Glutamate Dehydrogenase Functions in Glutamic Acid Metabolism and Stress Resistance in Pyropia haitanensis

Pyropia haitanensis is an important laver species in China. Its quality traits are closely related to the content of glutamic acid. Glutamate dehydrogenase (GDH) is a crucial enzyme in the glutamic acid metabolism. In this study, two GDH genes from P. haitanensis, PhGDH1 and PhGDH2, were cloned and successfully expressed in Escherichia coli. The in vitro enzyme activity assay demonstrated that the catalytic activity of PhGDHs is mainly in the direction of ammonium assimilation. The measured Km values of PhGDH1 for NADH, (NH4)2SO4, and α-oxoglutarate were 0.12, 4.99, and 0.16 mM, respectively, while the corresponding Km values of PhGDH2 were 0.02, 3.98, and 0.104 mM, respectively. Site-directed mutagenesis results showed that Gly193 and Thr361 were important catalytic residues for PhGDH2. Moreover, expression levels of both PhGDHs were significantly increased under abiotic stresses. These results suggest that PhGDHs can convert α-oxoglutarate to glutamic acid, and enhance the flavor and stress resistance of P. haitanensis.

Effects of Nutrient Availability on the Release of Dissolved and Particulate Organic Carbon by Pyropia haitanensis and Its Implications

As an important component of the coastal-offshore ecosystem, Pyropia haitanensis aquaculture is continually being challenged due to rapid environmental changes because of global climate change and anthropogenic pressures. To explore the effect of nutrient availability on carbon metabolism by P. haitanensis, two strains of thalli were incubated for 15 days under four different concentrations of nitrogen and phosphorus. Significant increases in carbon, nitrogen, and phosphorus contents were observed in the algal tissue after the nitrates and phosphates enrichment, leading to elemental stoichiometry gradually approaching the Redfield ratio. Our results also showed a positive correlation between carbon or phosphorous accumulation and growth rate. Furthermore, under the natural seawater conditions, the release rates of dissolved organic carbon (DOC) and particulate organic carbon (POC) were highest, and more DOC was released than POC. The release rates tended to decrease with nutrient enrichment. The average proportions of DOC compared to total carbon were 6.3%–25.7%, while the average proportions of POC compared to total carbon were 2.1%–5.4%. Our results support the proposed importance of P. haitanensis in contributing the DOC and POC that play a significant role in the biological carbon pump and in sustaining marine aquaculture ecosystems in eutrophic environments.

Transcriptome Co-expression Network Analysis Identifies Key Genes Regulating Conchosporangia Maturation of Pyropia haitanensis

Conchosporangia maturation is crucial for the yield of Pyropia/Porphyra. However, the molecular mechanisms underlying this process are poorly understood. In this study, we selected two strains of Pyropia haitanensis that show significant differences in conchosporangia maturation as materials to produce RNA-Seq libraries. Then, we identified key molecular pathways and genes involved in conchosporangia maturation by conducting a weighted gene co-expression network analysis. Two specific modules were identified, and included functions such as phosphorus metabolism, lipid metabolism, and the phosphatidylinositol signaling system. The hub genes that responded positively during conchosporangia maturation encoded diacylglycerol kinase (DGK) and phosphatidylinositol-3-phosphate-5-kinase, which are involved in the synthesis of phosphatidic acid, a key component of lipid metabolism. A full-length DGK sequence of P. haitanensis, designated as PhDGK1, was obtained by rapid-amplification of cDNA ends. Conserved motif and phylogenetic tree analyses showed that PhDGK1 belongs to DGK Cluster II. The transcript level of PhDGK1 increased during conchosporangia maturation in both strains, but increased earlier, and to higher levels, in the early-maturing strain than in the late-maturing strain. This pattern of gene expression was consistent with the patterns of maturity and changes in pigment contents. These results indicate that lipid metabolism plays a key role in regulating conchosporangia maturation in Pyropia spp., and that PhDGK1 might be a useful molecular marker for breeding new early-maturing strains.