Diatoms are a significant group of algae, playing a crucial role as primary producers in aquatic ecosystems. They contribute approximately one-fifth of the world's primary production, comparable to the net primary production of tropical rainforests. Unlike many other algae, diatoms mainly store energy in the form of oils or golden seaweed polysaccharides. These oils are accumulated as droplets inside the cells, with their content potentially reaching 40 to 60% of the cell's dry weight. Due to this high oil content, diatoms are considered one of the most promising sources for biodiesel production. However, the molecular mechanisms behind their oil accumulation remain poorly understood.
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 to explore how various metabolic pathways contribute to triglyceride synthesis during oil accumulation. Through subtractive hybridization, they identified that a gene associated with leucine degradation, named *MCC2*, was significantly upregulated during lipid accumulation.
It is commonly believed that oil accumulation occurs under nutrient limitation, which often leads to the breakdown of amino acids. But the exact relationship between amino acid degradation and oil production has remained unclear. Recent research using fluorescent quantitative PCR and non-labeled quantitative proteomics (in collaboration with the Gemini Institute of Fisheries Research) revealed that, in addition to amino acid degradation, several key metabolic pathways—including glycolysis, the tricarboxylic acid cycle, pyruvate metabolism, and fatty acid and triglyceride synthesis—were also significantly activated during lipid accumulation.
During this process, carbon from amino acid degradation and glycolysis enters the tricarboxylic acid cycle, then either via the malate shuttle or directly into the chloroplast as pyruvate for fatty acid synthesis. Functional verification showed that when *MCC2* was knocked down, the triglyceride content in the algae decreased by 28 to 37%. This suggests that up to 40% of the lipids in *Phaeodactylum tricornutum* may be derived from the breakdown of other cellular components under nitrogen-limited conditions. The study implies that the degradation of branched-chain amino acids, particularly leucine, plays a central role in oil accumulation.
Metabolite analysis further revealed that the degradation of three branched-chain amino acids was inhibited in *MCC2*-knockdown strains. Although levels of other amino acids like glutamine, arginine, glutamic acid, proline, alanine, ornithine, and aspartic acid were much higher, these also dropped rapidly. This indicates that these amino acids are closely related to the urea cycle, which is unique to diatoms. Before nitrogen deprivation, the urea cycle helps generate ammonia and carbon dioxide, or it synthesizes polyamines that are stored in the cell.
Interestingly, the study also found that proline degradation first produces leucine. Overall, the experiments provided the first evidence that the carbon flow from both glycolysis and branched-chain amino acid degradation contributes to lipid accumulation in diatoms. This discovery offers new insights into the metabolic pathways involved in oil synthesis and could help improve the efficiency of biofuel production from diatoms.
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