## 1 Introduction  
Lipases (EC3.1.1.3) belong to the family of lipolytic enzymes that perform hydrolysis and esterification of triacylglycerhydcerides and other water-insoluble esters [1]. Lipases are widely used in the detergent, food, dairy, pulp, and pharmaceutical industries because they possess a conserved catalytic triad structure, are stable in organic solvents, undergo interfacial activation in microaqueous environments, and exhibit high substrate specificity [2]. Based on amino acid sequence homology and fundamental biological properties, bacterial lipolytic enzymes have been classified into eight families [3]. Family II, also known as the GDSL family, has five consensus sequences (I-V) and four invariant important catalytic residues (Ser, Gly, Asn, and His in blocks I, II, III, and V, respectively). Ser residues are near the N-terminal end of the amino acid sequence [4]. To date, there have been few reports of GDSL family lipases in bacteria.  
Many different gram-positive and gram-negative bacterial strains produce lipases. Some of the most commercially important bacteria that produce lipases are _Bacillus_ sp., _Pseudomonas_ sp., and _Burkholderia_ sp. [5, 6, 7]. These microbial lipases operate at a wider temperature range, and their stability and activity are greater. The reported lipase from _Bacillus thermoleovorans_ ID-1 showed optimal activity at 70-75 \({}^{\circ}\)C and pH 7.5 and extracellular lipase activity on a variety of lipid substrates at elevated temperatures. This lipase has been applied in the biological treatment of oily wastewater [8]. In addition, researchers found that BL1 was active over a wide range of pH values (from 9.0 to 13.0), with an optimum pH of 11.0, and exhibited maximal activity at 55 \({}^{\circ}\)C. The enzyme showed excellent stability and compatibility with various commercial detergents [9]. Lipases have a wide range of industrial applications, and the detergent industry is an important field for the application of lipases. However, few lipases are currently used in the detergent industry.  
Therefore, more stable and efficient alkaline lipases need to be identified from nature. Thermophilic bacteria are potential sources of thermostable alkaline lipases. High-temperature compost can reach as high as 80 \({}^{\circ}\)C in the ultrahigh-temperature phase, and the microbial community tends to be homogeneous. The dominant bacteria are mainly _Saccharomycespora_ (28%), _Oceanobacillus_ (13%), _Thermobifida_ (13%), _Actinomadura_ (12%), _Bacillus_ (11%), and _Geobacillus_ (6%) [10; 11]. _Geobacillus_ spp. grow between 55 and 65 \({}^{\circ}\)C and have been extensively studied for bioremediation, bioproduction, and thermotolerant enzyme production [12]. Enzymes produced by _Geobacillus_ spp. include glycoside hydrolases, lipases, and proteases [13]. The thermoalkaliphilic lipases Lip29, TA, and B78 from _Geobacillus_ spp. exhibit good thermostability and are expected to be used in the detergent industry [14; 15; 16].  
At present, all lipases that can be added to detergents are alkaline, and their optimum pH is 8.0-12.0 [17]. An ideal lipase for detergent addition should have the following characteristics: high activity, high reaction temperature, high pH, and strong resistance to alkali substances, organic solvents, metal ions, and other chemical substances [18]. In this study, a novel GDSL family lipase, LipGt6, was cloned and characterized from the thermophilic bacterium _G. thermolevorans_ H9. The purified recombinant LipGt6 was found to have optimal activity at 50 \({}^{\circ}\)C and pH 9.0 and be thermostable up to 70 \({}^{\circ}\)C. The potential of this enzyme as a detergent additive was initially evaluated.