Alpha-linolenic acid and stearidonic acid are precursors of omega-3 polyunsaturated fatty acids, essential nutrients in the human diet. The ability of cyanobacteria to directly convert atmospheric carbon dioxide into bio-based compounds makes them promising microbial chassis to sustainably produce omega-3 fatty acids. However, their potential in this area remains unexploited, mainly due to important gaps in our knowledge of fatty acid synthesis pathways. To gain insight into the cyanobacterial fatty acid biosynthesis pathways, we analyzed two enzymes involved in the elongation cycle, FabG and FabZ, in Synechococcus elongatus PCC 7942. Overexpression of these two enzymes led to an increase in C18 fatty acids, key intermediates in omega-3 fatty acid production. Nevertheless, coexpression of these enzymes with desaturases DesA and DesB from Synechococcus sp. PCC 7002 did not improve alpha-linolenic acid production, possibly due to their limited role in fatty acid synthesis. In any case, efficient production of stearidonic acid was not achieved by cloning DesD from Synechocystis sp. PCC 6803 in combination with the aforementioned DesA and DesB, reaching maximum production at 48 h post induction. According to current knowledge, this is the first report demonstrating that S. elongatus PCC 7942 can be used as an autotrophic chassis to produce stearidonic acid.

Circadian clocks allow organisms to anticipate daily fluctuations in light and temperature, but how this anticipatory role promotes adaptation to different environments remains poorly understood. Here, we subjected the cyanobacterium Synechococcus elongatus PCC 7942 to a long-term evolution experiment under high light, high temperature, and elevated CO ₂ levels. After 1,200 generations, we obtained a strain exhibiting a 600% increase in growth rate. Whole-genome sequencing revealed three mutations fixed in the evolved population, two of which were sufficient to recapitulate the fast-growing phenotype in the wild type. A mutation in the promoter of the shikimate kinase aroK led to its overexpression, while a mutation in the central circadian regulator sasA disrupted both the phase and amplitude of the circadian rhythm. Changes in circadian control led to widespread perturbations in the transcriptome and metabolome. These included major shifts in the Calvin–Benson–Bassham cycle and glycogen storage dynamics. While these changes increased fitness under the experimental conditions, they caused maladaptation when light or CO ₂ levels were altered, revealing a trade-off between fitness and environmental flexibility. Our results demonstrate that mutations in circadian control can drive fast adaptation by modulating central metabolism, underscoring the circadian cycle as a cornerstone of cellular plasticity. Thus, targeting the circadian cycle could be key to engineering cyanobacterial strains optimized for carbon fixation and biomass production.