Background: Alginate is an important cell wall component and mannitol is a soluble storage carbon substance in the brown seaweed Saccharina japonica. Their contents vary with kelp developmental periods and harvesting time. Alginate and mannitol regulatory networks and molecular mechanisms are largely unknown. Results: With WGCNA and trend analysis of 20,940 known genes and 4264 new genes produced from transcriptome sequencing of 30 kelp samples from different stages and tissues, we deduced that ribosomal proteins, light harvesting complex proteins and “imm upregulated 3” gene family are closely associated with the meristematic growth and kelp maturity. Moreover, 134 and 6 genes directly involved in the alginate and mannitol metabolism were identified, respectively. Mannose-6-phosphate isomerase (MPI2), phosphomannomutase (PMM1), GDP-mannose 6-dehydrogenase (GMD3) and mannuronate C5-epimerase (MC5E70 and MC5E122) are closely related with the high content of alginate in the distal blade. Mannitol accumulation in the basal blade might be ascribed to high expression of mannitol-1-phosphate dehydrogenase (M1PDH1) and mannitol-1-phosphatase (M1Pase) (in biosynthesis direction) and low expression of mannitol-2-dehydrogenase (M2DH) and Fructokinase (FK) (in degradation direction). Oxidative phosphorylation and photosynthesis provide ATP and NADH for mannitol metabolism whereas glycosylated cycle and tricarboxylic acid (TCA) cycle produce GTP for alginate biosynthesis. RNA/protein synthesis and transportation might affect alginate complex polymerization and secretion processes. Cryptochrome (CRY-DASH), xanthophyll cycle, photosynthesis and carbon fixation influence the production of intermediate metabolite of fructose-6-phosphate, contributing to high content of mannitol in the basal blade. Conclusions: The network of co-responsive DNA synthesis, repair and proteolysis are presumed to be involved in alginate polymerization and secretion, while upstream light-responsive reactions are important for mannitol accumulation in meristem of kelp. Our transcriptome analysis provides new insights into the transcriptional regulatory networks underlying the biosynthesis of alginate and mannitol during S. japonica developments.

Glutathione S-transferase (GST) is a crucial enzyme for metabolism, detoxification, and stress resistance in organisms. Many GSTs have been identified in seaweeds, but the isolation and functional analysis of GSTs in Saccharina japonica have not been completed. In this study, a total of 32 SjGST genes, localized on 10 scaffolds and 6 contigs, were identified and categorized into three groups. Most of these SjGSTs were presumed to be distributed in the cytoplasm. Tandem duplication had a significant influence on the expansion of the SjGST gene family. Functional analysis of cis-acting elements in the promoter regions demonstrated that SjGSTs enhance the stress resistance of the kelp. Quantitative real-time PCR tests confirmed that SjGSTs positively influence S. japonica sporophytes under stress from low salinity, drought, and high temperature. Recombinant yeast tests further affirmed the role of SjGSTs in stress resistance; SjGSTs improved the growth rate of recombinant yeast under 1.5 M NaCl or 8 mM H2O2. Analysis of biochemical parameters indicated that the optimum temperatures for SjGST20 and SjGST22 were 20 °C, and the optimum pH values were 7.0 and 8.0 for SjGST20 and SjGST22, respectively. The Km values for the substrate 1-chloro-2,4-dinitrobenzene (CDNB) were 2.706 mM and 0.674 mM and were 6.146 mM and 3.559 mM for the substrate glutathione (GSH) for SjGST20 and SjGST22, respectively. SjGSTs are important stress resistant genes in S. japonica. This research results will enhance our understanding the function of GSTs in brown seaweeds, and explained its functional roles in stress resistance in marine environments.

Marine bacteria are crucial sources of alginate lyases, which play an essential role in alginate oligosaccharide (AOS) production. This study reports the biochemical characteristics of a new species of the Microbulbifer genus, Microbulbifer sp. HZ11. The strain HZ11 is Gram-negative, aerobic, flagellate-free, and rod-shaped. The genome of strain HZ11 is a 4,248,867 bp circular chromosome with an average GC content of 56.68%. HZ11 can degrade alginate and other polysaccharides. The carbohydrate-active enzyme (CAZyme) genes account for 4.57% of the total protein-coding genes of HZ11. Its alginate metabolism process is consistent with the characteristics of the polysaccharide utilization locus (PUL) system. The alginate lyase produced by strain HZ11 showed the highest activity at 50 °C, pH 8.5, and 0.1 M NaCl. The substrate preference was as follows: sodium alginate > poly mannuronic acid > poly guluronic acid. The thin layer chromatography (TLC) results revealed that the main enzymatic degradation products were monosaccharides or AOSs with a degree of polymerization (DP) of 2–3. These results help clarify the metabolism and utilization mechanism of alginate by marine bacteria and provide a theoretical reference for its application in the degradation of alginate and other polysaccharides.

Vibrio sp. is one of the main producers of alginate lyase; however, most strains have problems such as low and unstable enzyme production. In this study, the enzyme production conditions of V. sp. 32415, a marine bacterium capable of producing extracellular alginate lyase, were optimized through Response Surface Design. The optimized medium was as follows: NaCl 12 g/L, FeSO₄·7H₂O 0.067 g/L, NH₄Cl 7 g/L, alginate 11 g/L, K₂HPO₄·3H₂O 4 g/L, MgSO₄·7H₂O 1 g/L. Under 28 °C, 160 rpm, 30 mL/300 mL liquid volume, and an initial pH 5.5 culture condition, the extracellular enzyme activity was 51.06 U/mL, which was 2.8 times higher compared with the activity before optimization. The optimal temperature, pH, and NaCl concentration for the extracellular alginate lyase were 37 °C, 8.0, and 0.1 M, respectively. The enzyme remained more than 80% of its original activity at 30 °C for 4 h. 1 mM Fe³⁺, Ca²⁺, K⁺, Mg²⁺, and Na⁺ enhance enzyme activity, with a preference for polyG blocks. V. sp. 32415 has two circular chromosomes and one circular plasmid. Chromosome 2 has two polysaccharide utilization loci. It utilizes alginate through the Scatter pathway. The results of this study provide theoretical and data support for understanding the production of extracellular alginate lyase by marine Vibrio and their metabolism and utilization of alginate.