Effects of Shipping Traffic On Marine Microbial Community Composition

Abstract

Shipping traffic across the world’s oceans continues to increase, and with it discharges of hydrocarbons and nutrient rich wastewater. In this thesis, we used a dataset of 971 marine metagenomic samples and estimates of shipping traffic density from satellite altimetry detections to measure the impact on the marine microbiome. Whole genome shotgun sequences were given taxonomic assignments through k-mer matching and aligned to genes for putative hydrocarbon degrading enzymes identified from ocean gene catalogs. Ship density had a significant (p < 0.05) impact on taxonomic community composition as measured by beta diversity metrics, but the effect size was smaller than other environmental variables such as temperature, indicating it did not impact the most abundant taxa. The abundance of several species of cyanobacteria had a significant association with ship density in more than one cruise, but the directions of association were not consistent with the expected effects on nitrogen fixing and non-nitrogen fixing genera from wastewater eutrophication. Five species belonging to known hydrocarbon degrading genera increased in abundance with ship density in individual sampling cruises. Additionally, gene sequences associated with several hydrocarbon degrading gene families increased in abundance with ship density in several individual cruises, and the aromatic degrading monoaromatic dioxygenase was found by a mixed linear model to increase with ship density across cruises. These taxonomic and functional results supported the hypothesis that operational discharges of oil from ships would increase the abundance of hydrocarbon degrading taxa. Overall, this thesis found that ship traffic density had a significant impact on microbial community composition, among this being associated with the increased abundance of hydrocarbon degrading taxa.