straw samples showed alteration in morphology with exposure of cellulosic components. Enzymatically pretreated rice straw on saccharification by a commercial cellulase yielded about 400 mg of reducing sugar per gram, comparable to that released on chemical pretreatment. Hence, pretreatment based on M.unnanensis strain B4 and its ligninolytic enzymes can be an alternative to chemical pretreatment for saccharification of rice straw to fermentable sugars.