Rising CO2 concentration was reported to increase phytoplankton growth rate as well as lipid productivity. This has raised questions regarding the NADPH supply for high lipid synthesis as well as rapid growth of algal cells. In this study, growth, lipid content, photosynthetic performance, the activity, and expression of key enzymes in Calvin cycle and oxidative pentose phosphate pathway (OPPP) were analyzed in the marine diatom Phaeodactylum tricornutum under three different CO2 concentrations (low CO2 (0.015 %), mid CO2 (atmospheric, 0.035 %) and high CO2 (0.15 %)). Both the growth rate and lipid content of P. tricornutum increased significantly under the high CO2 concentration. Enzyme activity and mRNA expression of three Calvin cycle-related enzymes (Rubisco, 3-phosphoglyceric phosphokinase (PGK), phosphoribulokinase (PRK)) were also increased under high CO2 cultivation, which suggested the enhancement of Calvin cycle activity. This may account for the observed rapid growth rate. In addition, high activity and mRNA expression of G6PDH and 6PGDH, which produce NADPH through OPPP, were observed in high CO2 cultured cells. These results indicate OPPP was enhanced and might play an important role in lipid synthesis under high CO2 concentration. The oxidative pentose phosphate pathway may participate in the lipid accumulation in rapid-growth P. tricornutum cells in high CO2 concentration.

The long-term coexistence of sea cucumber-associated microorganisms with their host enables them to jointly withstand the unique marine ecological environment, and possess great potential for producing various natural products. However, under conventional laboratory conditions, most biosynthetic gene clusters (BGCs) in these microorganisms remain silent, necessitating the establishment of effective activation strategies for exploring bioactive secondary metabolites (SMs). Histone acetylation status regulates chromatin structure and plays a crucial role in cellular physiology and fungal secondary metabolism. Penicillium sclerotiorum SD-36 was isolated from sea cucumbers in our previous study. Genome sequencing results indicate that this strain harbors as many as 52 BGCs, suggesting it holds a wealth of genetic resources essential for synthesizing diverse SMs. Here, we describe the impact of a class 1 histone deacetylase (HDAC), PsHos2, on secondary metabolism of sea cucumber-associated Penicillium sclerotiorum SD-36. The colony morphology and SM profile of ΔPsHos2 exhibited significant changes, with the emergence of multiple new compound peaks. Six compounds, including five azaphilones, which are characterized by a pyranoquinone core structure, were isolated from ΔPsHos2, and seventeen unreported potential azaphilone-related nodes were obtained using molecular networking based on LC-MS/MS. Transcriptome analysis revealed that PsHos2 influenced the expression of 44 BGC core genes. Specifically, seven genes within cluster 86.1, the putative BGC for azaphilones, were upregulated, including two polyketide synthase (PKS) genes. The results indicate that regulation based on class 1 HDACs is an important strategy for enhancing SM synthesis in sea cucumber-associated fungi and expanding the resources of marine natural products.