UC Santa Barbara

Marine sediment microbial communities are significantly influenced by anthropogenic activities, which alter their composition and function. They also play an invaluable role in determining the fate of contaminants in the sediment environment. The San Pedro (SP) Basin in the Southern California Bight is a prime case study to explore this phenomenon, having been subjected to extensive contamination from sewage input to the bordering Palos Verdes (PV) Shelf and offshore dumping. Historical records have confirmed the occurrence of offshore dumping of industrial wastes, and we observe some of these waste products, such as petroleum (tar) and dichlorodiphenyltrichloroethane and its derivatives (DDX), in high abundance in the SP Basin. This study analyzes sediment cores from the SP Basin to assess the structuring of the microbial community and its relationship to contaminants. Using 16S rRNA gene sequencing, this study reveals variations in microbial community composition corresponding to a range of environmental gradients such as sediment depth, water depth, sedimentation rates, and inputs from the PV shelf, with emphasis on variations that correspond to anthropogenic impacts. To my knowledge, this work is the first to characterize microbial community structure of deep coastal basin sediments at high spatial resolution on a regional scale. Taxa such as Proteobacteria and Myxococcota decrease in abundance with sediment depth, while Chloroflexi, Firmicutes, and Archaea tend to increase. A decline in Shannon diversity indices with increasing sediment depth and distance from the shelf suggests that factors such as sediment age and organic matter (OM) characteristics play crucial roles. Non-metric multidimensional scaling (NMDS) analysis highlights correlations between sediment age, water depth, excess carbon load, and microbial community structure. Some taxa like Anaerolineae and Desulfobacterota show increased abundance in regions with high levels of contaminants as indicated by excess carbon or DDX, which may reflect their established roles in processes such as reductive dehalogenation and anaerobic hydrocarbon degradation. This research underscores the profound impact of anthropogenic inputs on sediment microbial communities and their essential roles in biogeochemical processes. Future studies will employ metagenomic analyses alongside additional geochemical analyses to further explore the functional potential of these communities. Understanding the dynamics of microbial communities in contact with toxic contaminants is crucial for developing effective strategies to mitigate pollution and preserve marine ecosystems.