2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Sessile polychaete worm of the genus Bispira
Marine biologists have shown that marine polychaete worms living in areas rich in methane enter into symbiosis with bacteria that oxidize methane, and at their expense, they, in fact, feed on methane themselves. As the authors of the article in Science Advances explain, this is one of the few known examples of methane use by multicellular organisms. It appears that benthic communities of animals and microorganisms make an important contribution to maintaining the balance of methane, which is a dangerous greenhouse gas.
Methane is one of the three most significant greenhouse gases that accelerate global warming. The circulation of methane in the biosphere is largely provided by microorganisms that are capable of both producing and oxidizing methane. However, the influx of methane also occurs due to human economic activities and geological processes, including the seepage of methane from under the ocean floor, where it is concentrated in huge quantities in the form of methane hydrates.
Deep-sea areas where methane seeps are called methane seeps and are characterized by distinct ecological communities. In particular, bacteria capable of oxidizing methane (methanotrophs) live there, and a few species of animals that benefit from symbiosis with these bacteria. These animals include certain types of pogonophores, molluscs and sponges.
Researchers from California noticed that several species of sessile polychaete worms from the families Serpulidae and Sabellidae, namely representatives of the genera Laminatubus and Bispira, live in large numbers in the area of the methane seep off the western coast of Costa Rica at a depth of almost two kilometers.
Community of polychaete worms in the methane seep area
Polychaetes (polychaetes) are widespread in benthic communities. These animals build tubes from bottom sediment particles and lead an attached lifestyle. They usually obtain food by trapping particles from the water using feathery palps covered with cilia. However, scientists have suggested that Laminatubus and Bispira inhabit methane vulture because they are able to enter into symbiosis with methane-oxidizing bacteria and use methane as a carbon source to build biomass.
Measurement of the ratio of carbon isotopes 12C and 13C (δ13C), which is significantly different for carbon dioxide and methane, in animal tissues showed that their biomass is actually built from methane. To directly prove this, biologists raised a group of animals from the bottom, put them in an aquarium, added methane with the 13C isotope and left them for 24-105 hours. During this time, polychaetes were able to process the labeled methane into carbon dioxide, as well as incorporate it into their tissues.
Bacteria (Bac) coat the epithelium of the Bispira worm (fluorescent orange and green staining)
Since eukaryotic organisms by themselves are unable to metabolize methane, scientists have focused on looking for microbial symbionts in animal tissues. Various methods of microscopy (light, fluorescence and transmission) have helped to distinguish bacteria from the Methylococcales group on the surface of the "tentacles" of worms. Apparently, polychaetes "tamed" methanotrophic bacteria and learned to use carbon compounds, which they make by oxidizing methane. The fact that worms use bacteria as symbionts, and not just feed on them, catching them out of water, scientists have confirmed by measuring δ13С for the surrounding water vulture. It turned out that there are too few methanotrophs there to explain the isotopic composition of polychaete tissues.
Another interesting type of symbiosis can be found in the sea slug Elysia chlorotica. This marine animal feeds on algae, but does not digest the chloroplasts obtained from the prey, but stores it in its tissues, as a result of which it acquires the ability to photosynthesize in the light.