Ramon, C. E. | Search bioRxiv

bioRxiv2026

Drought Dominates Engineered Lipid Sink Effects on Sorghum Physiology and Carbon Allocation

Determining the environmental limits that govern how engineered metabolic traits control whole-plant carbon (C) allocation is essential for developing climate-resilient bioenergy systems. Oil-enhanced sorghum offers a promising strategy to boost aboveground energy density, yet its system-wide effects on root C investment and soil C delivery, particularly under drought, are largely uncharacterized. Here, we provide the first comprehensive assessment of these interactions using whole-plant 13CO2 continuous labeling, depth-specific sampling of roots and rhizosphere soil, and coordinated measurements of photosynthesis, stomatal anatomy, tissue nitrogen, and root non-structural carbohydrates (NSCs). Under well-watered conditions, oil enhancement produced distinct aboveground physiological changes, including slightly longer stomata, lower stomatal density, significantly higher leaf nitrogen concentration and biomass, and greater leaf 13C enrichment. Importantly, these aboveground changes did not translate into detectable shifts in the allocation of recent photosynthate to belowground pools. In contrast, drought acted as a dominant regulatory factor, reorganizing C flow across both genotypes by suppressing photosynthesis and leaf water status, increasing root nitrogen and NSC reserves, and significantly promoting the retention of recent assimilates in shallow root systems. Depth-specific 13C patterns showed that drought reduced new C incorporation into deep roots while increasing 13C enrichment in deep rhizosphere soil, suggesting that drought altered the balance between C investment in root growth and rhizodeposit C inputs at depth. Ultimately, soil moisture availability was an overriding determinant of belowground C partitioning and vertical C delivery, superseding the influence of the engineered lipid sink. These findings provide new mechanistic insight into the environmental constraints on C flow in an engineered bioenergy crop and identify moisture-driven limitations as the primary bottleneck for translating synthetic metabolic innovations into robust, ecosystem-level C outcomes.

ecology↗

bioRxiv2026

Iron limitation alters diatom carbon flow through shifts in microbiome exometabolite consumption

Iron is required for photosynthesis, and thus affects biogeochemical cycling in widespread regions where its availability is limited. Turnover of aquatic photosynthetically-derived carbon is largely constrained by bacterial activity, but we lack a mechanistic understanding of how iron limitation influences this activity. We examined a bacterial enrichment community dependent on carbon from the diatom Phaeodactylum tricornutum to investigate how iron limitation alters the flow of carbon to bacteria, and exometabolite and community composition. Using stable isotope tracing, we quantified diatom exudate incorporation with single-cell-resolution. We identified a population of bacteria under iron limitation with high metabolic activity yet low incorporation of newly-fixed diatom carbon, indicating a shift in metabolism relative to the iron-replete control. Ultra-high-resolution exometabolomics revealed bacterial consumption of aromatics, lipid-like compounds, and purines and pyrimidines occurred under iron-limitation, when these compounds also exhibited increased exudation. We identified gene pathways for utilization of these compounds in taxa with increased abundance under iron limitation which may be responsible for carbon flow shifts. These results provide a mechanistic link between iron-limitation driven shifts in exudate composition and flow of carbon to the microbiome. This has important implications for predicting carbon flow in surface oceans and manipulating algal-bacterial interactions in engineered systems.

microbiology↗