Abstract :
[en] The circular economy has gained increasing attention in recent years. Particularly, the upcycling of liquid by-products of industrial processes rich in organic matter to produce high added-value compounds is promising. Purple non-sulphur bacteria (PNSB) are good candidates for the development of such circular bioprocesses due to their metabolic versatility [1]. Indeed, PNSB were extensively studied for bioindustrial purposes, such as wastewater treatment or the production of bioplastic and biohydrogen [2].
Sugar beet molasses, a co-product from sugar industries, is mainly composed of sucrose (~61%) but also contains fructose and glucose in very low proportions (<1%) [3]. The growth of PNSB on molasses has already been studied, especially in the field of biohydrogen production [4]. However, if macroscopic parameters of their growth have been determined with this carbon source, the sugar assimilation metabolism has been poorly investigated. In this work, we analysed the growth of two strains of PNSB with sucrose as a main carbon source under phototrophic conditions, either in pure culture or in co-culture.
In the case of Rs. rubrum, results showed very limited growth through OD680nm monitoring in a sucrose-containing medium (Fig. 1A). However, this strain was able to assimilate fructose and glucose sequentially, with fructose being first assimilated (Fig. 1B). Concerning Rh. capsulatus, it was observed that it is capable of assimilating sucrose, fructose and glucose (Figs. 1C and 1D). In co-cultures, bacterial growth reached a final OD680nm of 3.64±0.196 in the presence of sucrose and 3.95±0.179 in the presence of sucrose, fructose and glucose. Such a high bacterial growth had not yet been observed in pure cultures. Surprisingly, while we have previously shown that this strain was unable to assimilate sucrose, Rs. rubrum was still present at the end of the experiment, either in presence of sucrose only or in presence of sucrose, fructose and glucose (Respectively, fig. 1E and 1F, bar graphs). Based on these results, we hypothesised that Rh. capsulatus releases organic molecules into the extracellular environment, which would then be assimilated by rubrum as a carbon source. A further investigation is required to identify the organic molecule released by Rh. capsulatus following sugar assimilation.
