## 1 Introduction  
Lycopene, a naturally occurring carotenoid predominantly found in red or yellow pigmented edible plants such as tomatoes [1], watermelons [2], pomergnates [3], and papayas [4], exhibits significant antioxidant properties through its capacity to neutralize reactive oxygen species and scavenge free radicals [5, 6]. Given these potent bioactivities, global market demand for lycopene has surged substantially [7], driving intensified research efforts toward optimizing its biosynthesis pathways [8]. Extracting lycopene from plants, especially from tomatoes, is a traditional and commonly used method, often involving various extraction techniques such as organic solvent extraction, ultrasound-assisted extraction, and supercritical fluid extraction [8]. However, the lycopene content in tomatoes and other plants is relatively low [9], and there are issues with inefficient resource utilization [10]. Chemical synthesis of lycopene poses potential risks to food safety due to chemical residues. On the other hand, microbial production of lycopene, as a more efficient and safer production method, has been widely adopted [8].  
Photosynthetic bacteria, especially purple non-sulfur bacteria (PNSB), demonstrate remarkable adaptability to diverse environmental conditions. They can grow chemo-heterotrophically under aerobic conditions or photoheterotrophically under anaerobic light conditions. These bacteria naturally synthesize carotenoids, which makes them ideal candidates for lycopene biosynthesis [10]. Among the PNSBs, potential lycopene producers reported in the literatures include _Rhodospirillum rubrum_, _Rhodopsseudomonas faecalis_, and _Cerebacter gluneroides_ (formally _Rhodobacter sphaeroides_) [11, 10, 12, 13]. However, the accumulation of lycopene in wildtype strains is typically low (0\(\sim\)3.24 mg/g DCW), and metabolic engineering strategies are often required to achieve higher yields, usually by means of enhancing precursor synthesis or eliminating competing pathways. For instance, [PERSON] et al. increased the lycopene yield from 3.24 mg/g DCW to 32.06 mg/g DCW in the wildtype _Rhodopsseudomonas palustris_ by knocking out the acyclic carotenoid 1,2-hydratase gene (_cntC_) [13]. Similarly, [PERSON] et al. introduced the _cntI\({}_{4}\)_ gene from _Rhodospirillum rubrum_ into the wildtype _C. sphaeroides_ and knocked out the _cntC_ gene, thereby boosting the yield to 10.32 mg/g DCW [14].  
In PNSBs, dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP), the precursors of lycopene, are primarily synthesized through the methylerythitol 4-phosphate (MEP) pathway [15]. Within this synthetic pathway, phytoene desaturase (Crtl) is a key enzyme that catalyzes the four-step dehydrogenation of phytoene to form lycopene [15]. Therefore, the activity and functionality of Crtl significantly influence the bacteria's capacity of synthesizing lycopene [16, 17, 18, 19]. Currently, the production of lycopene using PNSBs is predominantly carried out under anaerobic light conditions. However, this strategy will be limited by the insufficient illumination that is caused by high cell density in large-scale fermentation. The purpose of this study is to explore the feasibility of synthesizing lycopene under aerobic conditions, investigate the effects of amino acids on bacterial lycopene synthesis, and delve into the underlying mechanisms.