A recent investigation has unveiled how a certain type of foraminifera, which are single-celled organisms found in nearly all marine environments, survives in areas devoid of light and oxygen.
While most life forms on Earth depend on sunlight for sustenance, what happens to those organisms dwelling in the deep sea where sunlight cannot penetrate? A recent study spearheaded by the Woods Hole Oceanographic Institution (WHOI) and published in The ISME Journal provides insights into how a specific foraminifera species manages to exist in a dark, oxygen-free environment.
This foraminifera species employs a metabolic strategy known as chemoautotrophy. This process uses inorganic substances, potentially including sulfide, to absorb carbon, enabling survival in environments lacking oxygen. Chemoautotrophy has been documented in Bacteria and Archaea, which are simpler microbial organisms that lack a true nucleus. However, foraminifera are classified as eukaryotes, meaning they possess a defined nucleus that contains their genetic material.
“Foraminifera, along with animals, plants, and seaweed, fall under the eukaryote category. Our interest in studying this particular foraminifera arises from its ability to thrive in environments akin to those on Earth during the Precambrian era, a time before animal life emerged,” stated Fatma Gomaa, a research associate in WHOI’s Geology & Geophysics Department. “In that period, the oceans had little to no available oxygen and contained high levels of harmful inorganic substances, resembling some modern habitats found on the ocean floor, particularly in sediments. Investigating the energy and carbon sources utilized by this foraminifera can help us understand how such species adapt to environmental changes and deepen our insights into the evolution of eukaryotic life on Earth.”
To collect foraminifera specimens, the research team used the remotely operated vehicle Hercules from the E/V Nautilus, managed by the Ocean Exploration Trust, to gather sediments approximately 570 meters (1,870 feet) beneath the ocean surface near California’s coast. The team employed two primary methods to study the life strategies of the foraminifera. The first method involved freezing samples in a preservative with a red dye to maintain the foraminifera in situ. They then examined the organisms’ metabolic pathways through gene expression analysis. Additionally, in situ incubations with an isotopic carbon tracer were conducted, enabling researchers to trace labeled metabolites in chemical reactions. These incubations remained on the ocean floor for about 24 hours before being retrieved and subsampled in red light.
“Our analysis of the seafloor tracer incubations revealed that the tracer moved from the water and became associated with the foraminifera biomass. This provided insights into the carbon sources these organisms utilize,” remarked Daniel Rogers, an associate professor of chemistry and department chair at Stonehill College. “It was crucial to make these observations at depth, where the organisms naturally exist. Bringing them to the surface exposes them to light, raises the temperature, and alters the pressure conditions. This in situ approach allows us to better understand how these organisms endure in such extreme conditions.”
The study received funding from NASA, which is exploring the potential for life on other planets and how it may survive. Although the deep sea and extraterrestrial environments differ in many ways, they share some characteristics like cold temperatures, darkness, and, in many places, a lack of oxygen. Joan Bernhard, a senior scientist in WHOI’s Geology & Geophysics Department and an expert on foraminifera, has spent decades studying this group of benthic foraminifera to understand their survival strategies in challenging environments throughout much of Earth’s history.
“Foraminifera are extremely plentiful on our planet, with most measuring only about 300 microns in diameter. In a space no larger than a pencil eraser, you could find up to 500 individuals of this species in a dark, oxygen-free, sulfidic habitat,” explained Bernhard. “This species incorporates chloroplasts from unrelated organisms — these organelles support photosynthesis when exposed to sunlight. This phenomenon is referred to as kleptoplasty, wherein one organism appropriates chloroplasts from another. Even though these foraminifera never receive sunlight, we recognize that kleptoplasty is occurring here; however, we require further research to uncover why these foraminifera are so successful in their dark, oxygen-deficient environment.”
In addition to their remarkable ability to adapt to extreme habitats, foraminifera shells are also valuable in studies related to climate change and the search for hydrocarbon deposits. “We possess fossil records of foraminifera that extend back over half a billion years, providing a longer history of this group than most other life forms on Earth,” Bernhard added. “By examining these fossils, we can trace how their shells have reacted to historical shifts in environmental parameters such as temperature, salinity, pH, and oxygen levels. By analyzing the geochemistry trapped in their shells, foraminifera serve as excellent indicators of the age and environmental conditions of geological deposits, which is crucial for accurately constructing climate records. The discovery that a foraminifera species is chemoautotrophic raises new questions about our interpretations of their geochemical records and whether we are understanding them correctly. It’s possible that other foraminifera species also exhibit similar behaviors.”
The researchers preserved specimens of two additional foraminifera species, and preliminary findings suggest biological differences among these types. Scientists are currently executing similar investigations to identify the energy and carbon sources for these species as well.
