## 3 Results
### Deduced Biosynthetic Pathway of Lycopene in Cereibacter sphaeroides DT.1  
Previous studies have shown that most prokaryotes synthesize lycopene through the MEP pathway, which differs from the MVA pathway in eukaryotes [15]. Comparision of the whole genome sequence with the KEGG database revealed the existence of a complete carotenoid biosynthesis pathway in strain DT.1, including gene clusters related to lycopene metabolism such as _dxs_, _ispD_, _crtlDCEF_, _pps_R and _prr_A. However, key enzymes of the MVA pathway, such as Mvd and PmK, were not detected, indicating that this strain synthesizes lycopene through the MEP pathway, consistent with early findings reported in the literature [13]. Additionally, metabolomic data detected precursor substances related to lycopene synthesis, such as IPP and GPP, as well as downstream metabolites derived from further conversion of lycopene, including \(\beta\)-carotene, zeaxanthin, lutein, canthaxanthin, astaxanthin, spheroidenone, and torulene (Table 4), further confirming the existence of the MEP pathway. Based on the integrated analysis of genomic and metabolomic data, the inferred lycopene synthesis pathway is illustrated in Figure 2.  
It has been generally accepted that phytoene desaturase (Crtl) is a key enzyme in the MEP pathway, respsnible for catalyzing the four-step desaturation of GPP to form lycopene [26]. However, in the wildtype strains of _C. sphaeroides_, Crtl only possesses three-step desaturation activity and is incapable of fulfilling the fourth step desaturation reaction to convert neurosoprene to lycopene [14]. Therefore, introducing the four-step desaturation gene (_crtl4_) is a common practice for constructing an efficient lycopene synthesis pathway. For instance, [PERSON] et al. [27] succeeded in introducing the _crtl4_ gene from _Erwinia herbicola_ into _Escherichia coli_ to construct a lycopene synthesis pathway. Nevertheless, in the newly isolated wildtype strain DT.1 of _C. sphaeroides_, lycopene accumulation has been detected in HPLC analysis (Table 5), and several downstream metabolites, such as \(\beta\)-carotene, zeaxanthin and torulene, have been identified through metabolomic analysis (Table 4). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Index** & **MS2\_Name** & **RT/min** & **CK** & **Glu** & **Pro** \\ \hline M537T173\_POS & Beta-carotene & 2.88 & 7781.43 \(\pm\) 2634.71 & 14,439.83 \(\pm\) 1235.09 & 15,336.38 \(\pm\) 3178.37 \\ M569T184\_POS & Zeaxanthin & 3.07 & 963,075.70 \(\pm\) 142,406.20 & 926,266.30 \(\pm\) 341,863.26 & 776,124.00 \(\pm\) 3178.37 \\ M569T206\_POS & Lutetin & 3.43 & 1,160,456.61 \(\pm\) 65,581.64 & 919,355.83 \(\pm\) 362,079.99 & 653,686.38 \(\pm\) 35,581.47 \\ M619T231\_POS & Canthaxanthin & 3.02 & 6857.63 \(\pm\) 757.37 & 18,838.77 \(\pm\) 1054.73 & 7727.39 \(\pm\) 632.04 \\ M619T231\_POS & Astaxanthin & 3.85 & 2039.10 \(\pm\) 321.32 & 2957.82 \(\pm\) 453.69 & 2380.18 \(\pm\) 832.02 \\ M582T33\_POS & Spheroidenone & 0.55 & 21,182.96 \(\pm\) 9215.70 & 175,102.50 \(\pm\) 24,123.34 & 458,504.69 \(\pm\) 256,812.61 \\ M535T173\_POS & Torulene & 2.89 & 11,520.06 \(\pm\) 1701.78 & 40,305.76 \(\pm\) 19,795.78 & 23,530.64 \(\pm\) 7258.71 \\ M313T443\_NEG & GPP & 7.38 & 23,260.55 \(\pm\) 3630.39 & 152,115.30 \(\pm\) 8159.49 \(\pm\) 330,939.50 \(\pm\) 10,876.51 \(\pm\) \\ M381T318\_NEG & GPP & 5.30 & 19,575.74 \(\pm\) 1199.62 & 5487.88 \(\pm\) 677.15 & 4712.32 \(\pm\) 1371.83 \\ \hline \hline \end{tabular} Note. * indicates that these data are significantly regulated (\(p\) value \(<\) 0.05, VIP \(>\) 1). The data in the table are integral quantitative values of metabolites expressed as mean \(\pm\) standard deviation (\(n\) = 3). RT: Retention time. CK stands for yeast extract medium, Glu and Pro stand for yeast extract medium supplemented with 400 mg/L glutamate or proline, respectively.  
\end{table} 
Table 4: Metabolites related to lycopene synthesis in strain DT.1.