Woods Hole, Mass. (Jan. 16, 2025) – Most life on Earth relies on the sun's energy for survival, but what about organisms in the deep sea that live beyond the reach of its rays? A new study led by Woods Hole Oceanographic Institution ( WHOI ), published in The ISME Journal, sheds light on how a species of foraminifera, single-celled organisms found in almost all marine habitats, thrives in a dark, oxygen-free environment.
For this foraminifera species, the answer is chemoautotrophy, a metabolic process that utilizes inorganic energy sources, perhaps sulfide, to take up carbon, enabling it to survive in oxygen-free environments. Chemoautotrophy has been observed within Bacteria and Archaea, which are microbial organisms without a true nucleus. However, foraminifera are eukaryotes, meaning they have a well-defined nucleus, which houses an organism's genetic material.
"Animals, plants, seaweed, and foraminifera are all eukaryotes. We were interested in studying this foraminifera because it thrives in a very similar environment to Earth during the Precambrian, a time before the evolution of animals," explained Fatma Gomaa , a research associate in WHOI's Geology & Geophysics Department. "During that time, there was very little to no available oxygen in the oceans and higher concentrations of toxic inorganic compounds; conditions similar to some modern environments found on the seafloor, especially within sediments. Understanding the energy and carbon sources used by this foraminifer helps us to answer questions on how these species adapt to environmental changes while advancing our knowledge on the evolution of eukaryotic life on Earth."
Using the remotely operated vehicle Hercules from the exploration vessel E/V Nautilus, operated by the Ocean Exploration Trust, the team collected sediments containing foraminifera about 570 meters (1,870 feet) below the ocean surface, off the coast of California. At depth, the team utilized two main methods to learn about the life strategies of the foraminifera. The first included infusing samples with a preservative (visible with red dye), preserving the foraminifera in situ. The researchers assessed their use of different metabolic pathways using gene expression analyses. Additionally, researchers used in situ incubations with an isotopic carbon tracer, a technique that allows tracking of labeled metabolites through chemical reactions. These incubations were kept on the seafloor for approximately 24 hours before being recovered and subsampled in red light.
"When we analyzed the seafloor tracer incubations, we could see that the tracer moved from the water and was associated with the foraminifera biomass. This gave us an idea of where these organisms were getting their carbon," said Daniel Rogers , an associate professor of chemistry and department chair at Stonehill College . "It was important for us to make these observations at depth, where these organisms are in their natural state. By bringing them to the surface, we expose them to light, increase the temperature of their environment, and change the amount of pressure they're under. This in situ approach gives us a more accurate depiction of how these organisms survive in such harsh environments."
This study was funded by NASA , which is interested in the possibility of life on other planets and how it might survive. While the deep sea couldn't be further from extraterrestrial planets, both environments share similarities such as cold temperatures, darkness, and in many locations, no oxygen. Joan Bernhard , a senior scientist in WHOI's Geology & Geophysics Department and foraminifera expert, has been studying this population of benthic foraminifera for decades to learn how these fascinating creatures survive in this challenging environment and have done so throughout a large portion of Earth history.
"Foraminifera are extremely abundant on earth. Most are only about 300 microns in diameter, so rather small. In a volume as small as a pencil eraser, there could be about 500 of this particular species in this dark, oxygen-free and sulfidic habitat." Bernhard explained. "This species takes up unrelated organism's chloroplasts—organelles that perform photosynthesis if exposed to sunlight. This process is called kleptoplasty, in which an organism steals chloroplasts from another type of organism, even though these foraminifera are never exposed to sunlight. We know kleptoplasty is happening here, but we needed more research to understand why this foraminifer is so successful in the dark, without oxygen."
Aside from their ability to thrive in what some consider to be an extreme habitat; the shells of foraminifera are also used in climate-change studies and for searching for hydrocarbon reserves. "We have fossil records of foraminifera dating back over half a billion years, which means we have a longer record of this group than most other life on Earth," Bernhard continued. "By studying these fossils, we can see how their shells have responded to changes in the environment, like temperature, salinity, pH, or oxygen. By studying the geochemistry preserved in their shells, foraminifera are excellent tools for showing the age and environment of a geologic deposit. All of this information is essential for building accurate climate records. The fact that a foraminifera species is chemoautotrophic raises questions about their geochemical records and whether we are interpreting them correctly. Other foraminifera species may also be performing this way."
Researchers also preserved specimens of two other foraminifera species and initial results suggest these types differ biologically. Scientists are presently conducting comparable analyses on these other species to pinpoint their energy and carbon sources.
About Woods Hole Oceanographic Institution
