## 3 Results
### C40 Carotenoid biosynthetic Pathways in the Antarctic Ocean as Elucidated by Metagenomic Analysis
Enclosed within boxes are numerical values representing the gene abundance of enzymes, quantified in transcripts per million (TPM), providing a quantitative assessment of gene abundance. Cases flagged with a question mark indicate missing genes encoding enzymes such as O-glycostransferase (CruG) in the carotenoid biosynthesis pathway (k000906). of hydroxy/keto groups and an \(\chi\)-end group, which are pivotal in defining odenone's chemical structure (Figure 5). Metagenomic analysis indicates the absence of genes encoding carotenoid 1,2-hydratase (CrtC), 1'-hydroxy-\(\gamma\)-carotene C-4' ketolase (CrtO), and carotenoid \(\chi\)-ring synthase (CrtU) at both sampled depths. Contrastingly, a gene encoding \(O\)-methyltransferasease (CrtF) showed a higher abundance at 16 m compared to 25 m.  
3.4 Assessment of the Prevalence of C40 Spheroidene Biosynthesis Genes in the Antarctic Ocean Microbiota  
Spheroidene is a carotenoid of the C40 family, known for its contribution to the photosynthetic apparatus of purple bacteria and its role as a photoprotective agent. Within the extreme photic conditions of the Antarctic Ocean, the biosynthesis of spheroidene suggests a highly specialized adaptation mechanism [32]. Our metagenomic analysis indicated that genes encoding C-3',4'-desaturase (CrtD) and \(O\)-methyltransferasease (CrtF) were more abundantly presented at the 16 m depth compared to 25 m (Figure 6), suggesting a stratification in biosynthetic activity that would correlate with environmental gradients, such as light or oxidative stress. Notably, gene encoding hydroxyneurosporene synthase (CrtC), which is critical in spheroidene biosynthesis, was not detected at both depths. Figure 6: Spheroidene and spirilloxanthin (and 3,4,3',4'-terahydroospirilloxanthin) biosynthesis as revealed by metagenomic data analysis. Each enzymatic step was documented with its corresponding KEGG identifier. Color coding shows carotenogenic genes with higher abundance at specific depths. The absence of genes encoding enzymes was indicated by ’NA’, representing the lack of detectable genes at those depths. Numerical values enclosed within boxes denote the gene abundance of enzymes, measured in transcripts per million (TPM), offering a quantitative assessment of gene abundance. 3.5 Assessment of the Prevalence of C40 Spirilloxanthin Biosynthesis Genes in the Antarctic Ocean Microbiota  
Spirilloxanthin, another member of the C40 carotenoid family, is recognized for its significance in bacterial photosynthetic processes and membrane stabilization [33]. The spirilloxanthin biosynthetic pathway is a series of enzymatic reactions that convert neurosporene into spirilloxanthin. This pathway is commonly found in purple non-sulfur bacteria like _Rhodospirillum rubrum_. Spirilloxanthin is synthesized through a cascade of enzymatic reactions starting from the precursor neurosporene: Iycopene synthase (Crtl), carotenoid-1,2-hydratase (Crtl), 1-hydroxycarotenoid 3,4-desaturase (Crtl), and \(O\)-methyltransferasearse (Crtf). This cascade facilitates sequential modifications that transform lycopene into rhodopsin, 3,4-dehydrorhodopin or 3,4-dihydrororhodovibrin, and eventually spirilloxanthin and 3,4,3',4'-terahydrospirilloxanthin (Figure 6).