Vertical distribution of intracellular protoporphyrin IX in coastal sediment cores: Implications for sedimentology and microbial community composition

Protoporphyrin IX (PPIX), a basic porphyrin system found in nature, all “porphyrin-type” tetrapyrroles with a biological function are biosynthetically derived thereof. PPIX is a metalloprosthetic group of numerous proteins involved in diverse metabolic and respiratory processes across all domains of life, and is thus considered essential for respiring organisms. Determining the biotic and abiotic factors that influence marine microbial growth and community structure is critical for understanding global biogeochemical cycles. Here, we present vertical profiles of intracellular PPIX and four derivative products (Chlorophyll-a/b and Pheophytin-a/b) from two coastal sediment cores, alongside ancillary geochemical and 16S rRNA microbial community data. Our findings indicated that PPIX is present in the natural sediment environment and displays a decreasing trend with depth, revealing a significant positive correlation with both organic matter and microbial abundance. Co-occurrence networks revealed that the environmental distribution of PPIX was positively correlated with the microbial porphyrin producer (high genetic completeness), but negatively correlated with auxotrophs (absence or low genetic completeness). It emphasized the critical role of PPIX as a biological molecule involved in key physiological processes. These results suggest that PPIX is a prominent component of the shared extracellular metabolite pool, especially in anoxic marine sediments where it exists at physiologically relevant concentrations for microbial metabolism. This study highlighted the significance of PPIX in microbial ecology and its potential impact on biogeochemical cycles in marine sedimentary environments.