The ecology of deep-sea chemosynthetic habitats, from populations to metacommunities

Abstract

Chemosynthetic ecosystems are habitats whose food webs rely on chemosynthesis, a process by which bacteria fix carbon using energy from chemicals, rather than sunlight-driven photosynthesis for primary production, and they are found all over the world on the ocean floor. Although these deep-sea habitats are remote, they are increasingly being impacted by human activities such as oil and gas exploration and the imminent threat of deep-sea mining. My dissertation examines deep-sea chemosynthetic ecosystems at several ecological scales to answer basic biology questions and lay a foundation for future researchers studying these habitats. There are two major varieties of chemosynthetic ecosystems, hydrothermal vents and cold seeps, and my dissertation studies both. My first chapter begins at cold seeps and at the population level by modeling the population dynamics and lifespan of a single species of tubeworm, Escarpia laminata, found in the Gulf of Mexico. I found that this tubeworm, a foundation species that forms biogenic habitat for other seep animals, can reach ages over 300 years old, making it one of the longest-lived animals known to science. According to longevity theory, its extreme lifespan is made possible by the stable seep environment and lack of extrinsic mortality threats such as predation. My second chapter expands the scope of my research from this single species to the entire cold seep community and surrounding deep-sea animals common to the Gulf of Mexico. The chemicals released at cold seeps are necessary for chemosynthesis but toxic to non-adapted species such as cold-water corals. Community studies in this area have previously shown that seeps shape community assembly through niche processes. Using fine-scale water chemistry samples and photographic mapping of the seafloor, I found that depressed dissolved oxygen levels and the presence of hydrogen sulfide from seepage affect foundation taxa distributions, but the concentrations of hydrocarbons released from these seeps did not predict the distributions of corals or seep species. In my third chapter I examine seep community assembly drivers in the Costa Rica Margin and compare the macrofaunal composition at the family level to both hydrothermal vents and methane seeps around the world. The Costa Rica seep communities have not previously been described, and I found that depth was the primary driver behind community composition in this region. Although this margin is also home to a hybrid “hydrothermal seep” feature, this localized habitat did not have any discernible influence on the community samples analyzed. When vent and seep communities worldwide were compared at the family-level, geographic region was the greatest determinant of community similarity, accounting for more variation than depth and habitat type. Hydrothermal vent and methane seeps are two chemosynthetic ecosystems are created through completely different geological processes, leading to extremely different habitat conditions and distinct sets of related species. However, at the broadest spatial scale and family-level taxonomic resolution, neutral processes and dispersal limitation are the primary drivers behind community structure, moreso than whether the habitat is a seep or a vent. At more local spatial scales, the abiotic environment of seeps still has a significant influence on the ecology of deep-sea organisms. The millennial scale persistence of seeps in the Gulf of Mexico shapes the life history of vestimentiferan tubeworms, and the sulfide and oxygen concentrations at those seeps determine seep and non-seep species’ distributions across the deep seafloor.