Diatoms are a significant group of algae, playing a vital role as primary producers in aquatic ecosystems. They contribute approximately one-fifth of the world’s total primary production, matching the net productivity of tropical rainforests. Unlike many other algae, diatoms primarily store energy in the form of oils or golden seaweed polysaccharides. These oils are accumulated within the cell as oil droplets, making up 40 to 60% of their dry weight. This high lipid content makes them a promising candidate for biodiesel production. However, the molecular mechanisms behind their oil accumulation remain largely unknown.
Recently, Hu Xiaohua, an associate researcher at the Institute of Hydrobiology, Chinese Academy of Sciences, conducted a study using *Phaeodactylum tricornutum* as a model organism. Their research aimed to uncover how different metabolic pathways contribute to triglyceride synthesis during oil accumulation. Through subtractive hybridization, they identified a gene involved in leucine degradation, named *MCC2*, which was significantly upregulated during lipid accumulation.
It is commonly assumed that oil accumulation occurs under nutrient limitation, leading to increased amino acid degradation. However, the exact relationship between this process and lipid production has not been clearly established. Using fluorescent quantitative PCR and non-labeled quantitative proteomics (in collaboration with the Gemini Institute of Fisheries Research), the researchers found that not only was amino acid degradation increased, but also key metabolic pathways such as glycolysis, the tricarboxylic acid cycle, pyruvate metabolism, and fatty acid and triglyceride synthesis were all significantly upregulated.
During lipid accumulation, carbon streams from both amino acid degradation and glycolysis enter the tricarboxylic acid cycle. From there, they either pass through the malate shuttle or directly enter the chloroplast as pyruvate to support fatty acid synthesis. Functional verification experiments showed that when *MCC2* was knocked down, triglyceride synthesis in the algae decreased by 28 to 37%. These findings suggest that up to 40% of the lipids in *Phaeodactylum tricornutum* may be derived from the breakdown of other cellular components under nitrogen-limited conditions. This implies that the degradation of branched-chain amino acids, particularly leucine, plays a central role in oil accumulation.
Metabolite analysis revealed that the degradation of three branched-chain amino acids was significantly reduced in *MCC2*-knockdown strains. Although levels of other amino acids like glutamine, arginine, glutamic acid, proline, alanine, ornithine, and aspartic acid were much higher, their concentrations rapidly dropped to very low levels. This suggests that these amino acids are closely linked to the urea cycle unique to diatoms. Before nitrogen deprivation, cells use the urea cycle to produce ammonia and carbon dioxide or synthesize polyamines, which are stored within the cell. The study also found that proline degradation leads to the formation of leucine, further supporting the idea that branched-chain amino acid metabolism is deeply connected to lipid production.
Overall, this research provides the first direct evidence that carbon flow from glycolysis and branched-chain amino acid degradation contributes significantly to lipid accumulation in diatoms. These insights could help improve biofuel production strategies by enhancing lipid yield in diatom-based systems.
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