Sustainable alginate lyases catalyzed degradation of bio-based carbohydrates

Alginate is a water-soluble and acidic polysaccharide derived from the cell wall and intercellular substance of brown algae. It is widely distributed in brown algae, such as Laminaria, Sargassum, and Macrocystis, etc. Alginate lyase can catalytically degrade alginate in a β-eliminating manner, and its degradation product-alginate oligosaccharide (AOS) has been widely used in agriculture, medicine, cosmetics and other fields due to its wide range of biological activities. This article is mainly to make a brief introduction to the classification, source and application of alginate lyase. We hope this minireview can provide some inspirations for its development and utilization.

At present, the preparation of algal oligosaccharides mainly adopts three kinds of degradation methods: chemical, physical and biological. Chemical methods include acid hydrolysis, alkaline hydrolysis and oxidative degradation, and physical methods mainly include hydrothermal method, ultrasonic method and radiation method, but these two methods have many drawbacks, which are not conducive to large-scale production. Biological methods mainly use microbial fermentation or the action of enzymes to degrade the prepared enzymes. It has the advantages of mild conditions, easy control, and strong product specificity, which has attracted people's attention, it may be an important direction of industrial production. (Guo et al., 2016 . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Source of alginate lyase
The sources of alginate lyase are extensive, and it has been reported that the production of alginate lyase mainly comes from marine algae, marine mollusks and microorganisms (including bacteria, fungi and some viruses). Among them, there are the most reports on the source of microorganisms, including (Pseudoalteromonas sp.) (Ma et al., 2008) (Vibrio sp.), (Kawamoto et al., 2006) (Flavobacterium sp.) ) (Paenibacillus sp.) (Zhu et al., 2020;Huang et al., 2022).

Application of alginate lyase
Alginate lyase is an important marine biological enzyme. AOS has various biological activities due to the differences in degradation mode, G content of degradation products (G/M ratio), molecular weight and spatial conformation. As an excellent natural antioxidant, alginate oligosaccharide has great application potential in the fields of human, animals and plants health (Yokose et al., 2009;Tondervik et al., 2014;Saberi Riseh, 2022). It can promote growth, improve stress resistance, increase yield, and inhibit fungal growth. With the implementation of the national strategy of regulating the use of chemical fertilizers and pesticides, it may become an environment-friendly bio-fertilizer and bio-pesticide in the future.
Alginate lyase is not only widely used in agriculture, but also in the fields of medicine and food. It can be used as a growth promoter for therapeutics such as antioxidants and tumor suppressors (Tusi et al., 2011;Hu et al., 2004), and can also induce cytokine production, regulate blood sugar and lipids (Iwamoto et al., 2005), which are widely used in the food and pharmaceutical industries. For example, Pseudomonas aeruginosa is one of the main pathogens of many chronic infectious diseases, such as chronic lung infection and urinary Frontiers in Chemistry frontiersin.org 02 tract infection (Jain and Ohman, 2005). The study found that algin is an important component of Pseudomonas aeruginosa biofilm, and Albrecht used lyase as an adjunct therapeutic agent together with antibiotics, which made the antibiotics come into direct contact with pathogenic bacteria to achieve the therapeutic effect (Albrecht and Schiller, 2005). This shows that the enzyme has great potential in the bactericidal application of biofilm pathogens.
Due to fuel consumption, researchers have begun to pay attention to the production of biofuel. At present, although chemical catalysis has high efficiency, its high cost, complex synthesis process, and high energy consumption under synthetic conditions limit its industrial application to a certain extent (Pan et al., 2022a;Pan et al., 2022b). Seaweed is considered an ideal source for bioethanol production due to its advantages of not occupying arable land and being non-polluting. According to reports, Wargacki et al. (2012) transformed Escherichia coli to establish a system for directly fermenting brown algae to produce ethanol. However, Escherichia coli has insufficient tolerance to ethanol, making it impossible for large-scale production. Sasaki et al. (2018) developed a co-cultivation platform for bioethanol production from brown algae, consisting of engineered yeast AM1 and CDY strains, which produced 2.1 g/L of ethanol when the brown algae Ecklonia kurome was used as the sole carbon source. This research has made significant progress in the biotechnology of brown algae to bioethanol, but it is still insufficient for industrial production. Studies have shown that the synergistic effect of multiple microorganisms on the ethanol fermentation system of macroalgae will be the trend of future research.

Conclusion and outlook
Alginate lyase has attracted the attention of researchers because of its unique properties and has great potential for application in various fields. At present, the related research on alginate lyase mainly focuses on the screening of strains, the mining of genes, and the analysis of the degradation substrate and product structure. With the continuous advancement of science and technology, people have gradually deepened the research on the analysis of alginate lyase protein crystal analysis, the catalytic mechanism of the active center and the transformation strategy. This will likely improve the problems of low enzyme-producing strains, low tolerance, and unstable properties in the large-scale industrial production process, and has great application potential in agricultural protection, biofuel production, and environmental protection.

Author contributions
ZZ and AD contributed equally to this work and drafted the manuscript. YL participated in some manuscript writing and checking. TL conceptualized and directed the whole project. All of the authors contributed to scientific discussions.

Funding
We acknowledge financial support from the Program of Introducing Talents