By Julie A. Huber, PhD, Assistant Scientist - Marine Biological Laboratory
and Julie Meyer, PhD, Postdoctoral Researcher - Marine Biological Laboratory
As a refresher, all of life on earth is divided into three domains- Bacteria, Archaea, and Eukarya. Bacteria and Archaea are single celled organisms with no true nucleus, no membrane-bound organelles, and they divide by binary fission. They are very, very small—invisible to the naked eye, whereas most eukaryotes tend to be bigger things, like humans, tigers, shrimp, and zooplankton. Microorganisms are everywhere on Earth—in your belly, dirt, the oceans—and they have had a profound impact on Earth’s habitability and biodiversity, especially considering they have been around for about 3.5 billion years. Yet their diversity and distribution remain under-sampled and uncharted, especially in the deep sea.
Since the discovery of hydrothermal vents in the late 1970s, it has been shown that microbes are ubiquitous in and around hot and warm sea-floor vents driven by volcanic heat. Microbes have the ability to capture energy from a huge range of chemical processes, and many of the microbes at deep-sea hydrothermal vents do not need sunlight or oxygen to survive. Some live off of sulfur, hydrogen, or iron, while others produce methane. Many of these organisms fix their own carbon from carbon dioxide and use energy sources like hydrogen sulfide being emitted in vent fluids to grow. In turn, they serve as a carbon source for the larger creatures often seen at deep-sea vents, like tube worms or mussels. These chemosynthetic microbes are considered the primary producers at vents, like plants in the sunlit world, providing energy for all other life forms found there. This is a fundamentally different ecosystem than the photosynthetic light-driven ecosystem that most of us are familiar with at the surface of the ocean or in our backyard gardens. Until vents were discovered, nobody even knew this way of building an ecosystem was possible!
There are many different “niches” or habitats that microbes can live in at vents—sediments, black smoker chimneys, venting fluids, mats, even on rocks beneath the seafloor! Some microbes are heat-loving, or thermophilic, meaning they grow at very high temperatures, from a “mild” 40 °C (104 °F) all the way up to 121 °C (250 °F)! We study these communities using a suite of molecular, microscopic and enrichment-based techniques to examine the adaptation of microbes to their geologic and chemical habitat.
On this mission to the Mid-Cayman Rise, we are especially interested in documenting the microbial habitats in ultramafic-hosted sites—sites with rocks that are extremely rich in hydrogen, magnesium and iron. There is very little known about microbes in this type of environment and our interests in hydrothermal vent microbial communities spans much further than just the bottom of the ocean. It has been hypothesized that life may have originated and evolved near deep-sea hydrothermal systems, and that organisms currently living in these likely analogues of early habitats may still harbor characteristics of early life. Microbes unique to this environment could provide insight into metabolic processes, strategies for growth, and survival of life forms in the subsurface of solar bodies with a water history. For example, Jupiter’s satellite Europa may harbor a liquid ocean with life-supporting hydrothermal systems beneath its icy shell. And the recent detection of methane in the Mars atmosphere has brought considerable attention to methane generation, both abiotic and biotic, and in general, determining if Mars can feasibly support microbial metabolisms that use or generate methane. With a sample size of only one, the search for life beyond Earth must begin with life as we know it on Earth, and deep-sea hydrothermal vents are a great place to begin our search for unusual life that pushes the limits of life as we know it.