Curtis, N. | Search bioRxiv

bioRxiv2026

Decreased Damage for proton FLASH vs Conventional Dose Rates in Mouse Jejunum Shown by Quantitative Assessment of γ-H2AX

Purpose: FLASH radiation with ultra-high dose rate delivery is less damaging to normal tissue than conventional radiation ( <1 Gy/s). Since radiation depletes oxygen (ROD), this damage reduction might occur via the oxygen effect. ROD experiments have shown an oxygen-independent reduction in dose effectiveness at FLASH dose rates. However, prior in vivo ROD measurements relied on extracellular oxygen probes that could not penetrate cell membranes, leaving intracellular effects unresolved. To investigate the ROD hypothesis more directly, we developed a novel three-component immunohistochemical assay with algorithmic image processing to quantitatively compare DNA damage following FLASH and conventional irradiation in mouse jejunum. Methods: Mice received intravenous EF5 2 hours before proton irradiation at FLASH (103.63 +/- 17.2 Gy/s) or conventional (0.73 +/- 0.1 Gy/s) dose rates of 2.5 Gy or 5 Gy, with unirradiated controls. Mice were euthanized 30 minutes post-irradiation, and 10 cm of jejunum was frozen as a 'Swiss Roll', sectioned, stained, and imaged. Tissue sections were stained for {gamma}-H2AX, DRAQ5, and EF5 to assess DNA double-strand breaks, total DNA content, and hypoxia, respectively. An in-house algorithm identified individual cell nuclei and registered each nucleus with its corresponding {gamma}-H2AX and EF5 signals, enabling quantitative measurement of DNA damage as a function of local tissue hypoxia. Results: Hypoxia was greatest in the villi and, to a lesser extent, the outer jejunal musculature, with substantial inter-animal variation. DNA damage decreased in hypoxic regions. FLASH enhanced the hypoxia-associated reduction in DNA damage compared with conventional dose rate and, separately, revealed an oxygen-independent reduction in DNA damage, suggesting an additional FLASH sparing mechanism. Conclusion: Current results suggest that FLASH compared to conventional dose rate radiation caused less DNA damage with increasing effect at low oxygen levels, a result consistent with ROD as a mechanism. Pronounced tissue heterogeneity in murine jejunum requires further studies to segment the effect for each tissue type.

biophysics↗

bioRxiv2026

Quantitative metagenomics reveals fine-scale population dynamics across bacteria, archaea, and microbial eukaryotes in an estuarine-coastal continuum

Microbial communities are foundational to marine ecosystem function, yet their diversity is often obscured by broad taxonomic groupings and relative-abundance surveys that mask the dynamics of individual populations. This limitation is especially important across estuarine-coastal gradients, where microbial standing stocks, environmental conditions, and community composition vary sharply over space and time. Here, we used quantitative, genome-resolved metagenomics to examine microbial population dynamics across a one-year estuary-to-ocean transect spanning the Neuse River Estuary, Pamlico Sound, and adjacent North Atlantic shelf waters. Internal standard normalization enabled absolute abundance estimates for single-copy genes and metagenome-assembled genomes (MAGs), allowing individual populations to be tracked as genome equivalents per liter. Bacterial standing stocks were higher in estuarine waters, and communities varied primarily with salinity and season. We recovered 415 MAGs, including 386 bacterial genomes that represented, on average, 52% of bacterial genome equivalents, along with archaeal and eukaryotic representatives. Many abundant MAGs lacked close reference genomes, demonstrating that numerically important coastal populations remain poorly characterized. Genome-resolved abundances revealed pronounced niche partitioning among closely related taxa, including seasonal and spatial turnover of Synechococcus, Cyanobium, and Vulcanococcus populations associated with distinct pigment-defined cytometric groups. Rhodobacteraceae MAGs also showed population-specific correlations with picoeukaryotic MAGs, including a winter offshore Planktomarina population that reached 18% of total bacterial genome equivalents during a Micromonas-associated bloom. By providing absolute population abundances, this study transformed coastal microbiome surveys into numerical frameworks for resolving microbial population structure, ecological interactions, and biogeochemical relevance across dynamic environmental gradients. IMPORTANCEEstuarine and coastal waters contain diverse microbial communities that help regulate food webs and the cycling of carbon and nutrients, but many of the individual microbial populations responsible for these processes remain poorly understood. In this study, we examined bacteria, archaea, and small algae across the Neuse River Estuary, Pamlico Sound, and nearby coastal ocean waters over one year. By measuring the absolute abundance of individual microbial genomes, rather than only their relative proportions, we showed that closely related populations can have very different seasonal and spatial patterns. This was especially clear for cyanobacteria related to Synechococcus and heterotrophic bacteria in the family Rhodobacteraceae, which showed distinct population dynamics and associations with small algae. These results demonstrate how quantitative genome-resolved measurements can reveal hidden microbial population structure and improve our understanding of how microorganisms shape coastal ecosystem function.

genomics↗