**4. Conclusion**

This work reinforces the importance of the sulfate concentration, the nature and ratio of the sugars used as C-source, the presence of light, and the growth time for the production of biomass and carotenoids by *G. alkanivorans* strain 1B, leading to a better understanding of how these factors interact with each other to influence different metabolic responses.

In terms of sulfate, it shows that >20 mg/L are needed to consume 10 g/L C-source (glucose/fructose), while guarantying efficient biomass and carotenoid production. However, this concentration is enough to cause significant biodesulfurization inhibition, making it difficult to conjugate high carotenoid production with high desulfurization ability, without adapting the biocatalyst production method.

In terms of carbon, as expected, the presence of fructose leads to faster growth rates and greater biomass production. However, at later times, after growth has ended, it also led to a loss of biomass, especially in the absence of light, probably due to the production of extracellular compounds, such as biosurfactants. Moreover, fructose seems to inhibit carotenoid production to some extent, since even 25% of fructose can result in a great loss in carotenoid production. Glucose, on the other hand, hinders growth rates and stimulates carotenoid production and conversion of carbon to carotenoids and biomass, possibly indicating the accumulation of reserve substances, thus justifying the longer growth rates. The presence of light also seems to reduce growth rates, and stimulate carotenoid production, while making the fructophilic behavior more evident. In addition, the growth time period is especially important to generate specific carotenoids without light, and biomass in the presence of either light or greater glucose concentrations. All these responses seem to indicate that the fructophilic behavior of this strain is not simply a matter of sugar transport, since there are different metabolic behaviors in the presence/absence of both sugars.

The overall results indicate that higher glucose concentrations combined with more light, or a better-adapted system, with a higher surface to volume ratio, could drastically increase carotenoid production. In addition, this study confirms that is possible to produce carotenoids under dark conditions, and that production can be greatly stimulated by culture medium conditions. Moreover, it also reinforces that even without sugar consumption it is possible to induce carotenoid production in cells of strain 1B, opening the possibility of developing two-phase systems of biomass production based on fructose without light, and further carotenogenesis induction with light exposure, under optimal conditions.

Ultimately, these results may help in the development of a future biorefinery, either pointing the way to generate carotenoids under dark conditions, as an added value byproduct, or further increment carotenoid production in the light, as the main bioproduct.
