br Declaration of competing interests br Acknowledgements Th

Declaration of competing interests
Acknowledgements The authors wish to thank the Natural Science Foundation of Jiangsu Province (BK20171261), the project of outstanding scientific and technological innovation group of Jiangsu Province, the National Science Fund for Distinguished Young Scholars (31425020), the National Natural Science Foundation of China (31571776), the project of outstanding scientific and technological innovation group of Jiangsu Province and the 111 Project (No. 111-2-06) for financial support.
Introduction The deep-sea is a low-temperature, high-pressure, and oligotrophic dark environment. In order to adapt to the deep-sea environment, microorganisms have undergone long-term evolutionary changes that allow them to maintain thermal equilibrium despite being in low temperature environments [1]. Cold-adapted microorganisms have KH 7 synthesis with high structural flexibility, where the rapid change of conformation at low temperatures allows for substrates to access the active center of the enzyme to complete reactions in an effective manner [2]. In addition, they generally have high kcat and kcat/Km values that can compensate for the negative effects of low temperature on the speed of the biochemical reaction [3]. These enzymes also have interesting properties, such as salt tolerance, alkali resistance and pressure resistance [4,5], therefore have high potential for industrial and biotechnological applications. β-galactosidase (EC.3.2.1.23) is a hydrolase that exists in animals, plants and microorganisms [6], including bacteria, fungi, and yeast. β-galactosidase hydrolyzes lactose to galactose and glucose, and also has transgalactosylation effects. In the food industry, commercialized β-galactosidases are mainly extracted from Kluyveromyces and Aspergillums, primarily due to the ease of culturing these species and their high production capacity [7]. The available commercial β-galactosidases (DSM, Novozymes) have low activity levels at low temperature. For the dairy industry, the ideal variant of β-galactosidase for lactose hydrolysis would have high enzyme activity at 4–8 °C and pH and would not be inhibited by the presence of Na+, Ca2+ and glucose [8,9]. Cold-adapted β-galactosidases would hydrolyze lactose in milk and dairy products at low temperatures, maintaining the original taste and nutritional value of the product for individuals who are lactose intolerant. In addition, cold-adapted β-galactosidases are likely to deactivate at moderate temperatures, and this would save energy consumption that is often required for high temperature inactivation of enzymes [10]. Whey produced during cheese production contains lactose that is purified by crystallization; however, this process remains inefficient compared to the hydrolysis of lactose from milk [11]. At this time, the use of β-galactosidase to hydrolyze lactose is a good alternative solution. The monosaccharide produced by the hydrolysis of lactose can be used as a carbon source for microbial culture media, and many valuable metabolites can be obtained during fermentation, such as ethanol, D-tagatose, and biopolymers [[12], [13], [14]]. Therefore, in recent years, a great deal of effort has been invested in the isolation and characterization of novel cold-active β-galactosidases from new sources. To date, most cold-active β-galactosidases have been isolated from the bacterial species Arthrobacter sp. [[15], [16], [17]], Planococcus sp. [18,19], Pseudoalteromonas sp. [[20], [21], [22]], Halomonas sp. [23], Rahnella sp. [24], Paracoccus sp. [25], and Halorubrum lacusprofundi [26]. Some microorganisms harbor more than one gene encoding for β-galactosidase [27,28]. Arthrobacter sp. ON14 has two genes encoding β-galactosidases belonging to the glycoside hydrolase family GH2 and glycoside hydrolase family GH42 [29]. Here, we report a new β-galactosidase from the marine bacterium Alteromonas sp. ML117, which was isolated from the deep sea. This novel cold-adapted β-galactosidase belongs to the GH2 family, and we describe its purification and biochemical characterization.