Marine Archaea: What Are They and Why Do They Matter?

All life on Earth belongs to one of three domains: Bacteria, Eukaryota, and Archaea. Initially mistaken for bacteria that lived in extreme environments, scientists later discovered archaea represent a separate lineage of life. While their reputation as “extremophiles” is well-earned, this classification is not entirely representative. Many species are found in a wide range of environments, including the ocean, where these marine archaea are abundant and play a significant part in marine ecosystems.

Unveiling Marine Archaea

Marine archaea are single-celled microorganisms that are distinct from bacteria on a molecular level. A fundamental difference is their cell membranes, which are composed of ether-linked lipids. This structure is more stable than the ester-linked lipids found in bacteria and eukaryotes, providing archaea with enhanced durability.

Their cell walls also set them apart, as they lack the peptidoglycan found in bacteria and instead contain substances like pseudopeptidoglycan or proteins. Genetically, archaea share some traits with eukaryotes, particularly in DNA replication and protein synthesis. Prominent marine groups include the Thaumarchaeota in oceanic water columns and the Euryarchaeota in sediments.

Diverse Habitats in the Marine Realm

Archaea are found throughout the world’s oceans, from the sunlit surface to the darkest depths. In the upper ocean, planktonic archaea are among the most abundant microorganisms on the planet, existing alongside marine bacteria. Their presence is not limited to the water column, as they also thrive in diverse and often extreme marine habitats. These include hydrothermal vents, cold seeps where hydrocarbons escape from rock, and even within sea ice. Nutrient-rich coastal sediments and highly saline bodies of water also host unique archaeal communities, highlighting their ability to occupy nearly every available niche.

Functions in Ocean Ecosystems

Marine archaea perform functions connected to the chemical balance of the oceans, playing roles in the nitrogen and carbon cycles. In the nitrogen cycle, marine Thaumarchaeota are responsible for a large portion of ammonia oxidation, which converts ammonia to nitrite. This step in nitrification is important for making nitrogen available to other marine life.

In the carbon cycle, some archaea are capable of carbon fixation, converting carbon dioxide into organic matter. In oxygen-free environments like deep-sea sediments, methanogenic archaea produce methane as a byproduct of their metabolism. Other groups consume methane, which helps regulate its concentration. These metabolic activities influence nutrient availability and the productivity of marine food webs.

Masters of Adaptation

The ability of marine archaea to thrive in such a wide array of habitats is due to specialized adaptations. For those in the deep sea, an adaptation is piezophily, the ability to withstand high hydrostatic pressure. This is achieved through changes in protein structure and cell membrane composition. In environments with extreme temperatures, archaea exhibit thermophily (heat-loving) or psychrophily (cold-loving) adaptations.

Thermophilic archaea have enzymes that function above 100°C, while psychrophiles have flexible enzymes and membranes that remain fluid in near-freezing conditions. In highly saline environments, halophilic (salt-loving) archaea balance the salt concentration inside their cells with the surrounding water. Many are also adapted to oligotrophy (low nutrient levels) in the open ocean, and others use anaerobic respiration to survive in sediments without oxygen.

Broader Implications and Discoveries

The influence of marine archaea extends beyond their ecosystems, impacting global climate and offering new avenues for scientific discovery. Through their involvement in the carbon and nitrogen cycles, they help regulate greenhouse gases like carbon dioxide, methane, and nitrous oxide. The unique enzymes that allow archaea to function in extreme conditions, known as extremozymes, are of interest to biotechnologists. These stable enzymes have potential applications in industries from pharmaceuticals to biofuels.

The study of marine archaea also provides insights into the origins of life, as many inhabit environments similar to those on the early planet. Studying these organisms is challenging, as many cannot be grown in a laboratory. This led to the development of culture-independent techniques like metagenomics, which allows scientists to study their genetic material directly from environmental samples. These methods continue to reveal the diversity of marine archaea, promising further discoveries.