Life in total darkness: Foraminifera survive with no light or oxygen

Most life on Earth depends on sunlight, yet some organisms flourish in the dark depths of the ocean where sunlight never reaches. 

A study led by the Woods Hole Oceanographic Institution (WHOI) provides new details about the survival of foraminifera – tiny, single-celled eukaryotes found in diverse marine environments.

The experts investigated how a species of foraminifera thrives in an oxygen-free, lightless, deep-sea habitat by employing a remarkable metabolic strategy: chemoautotrophy.

Organisms in the depths of the ocean

Chemoautotrophy is a process where organisms harness energy from inorganic compounds, such as sulfide, to assimilate carbon and sustain life in the absence of sunlight.

While this metabolic pathway is common among prokaryotic bacteria and archaea (which have no nucleus in their cells), it is unusual in eukaryotes (organisms with a nucleus in each cell), that typically rely on sunlight or organic matter for their source of energy. 

Foraminifera, though eukaryotic, have adapted to use chemoautotrophy to enable them to survive where oxygen and sunlight are absent.

“Animals, plants, seaweed, and foraminifera are all eukaryotes. We were interested in studying this foraminiferan because it thrives in a very similar environment to [that present on] 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.”

According to Gomaa, understanding the energy and carbon sources used by this foraminiferan helps to answer questions on how these species adapt to environmental changes, as well as advancing our knowledge on the evolution of eukaryotic life on Earth.

Investigating life in an extreme habitat

The research team collected sediment samples containing these foraminiferans approximately 570 meters (1,870 feet) beneath the ocean’s surface off the coast of California. For this, they used the remotely operated vehicle Hercules, which was deployed from the vessel E/V Nautilus. 

To study these organisms in their natural state, scientists infused samples with a preservative containing red dye and preserved them in situ, which avoided the destructive changes that can occur when organisms are brought from depth to the surface.

Two primary techniques were used to understand the foraminiferan’s metabolic strategies. First, gene expression analyses were conducted on preserved samples to identify metabolic pathways. 

Second, in situ incubations were carried out using an isotopic carbon tracer, which allowed researchers to track how the organisms incorporated carbon over a 24-hour period while still on the seafloor.

“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. 

Professor Rogers noted that it was important 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.”

Stealing chloroplasts in the dark

Another intriguing discovery is the process of kleptoplasty observed in these foraminiferans. Despite living in complete darkness, this species sequesters chloroplasts – organelles typically used for photosynthesis – from other organisms. 

Although these stolen chloroplasts cannot perform photosynthesis without light, their presence may confer other advantages, aiding the foraminiferan in nutrient acquisition or other metabolic processes.

“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,” explained Joan Bernhard, a senior scientist at WHOI, and an expert on foraminifera. 

“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 foraminiferan is so successful in the dark, without oxygen.”

Broader implications for science

The ability of foraminiferans to thrive in extreme environments not only expands our understanding of eukaryotic survival strategies but also has implications for climate change research and the search for extraterrestrial life. 

Foraminifera shells serve as important climate proxies, preserving a fossil record that spans over half a billion years. These fossils help scientists reconstruct past environmental conditions, but the discovery of chemoautotrophy prompts a reevaluation of how these organisms acquire energy and how their shells reflect ancient climates.

“By studying these fossils, we can see how their shells have responded to changes in the environment,” Bernhard noted. 

The metabolic adaptations of deep-sea foraminifera may require scientists to reconsider interpretations of geochemical records in their shells, potentially revealing new insights into Earth’s history.

Additionally, the funding interest from NASA highlights the relevance of this research beyond Earth. 

The deep-sea environment shares similarities with extraterrestrial settings – such as low temperatures, darkness, and an absence of oxygen – providing a model for how life might survive on other planets.

Future research on foraminifera

The study preserved specimens of two other foraminiferan species, with preliminary results suggesting distinct biological differences in their energy and carbon acquisition strategies. 

Ongoing research aims to compare these species, and to illuminate how various foraminifera thrive in diverse and extreme marine habitats.

As scientists continue to investigate these remarkable organisms, their findings not only advance our understanding of deep-sea life but also offer a glimpse into early eukaryotic evolution on a young, oxygen-poor Earth. 

Through the study of foraminifera, researchers gain insights into life’s resilience under harsh conditions – a knowledge that could inform our search for life beyond our planet and deepen our comprehension of Earth’s complex biological history.

The study is published in The ISME Journal.

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