...bacteria, archaea, and fungi plus protists account for greater than 90% of oceanic biomass...

Vent fluids as a window into subseafloor biosphere

Low temperaure hydrothermal vent fluids below 120°C can be used as a winto into the microbial communities and biogeochemical processes occurring beneath the seafloor. (image credit: WHOI ROV Jason/Chris German)

Vent fluids as a window into subseafloor biosphere

Vent fluids as a window into subseafloor biosphere

Microscopic images of subseafloor microbes

image courtesy of Dr. Julie Huber, MBL

Microscopic images of subseafloor microbes

Microscopic images of subseafloor microbes

The world’s oceans are teeming with microscopic life forms. Communities of bacteria, archaea, and fungi plus protists account for greater than 90 percent of oceanic biomass and 98 percent of primary production. Although they are largely invisible to the naked eye, microorganisms are pervasive in all marine environments where they have had a profound impact on earth’s habitability and biodiversity.  Yet their diversity and distribution remain under-sampled and uncharted, including at deep-sea hydrothermal vents.

The base of the food chain

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 the carbon dioxide being emitted in vent fluids, and they in turn serve as a carbon source for the larger creatures often seen at deep-sea vents, like tube worms or mussels.  These microbes are considered the primary producers at vents, 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. 

Some like it hot

Other microbial members of the hydrothermal vent community 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)!  My research focuses on these high temperature organisms and how they interact with the complex and dynamic geochemical vent environment.  We are especially interested in the subseafloor microbial community.  The circulation of hydrothermal fluids and seawater occurs within the upper 500 m of porous oceanic crust and provides a rich environment for microbial growth beneath the seafloor.  To access this “invisible” environment, we collect hydrothermal vent fluids as a window into the subseafloor habitat.  The MCR offers a unique opportunity to study the microbial populations at a very different kind of submarine volcanic system compared to more “traditional” mid-ocean ridge or hot spot systems, including those at great depth (Piccard) and those in ultramafic hosted conditions (Von Damm). Based on our previous work here, we have designed new a number of experiments to probe further into the generation and consumption of methane by microbes and the influence of temperature and hydrogen on these important metabolic processes. In addition, because many vent organisms are difficult or nearly impossible to culture, we will also continue our molecular-biology DNA and RNA-based approaches begun in 2011 to study the subseafloor microbial community.  This includes direct approaches for determining the microbial diversity of the population, as well as more advanced genomic approaches that allow one to determine the genetic and functional composition of the microbial community, as well as individual organisms. We are also excited to continue our time series at these two sites, as well as extend our studies to new hydrothermal systems along the MCR that we hope will be found on Leg 1 before we join the trip!

From the beginning to space?

Finally, our interest in hydrothermal vent microbial communities spans much further than 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.

- by Dr. Julie Huber & Dr. Julie Reveillaud (FK008 - 2013)