The Underworld of Hydrothermal Vents

One of the most common reproductive strategies in the Ocean is larva dispersal, a process where animals release their young into the open sea. Once eggs, sperm, or fertilized eggs develop into larvae and disperse, the open seas serve as a nursery ground. Species commonly affiliated with seafloor environments will settle as they mature into a more “teenage” stage. Some animals like corals or tubeworms adhere themselves to rocks, either in shallow waters or in the deep sea. Dispersal and settlement are important biological mechanisms for maintaining genetic diversity in the Ocean. Parents cast their young to the sea and rely on the Ocean’s physics to help ensure they reach new areas and maintain the genetic health of the population. While these processes are well-studied in coastal environments, there is little research on dispersal and settlement in the deep sea, especially for hydrothermal vent communities.

Hydrothermal vents are dynamic ecosystems prone to regular disturbances like volcanic eruptions and earthquakes. Some hydrothermal vents have brief lifespans, yet, wherever a hydrothermal vent forms, a biological community almost always follows, and how these animals arrive is yet to be determined. Even more mysteriously, while scientists have plentiful evidence that hydrothermal animals are reproducing, and water samples taken from the surrounding area contain plenty of larvae, those of tubeworms have not been found yet. This suggests that these hydrothermal animals may not only rely on the open Ocean to disperse their young.

Dr. Monika Bright and her team have a different hypothesis for how larvae are transported and create new biological communities on hydrothermal vents. They believe that the larvae, viruses, fungi, and bacteria travel beneath the seafloor and through the vents rather than in the overlying water—this would be a completely new phenomenon in science. To determine if this is the case, they will travel to the East Pacific Rise to conduct experiments with the potential to uncover those potential processes.

Beneath the Vents
The subseafloor is the underground space below the ocean floor, and is made from mud, sand, or volcanic rock. Scientists estimate that the total habitable space for life in the Ocean’s subseafloor is equivalent to 6% of the Ocean’s volume (Hondt et al., 2019). The subseafloor environment is poorly studied, especially in the deep sea, but is suspected to be vital in nutrient cycling and carbon sequestration for our planet. Beyond exploring dispersal at hydrothermal vents, Bright and her team will study the entire community living in the subseafloor below hydrothermal vents, including the bacteria, fungus, and viruses. They will work to determine the role this microscopic, underground community plays in supporting life in the world above them, and suspect it is in this underworld where some hydrothermal animals may acquire their larval stage symbionts.

Additionally, Bright and her team hypothesize that while the subseafloor is a hostile environment for most life, the bodies of smaller organisms are more likely to withstand it. The team believes that their travel time through the earth's crust from one vent to another may be relatively short, taking only a few hours before the larvae reach the surface again.

Tica and Research Methodologies
The Underworld of Hydrothermal Vents team will travel to the East Pacific Rise to conduct their experiments, specifically on a well-documented vent site known as Tica. They have chosen a well-studied area so that their research may contribute to the body of knowledge on these vents. First, they will attach an in situ mass spectrometer, a device for measuring molecules and isotopes, to ROV SuBastian. The scientists will use this sensor to create complete environmental and chemical profiles of the subseafloor around the vents.

The team will also collect rock, fluid, and animal samples from the surface and fluid samples from the subsurface, using ROV SuBastian’s pumps and robotic arms. They will cover then the surface with a custom-designed mesh box and a staining box to dye all the remaining animals. Leaving the mesh box for some time will allow larvae and small animals from the subsurface to settle in the mesh. Collecting fluid in the mesh box and rocks up to 10 cm below the seafloor afterwards will allow to distinguishing between previously stained and newly settled unstained animals on the rocks to examine the communities living in fissures in the earth’s crust. Through these collections and experiments, the team will paint a holistic picture of ecosystems in the subseafloor around Tica, potentially providing answers to the question of how hydrothermal vent communities form