Klau, L. J. | bioRxiv Science

bioRxiv2024

Phosphate starvation stops bacteria digesting algal fucan that sequesters carbon

Algae provide a solar powered pathway to capture and sequester carbon by injecting stable fucan made from carbon dioxide into the ocean 1-4. Stability of the pathway is at odds with the presence of marine bacteria with genes of enzymes that can digest fucan and release the carbon dioxide 5. Biochemical explanations for stable fucan remain hypothetical 6. We assembled a biological carbon cycle model and found phosphate limitation enhanced fucan synthesis by algae, stopped digestion by bacteria and thereby stabilized the fucan carbon sequestration pathway. Marine microalgae Glossomastix sp. PLY432 increased synthesis of fucan, a part of its extracellular matrix, under nutrient-growth limiting conditions. Rate and extent of fucan digestion by a marine, isolated bacterium of the Akkermansiaceae family decreased with decreasing phosphate concentration. Phosphate starvation restricted bacterial growth rate, biomass yield and in turn increased the amount of stable fucan. Phosphate is universally required for growth but rare relative to glycan carbon in photosynthesis-derived ecosystems. The fact that phosphate is required for replication, transcription and translation explains why bacteria can digest gigatons of laminarin with a few enzymes, but not fucan during nutrient limited algal blooms. We conclude phosphate starvation constrains the ability of bacteria to digest fucan, which evolves to maintain stability around algal cells and consequentially also to keep carbon dioxide in the ocean.

microbiology↗

bioRxiv2023

Metagenomic Data Reveal Type I Polyketide Synthase Distributions Across Biomes

Microbial polyketide synthase (PKS) genes encode the biosynthesis of many biomedically important natural products, yet only a small fraction of natures polyketide biosynthetic potential has been realized. Much of this potential originates from type I PKSs (T1PKSs), which can be delineated into different classes and subclasses based on domain organization and structural features of the compounds encoded. Notably, phylogenetic relationships among PKS ketosynthase (KS) domains provide a method to classify the larger and more complex genes in which they occur. Increased access to large metagenomic datasets from diverse habitats provides opportunities to assess T1PKS biosynthetic diversity and distributions through the analysis of KS domain sequences. Here, we used the webtool NaPDoS2 to detect and classify over 35,000 type I KS domains from 137 metagenomic data sets reported from eight diverse biomes. We found biome-specific separation with soils enriched in modular cis-AT and hybrid cis-AT KSs relative to other biomes and marine sediments enriched in KSs associated with PUFA and enediyne biosynthesis. By extracting full-length KS domains, we linked the phylum Actinobacteria to soil-specific enediyne and cis-AT clades and identified enediyne and monomodular KSs in phyla from which the associated compound classes have not been reported. These sequences were phylogenetically distinct from those associated with experimentally characterized PKSs suggesting novel structures or enzyme functions remain to be discovered. Lastly, we employed our metagenome-extracted KS domains to evaluate commonly used type I KS PCR primers and identified modifications that could increase the KS sequence diversity recovered from amplicon libraries. ImportancePolyketides are a crucial source of medicines, agrichemicals, and other commercial products. Advances in our understanding of polyketide biosynthesis coupled with the accumulation of metagenomic sequence data provide new opportunities to assess polyketide biosynthetic potential across biomes. Here, we used the webtool NaPDoS2 to assess type I PKS diversity and distributions by detecting and classifying KS domains across 137 metagenomes. We show that biomes are differentially enriched in KS domain classes, providing a roadmap for future biodiscovery strategies. Further, KS phylogenies reveal both biome-specific clades that do not include biochemically characterized PKSs, highlighting the biosynthetic potential of poorly explored environments. The large metagenome-derived KS dataset allowed us to identify regions of commonly used type I KS PCR primers that could be modified to capture a larger extent of KS diversity. These results facilitate both the search for novel polyketides and our understanding of the biogeographical distribution of PKSs across earths major biomes.

microbiology↗