Tryptophan, thiamine and indole-3-acetic acid exchange betweenChlorella sorokinianaand the plant growth-promoting bacteriumAzospirillum brasilense

Abstract

During synthetic mutualistic interactions between the microalga Chlorella sorokiniana and the plant growth-promoting bacterium (PGPB) Azospirillum brasilense, mutual exchange of resources involved in producing and releasing the phytohormone indole-3-acetic acid (IAA) by the bacterium, using tryptophan and thiamine released by the microalga, were measured. Although increased activities of tryptophan synthase in C. sorokiniana and indole pyruvate decarboxylase (IPDC) in A. brasilense were observed, we could not detect tryptophan or IAA in the culture medium when both organisms were co-immobilized. This indicates that no extra tryptophan or IAA is produced, apart from the quantities required to sustain the interaction. Over-expression of the ipdC gene occurs at different incubation times: after 48 h, when A. brasilense was immobilized alone and grown in exudates of C. sorokiniana and at 96 h, when A. brasilense was co-immobilized with the microalga. When A. brasilense was cultured in exudates of C. sorokiniana, increased expression of the ipdC gene, corresponding increase in activity of IPDC encoded by the ipdC gene, and increase in IAA production were measured during the first 48 h of incubation. IAA production and release by A. brasilense was found only when tryptophan and thiamine were present in a synthetic growth medium (SGM). The absence of thiamine in SGM yielded no detectable IAA. In summary, this study demonstrates that C. sorokiniana can exude sufficient tryptophan and thiamine to allow IAA production by a PGPB during their interaction. Thiamine is essential for IAA production by A. brasilense and these three metabolites are part of a communication between the two microorganisms.

Impact of thiamine metabolites and spent medium from Chlorella sorokiniana on metabolism in the green algae Auxenochlorella prototheciodes

“…Overall, 5-day spent medium appears to offer limited thiamine metabolites. This finding is consistent with a hypothesis posited by Bashan et al in which C. sorokiniana provides thiamine to Azospirillum bacteria in co-culture [38]. Furthermore, C. sorokiniana's spent medium composition appears to change as the culture ages.…”

Fungal mycelia and bacterial thiamine establish a mutualistic growth mechanism

“…Thiamine and riboswitch have the potential to be used widely and universally stimulating symbiosis among microbes and even inter-kingdom interactions in nature. Other vitamins besides thiamine, which are essential for growth but sufficient in small amounts, are likely the seeds of commensal and mutualistic interactions of microbes in nature (Klein et al, 2013;Magnúsdóttir et al, 2015;Palacios et al, 2016;Sokolovskaya et al, 2020).…”

Symbiotic Co-Culture of Scenedesmus sp. and Azospirillum brasilense on N-Deficient Media with Biomass Production for Biofuels

“…It is already known that A. brasilense is able to establish cell to cell interactions with microalgae to sustain mutualistic symbiotic relations [17,54]. Also, significant increments in enzymatic activities were observed in literature studies [55] during the co-culturing of A. brasilense and Chlorella sorokiniana. This shows that the co-culturing of these two species involve other mechanisms different than the abiotic stresses, such as nitrogen deprivation.…”

“…The two main factors affecting protein content in co-culturing are the production of growth promoters and the biological nitrogen fixation from A. brasilense [26]. Based on growth promoters produced by A. brasilense it is known that during co-culturing with Chlorella sorokiniana, the microalgae provides tryptophan and thiamine to bacteria in exchange for IAA [55]. The exchange of amino acids to produce growth promoters and the effect of growth promoters itself can affect biomass growth and composition, including protein content, thus explaining the differences between protein content with and without A. brasilense in the culture.…”