Metabolites cross the membrane, the transcriptome does not follow: co-localization is required for the cyanobacterial response to a yeast partner

Synechococcus elongatus shows a time-escalating transcriptome response to Rhodotorula toruloides only in mixed co-culture, with 51 genes changing (day-averaged q < 0.05) and none behind a 0.22 m membrane.

• Why it matters: Cross-feeding models often assume metabolite exchange alone explains phototroph responses, but it is unclear whether spatial proximity is required for gene regulation in engineered or natural partnerships.
• What they did: The authors co-cultured sucrose-secreting Synechococcus elongatus with Rhodotorula toruloides either together or separated by a 0.22 m pore membrane, using matched axenic controls, then tracked metabolite flow and cyanobacterial gene expression over days 4, 6, and 8.
• The result: Yeast metabolites reached the cyanobacterium in both setups, yet cyanobacterial iModulons (nine programs, mainly nitrogen and photosynthesis) responded only with co-localization, while membrane separation yielded just one changed gene and fewer differentially expressed genes.

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