Highlight authors: Lindsey Biondo¹, Marialena Christopoulou¹, Shannon Klein, Ph.D.², Dr. Larissa Frühe3, Kailani Acosta, Ph.D.3
Dissolved oxygen is critical in aquatic ecosystems, enabling marine organisms to survive and perform basic biological processes. However, in specific marine environments, there are oxygen depleted zones - areas in which the dissolved oxygen level is low enough that it becomes difficult to sustain most marine life. Historically, such zones have largely been documented in temperate regions, either within enclosed basins or out in the open ocean. In tropical regions, coral reefs have long-been regarded as well-oxygenated ecosystems, so research on the existence of persistent oxygen depleted zones in these systems is scarce. Yet, warmer waters and their three-dimensional complexity make these systems more likely to host such zones, especially in deeper waters.
Coral reef in the Red Sea
Led by the National Center for Wildlife (NCW), the Saudi Arabian governmental agency responsible for biodiversity assessments, conservation strategy, and more in the region - including oversight of both terrestrial and aquatic environments - a team of expert scientists was assembled to investigate unexplored, deep waters in the Red Sea, which is bordered by Saudi Arabia along with Djibouti, Egypt, Eritrea, and Yemen. Shannon Klein, Ph.D., Research Scientist from King Abdullah University of Science and Technology (KAUST), and Dr. Larissa Frühe, Postdoctoral Associate from OceanX, led studies with a focus on eDNA and oxygen dynamics, alongside other leading experts.
The NCW was instrumental as a principal partner on this expedition: providing scientific resources, guidance, and expertise. Goals for this research were identified based on conservation and biological management interests as part of the Red Sea Decade Expedition.
KAUST provided scientific expertise by contributing multiple scientific working groups and access to laboratories with different specialties for this integrative research. OceanX focused on fostering its partnerships with regional scientists and institutions to map and characterize the seafloor onboard the R/V OceanXplorer – an 87 meter research vessel equipped with scientific labs for data collection and analyses as well as a media lab for editing photos and videos from their discoveries in real time. These critical collaborations helped advance oceanography and marine science, while equipping local experts with cutting-edge tools onboard and skills to drive long-term protection and sustainable use of the ocean.
“OceanX is an impact-driven organization. The science we lead is designed to go beyond discovery. We want to place any critical knowledge we find in the field, directly in front of decision-makers to inform action, protect the ocean, and help build a more sustainable future for us all.”
- Kailani Acosta, Ph.D., Science Program Associate at OceanX
What began as a chance encounter with an unexpected pocket of low oxygen concentrations quickly turned into a larger scientific pursuit. While surveying the ocean floor of the Red Sea to investigate previously studied seafloor depressions, the team became particularly interested in deep depressions where oxygen levels were markedly depleted. This discovery prompted many questions about the biology, chemistry, and geology of these regions and the species that inhabit these extreme conditions. Led by Klein, the team addressed this work showcased in their 2025 PNAS Nexus study, Deep oxygen-depleted depressions in a Red Sea coral reef sustain resistant ecosystems. Drawing on patterns observed in other marine systems, they hypothesized that the deep structures and warm tropical environment would act to restrict vertical mixing to deplete dissolved oxygen. A second critical question followed: what processes set these systems in motion in the first place? Identifying the catalyst behind the formation and persistence of these low-oxygen depressions became central to understanding not only their physical dynamics, but also how life manages to endure within them.
The research was initiated and conducted as part of the Red Sea Decade Expedition, an initiative championed by NCW, to serve as the infrastructure to develop a clearer vision of the Red Sea’s environments. One of the main objectives of the Red Sea Decade Expedition was to conduct extensive research to generate data to identify areas of high biodiversity, fuel marine conservation policies, and better understand ecological relationships within the Red Sea.
Saudi Arabia aims to protect at least 30% of its land and seas by 2030, a target supported through the mandate of the NCW. This initiative is one amongst many from the Saudi Arabian government to propel Saudi Vision 2030 - a strategy aimed at investing in key sectors and supporting ambitious projects. The focus is to create a Saudi Arabia where, “tradition, innovation, and sustainability go hand-in-hand.”
With the Red Sea Decade Expedition as the impetus, this multi-organizational collaboration between the NCW, KAUST, and OceanX showcases the possibilities for discovery when government, academia, and research institutions come together to garner greater understanding of the natural world.
In addition to the primary partners, additional collaborators were involved to help make this research a success, including Beneath the Waves, King Abdul Aziz University, Ministry of Earth, Water and Agriculture, NEOM, NHK, Red Sea Global, and VICE News.
With the international, multi-disciplinary team aboard the R/V OceanXplorer, they began to map the area they were interested in researching, the Farasan Bank, to investigate deep depressions beneath the surface of shallow coral reefs. Using acoustic mapping alongside remotely operated vehicles (ROV) and submersibles, they began to put together a clearer picture of the seafloor of the Farasan Bank, specifically looking at the areas they suspected had depressions deep enough to contain ecosystems that are fundamentally different from those found above. They pinpointed two specific locations on the bank that they hoped to investigate.
Fig. 1 (from Deep oxygen-depleted depressions in a Red Sea coral reef sustain resistant ecosystems 2025) Locations of the depressions in the Difaht Farasan and their structural features. a) Overview map of the Red Sea, highlighting the location of the Difaht Farasan. b) Map of the Difaht Farasan, the geological formation from which the Farasan Islands emerged. The positions of the depressions discussed in this work are marked. c and d) 20-m resolution bathymetry collected in March 2022 showing seafloor morphology of the two depressions (Farasan Deep and Amq Deep). The color scale ranges from 60- to 800-m depth, and the locations of Farasan Deep c) and Amq Deep d) are also shown in b). On the bathymetric maps, triangle symbols mark the location of the CTD casts, circles represent the ROV dives, and the diamond symbols denote the manned submersible dives. All assets were deployed at the deepest accessible points of the surveyed depressions.
The team was excited to find the previously documented locations were correct – they found numerous deep depressions beneath the surface waters of shallow reefs. With the exact locations of the coral reef depressions confirmed and the bathymetry maps created, they began to identify points of interest in which they would need to take samples to determine dissolved oxygen concentrations.
Photo courtesy of NCW and OceanX: Sea-Bird Scientific’s SBE 9/11plus CTD rosette profiler and SBE 43 oxygen sensor
Sea-Bird Scientific’s tried and trusted SBE 9/11plus CTD rosette profiler and SBE 43 oxygen sensor were chosen for this mission and deployed using an oceanic rosette system off the R/V OceanXplorer. Taking 143 profiles over the four and a half months of the expedition, including both up and down casts, the system was deployed at depths of up to 620 meters at the deepest profiling location in this study. Sea-Bird Scientific instrumentation was chosen as the primary profiling solution due to its high reliability and accuracy over time. According to Klein,
“...it’s incredibly important that we have reliable sensors with high accuracy. These sensors were incredibly stable going through the water column,”
and notes that limited fluctuation is key to stable measurements and defensible data.
To validate the oxygen measurements, they used Winkler titrations, a method for measuring dissolved oxygen using chemical reactions, and found that the oxygen measurements from the SBE 43 performed better than the Winkler titrations at very low oxygen concentrations, emphasizing the criticality of both reliable instrumentation and data validation.
Fig. 2 (from Deep oxygen-depleted depressions in a Red Sea coral reef sustain resistant ecosystems 2025) Depth profiles of environmental parameters in the Farasan depressions. a–c) Full-depth profiles of dissolved oxygen concentrations, temperature, and salinity measured by the Sea-Bird Scientific CTD profiler (SBE 9/11plus), with oxygen (SBE 43) verified against the Winkler titration method at high oxygen levels. d–h) Profiles of measured (TA and DIC) and calculated (pHT, pCO2, and ΩAr) parameters of the seawater carbonate system. i–l) Depth profiles of measured concentrations of inorganic nutrients. d–l) Parameters measured in discrete water samples from Niskin bottles. Blue lines and dots represent data collected at Farasan Deep (from 3 to 491 m), whereas data collected at Amq Deep (3–619 m) are indicated by purple lines and dots. a–l) Dashed lines represent the approximate depth of the carbon maximum zones. Error bars represent ±1 SD.
One of the main objectives of this research was to better understand what drives the formation of persistent low oxygen zones in the tropics, such as those observed in the Red Sea. The team of interdisciplinary experts aboard the R/V OceanXplorer found no evidence that these oxygen depleted zones in the Red Sea were from anthropogenic influences.
When thinking about oxygen depletion zones, one would initially hypothesize that life does not thrive in low oxygen extremes, but what the team found was contrary to this hypothesis. Klein shares,
“..one of the most surprising angles of the study is that we found diverse marine life living in these warm, oxygen depleted zones.”
The team was excited to share one of their most surprising discoveries: lanternfish in this region had developed a behavioral adaptation that enabled them to persist in low oxygen environments by slowing down their swimming speed. The team believes this is a method to preserve energy and oxygen usage to enhance chances of survival. The team also found another behavioral adaptation of the lanternfish – it can exist in these low-level oxygen conditions at the high temperatures of 22 degrees Celsius, while in other regions, fish survive at similar oxygen levels but at much cooler temperatures, which reduces their requirements for oxygen.
Fig. 3 (from Deep oxygen-depleted depressions in a Red Sea coral reef sustain resistant ecosystems 2025). High-density lanternfish (Benthosema sp.) aggregation exhibits behavioral adaptations in stable suboxic conditions. a) Small aggregation (up to six individuals frame−1) of an unidentified cryptobenthic fish in Farasan Deep (at 286-m water depth, <2 µmol O2 kg−1). b) Comparison of the maximum number of lanternfish (Benthosema sp.) observed in any single video frame (Max N) of ROV footage between 13 open-water reference sites and Farasan Deep and Amq Deep. c) Large aggregation (up to 295 individuals frame−1) of Benthosema sp. observed in Farasan Deep under suboxic conditions (10.9 µmol O2 kg−1) at 619-m water depth. Inset: microscope image of a Benthosema sp. specimen (scale bar: 1 cm). d) Comparison of estimated swimming speeds of Benthosema sp. individuals in the reference sites (n = 29 individuals) and Amq Deep (n = 10 individuals), where Benthosema sp. aggregations were observed. a, c) Image credit: the National Center of Wildlife (NCW, Kingdom of Saudi Arabia).
While life was diverse in these oxygen deprived environments, the team notes that as soon as oxygen levels increased further up the water column, so did the diversity of organisms.
Furthermore, the team found that the two study sites, separated by several hundreds of kilometers, exhibited different degrees of oxygen depletion due to differences in their geomorphology. At one of the sites, the SBE 43 oxygen sensors could not detect any oxygen, while at the second observed site, the sensor could detect very low levels of oxygen indicating a suboxic environment. Klein goes on to describe the conditions that make up a suboxic environment as a place that contains,
“...oxygen concentrations that are obviously detectable, but very restrictive or in theory should be very restrictive of marine life at such warm temperatures.”
Photo courtesy of NCW and OceanX: Klein, Frühe, Parry (all authors of the study) taking water samples from the CTD rosette system for oxygen and nutrient analysis.
Dr. Larissa Frühe, a Postdoctoral Associate at OceanX who specializes in molecular ecology and marine microbiology, draws attention to the changes in oxygen levels alongside the variation in biological diversity. She notes,
“...[the variation] makes it even more interesting to see that we got persistently low or zero oxygen in areas so close to a highly diverse coral ecosystem.”
The variety of organisms found in these unsuspecting areas highlights the importance of ecosystems in these oxygen depleted zones for biodiversity and provides a basis for increased conservation and policy to protect these fragile environments.
Furthermore, accumulation of nitrate, phosphate, and silica at the bottom of these depressions show similar concentrations to those found in other oxygen minimum areas around the globe. According to Frühe, this specific variety of nutrient concentrations typically indicate metabolic processes that break down organic matter, which rains down through the water column as ‘marine snow’ and eventually deposits on the seafloor. These metabolic breakdowns change the nutrient profile when all of this organic matter accumulates in a specific spot. While this sounds like it could upset the nutrient profile balance, Frühe notes that it can create a sort-of micro niche environment - where specific microbes that survive on those particular types of nutrients can thrive. Frühe was excited to share that it is,
“...really interesting to see how the community of the microbes changes throughout the water column following the changes in the nutrients and organic matter, kind of like a biological gradient that we can see.”
These observations demonstrate the nature of environmental feedback – the decomposition of organic matter can contribute to the thriving of other organisms. She notes an example of an organism not particularly liking a low oxygen environment, but thriving with the nutrients present there, which continues to emphasize the variability in inputs needed for a successful ecosystem.
This research demonstrates the specific adaptations organisms have developed to be able to thrive in the oxygen depleted zones discovered in the Red Sea. According to Klein, naturally occurring sources of nutrients and elevated temperatures are the two main drivers that can change these ecosystems quickly and therefore will need to be monitored to understand potential changes to the biota.
The findings that these unique organisms can thrive in oxygen deprived environments can now be included as part of the NCW and overall Saudi Arabian conservation strategy as part of the Red Sea Decade Expedition. As Klein notes,
“...the NCW has expanded marine protected areas to include these sites and will also keep an eye on how these sites might change over time.”
These observations will help characterize the findings of the Red Sea Decade Expedition and inform future policy with the backing of defensible data as it relates to marine management and conservation.
Not only do findings related to biodiversity help characterize a specific region; it also helps to inform what other regions with similar geomorphology and ecosystems may experience. The team notes that the transdisciplinary nature of this research enabled the team to truly garner a holistic picture as to the dynamics of these Red Sea locations through varying areas of expertise and types of data gathered. It is through international collaboration and partnership, such as between NCW, KAUST, OceanX, and more, that enables these ground-breaking discoveries to be made.
Sea-Bird Scientific greatly appreciates the collaboration and partnership of NCW, KAUST, and OceanX to make this feature come to life.
This highlight was developed in partnership with NCW, KAUST, and OceanX, to showcase the research done as part of a scientific article released in March of 2025 in PNAS Nexus:
Deep oxygen-depleted depressions in a Red Sea coral reef sustain resistant ecosystems
The teams involved in this expedition used instrumentation from Sea-Bird Scientific to help characterize oxygen depletion zones in the Red Sea: the SBE 9/11plus and SBE 43 oxygen sensor.
Instruments and equipment used in this case study:
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