Welcome Photo: A deep-sea hydrothermal ecosystem (Photo courtesy of Stefan Sievert, WHOI/NSF/HOV Alvin, © Woods Hole Oceanographic Institution)
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Site opening on 29 July 2024
Final Call: For the last content of Fish of the Month (FoM), we have chosen a topic related to the deep sea. It is an area that is not easy to reach even if one wants to visit it, which is why it has attracted the interest of so many people. We have asked researchers from the Faculty of Fisheries Sciences, Hokkaido University, who have been studying the "deep sea" using various approaches, to introduce some of their insights and knowledge in this field. The lineage that once pioneered the ocean frontier with the submersible "Kuroshio" has certainly been carried over to the current faculty, although has approaches to research have changed. New findings related to the deep sea are being obtained one after another from various approaches based on physics, chemistry, and biology. We hope that this "deep-sea" content will be the "beginning of the end" and awaken a new generation to lead research into the ocean and its life. In October 2024, the grand opening of the new library museum complex for the Faculty of Fisheries Sciences, Hokkaido University, is scheduled. In the library, a wealth of academic materials will be housed and stored, including fish specimens and information on "Kuroshio" that have been introduced in FoM. Seeing is believing. We look forward to your visit. FoM Editorial 29 July 2024 posted
Investigating The Deep-Sea Floor Using Chemical Approach
Phytoplankton produce organic matter through photosynthesis in the surface layer of the ocean. The slow sinking of these organic particles can be seen in the video taken by an underwater camera (background photo). It is called“marine snow”because it resembles falling snow. Marine snow is thought to be a transparent, viscous polysaccharide derived from phytoplankton and macro algae. Light is reflected off the transparent particles, which is why they appear white. A little beyond the marine snow is darkness.
When the seafloor surface gets a few meters closer, it finally becomes visible, and a little closer, it becomes clearly visible (background photo). The circular objects on the seafloor are “Sand dollar” in English or “Kashi-pan” in Japanese which is one of echinoderms. They live by feeding on the marine snow that falling from above. This is a video and photographs taken while collecting samples of marine sediments on the continental shelf of the northern Bering Sea (50 m depth).
Background photo: benthic view of the North Bering.
Deeper seafloor surfaces can also be inhabited by marine organisms in high densities. The background photo shows the seafloor surface on the continental shelf slope off Kushiro, Hokkaido (at a depth of around 300 m). Brittle stars can be seen all over the seabed. Since this is also an area of high biological productivity, large amounts of organic particles (marine snow) fall from the surface and support a rich seafloor ecosystem.
We, marine chemists at the Graduate School of Fisheries, Hokkaido University, have been collecting core (columnar) samples of seafloor sediments to investigate the chemistry of the seafloor surface where such organic matter accumulates (background photo and LASBOS movie). In seafloor sediments, oxygen is consumed by microorganisms as they break down organic matter. When oxygen is depleted in the sediment, nitrate and sulfate ions are used as oxidants for microbial respiration.
Background photo: Core sample collection at the bottom of sea (Pacific continent at Kushiro, 300 m depth).
In this unique environment, chemical reactions occur that are not present in ordinary seawater. The chemical changes in iodine compounds are particularly interesting. Iodoethane, the most minor organic iodine gas in the atmosphere and seawater, somehow becomes the major component of organic iodine gas in marine sediments. Laboratory experiments have confirmed that iodoethane is produced in large quantities when phytoplankton is collected, bottled, and left to degrade. Phytoplankton (mainly diatoms) deposited on the sea floor is the true source of iodoethane (Ooki et al., 2022). This is an interesting discovery for a few marine chemists who have focused on iodine cycle in the ocean.
OOKI Atsushi・Faculty of Fisheries Sciences, Hokkaido University・Professor
References
29 July 2024 posted
Zooplankton in the Deep Sea
Zooplankton is widely distributed throughout the world's oceans, from the surface to the deep sea. In the ocean, zooplankton feeds on carbon fixed by phytoplankton in the surface layer and excretes it in the form of large fecal particles, thereby acting as a driver of carbon transport from the surface to the deep sea. This zooplankton-mediated material transport to the deep sea is called a "biological pump" and contributes to global material transport. The western North Pacific Ocean has three marginal seas around Japan: the Sea of Okhotsk, the Sea of Japan, and the East China Sea. During the voyage of the Oshoro Maru, the training ship of the Faculty of Fisheries of Hokkaido University, vertical stratified zooplankton samples were collected between the sea surface and a depth of 3000 m using a vertical multiple plankton sampler (VMPS) at seven stations including the western North Pacific and the three marginal seas (figure on back). The zooplankton abundance and biovolume were higher in the surface layer and decreased with increasing depth. The inter-station difference in zooplankton abundance was small, but the difference in the biovolume varied greatly depending on the station, indicating that the high zooplankton volume at the surface layer was also high in the deep layer. This difference in biovolume is proportional to the difference in carbon content, indicating that areas with high carbon fixation in the surface layer also have high carbon content in the deep sea and that production on the surface layer is proportional to biomass in the deep sea.
YAMAGUCHI Atsushi・Faculty of Fisheries Sciences, Hokkaido University・Associate Professor
Photo: Deep-sea plankton（Paraeuchaeta）
References
Yamamae, K., Y. Nakamura, K. Matsuno, A. Yamaguchi (2023) Vertical changes in zooplankton abundance, biomass, and community structure at seven stations down to 3000 m in neighboring waters of Japan during the summer: Insights from ZooScan imaging analysis. Progress in Oceanography, 219, 103155. PR
29 July 2024 posted
Microbes Supporting Ecosystems in Deep-Sea Hydrothermal Vent Fields
The deep sea harbors a surprisingly large number of microorganisms. In particular, an ecosystem based on the primary production of chemosynthetic autotrophic microorganisms has been formed around hydrothermal vents on the deep-sea floor. Where hot hydrothermal water emerging from beneath the seafloor mixes moderately with cold seawater, invertebrates such as tubeworms and bivalves, which live together with microorganisms, occur in dense populations. I have vivid memories of the richness of the deep-sea ecosystem when I had the opportunity to participate in a submersible expedition using an upgraded Alvin to the Crab Spa hydrothermal site (2,506 m depth) in the Eastern Pacific Rise (EPR).
The class Epsilonproteobacteria is a group of bacteria known to dominate in deep-sea hydrothermal systems. This group includes phylogenetically and physiologically diverse bacterial species, and it is known that Epsilonproteobacteria plays significant roles as primary producers in hydrothermal ecosystems. To date, new Epsilonproteobacteria have been isolated from hydrothermal vent sites around the world and have been proposed as new species, including thermophilic bacteria with optimum growth temperatures exceeding 45 ºC and mesophilic bacteria at lower temperatures. The former four species are Nitrosophilus labii、Nitrosophilus alvini、Nitrosophilus kaiyonis and Hydrogenimonas urashimensis; the latter are Hydrogenimonas cancrithermarum、 Sulfurovum aggregans. In addition to those, new Hydrogenimonas and Sulfurimonas bacteria have been isolated and are in the process of being described as new species.
The chemolithoautotrophic species of Epsilonproteobacteria living in deep-sea hydrothermal environments also provide excellent biological material for studying biological evolution. Hydrogenimonas cancrithermarum strain ISO32 is the first isolate of the genus Hydrogenimonas that (1) shows an optimum growth at temperatures below 45 ºC and (2) can utilize reduced sulfur compounds in addition to hydrogen as an energy source. In addition, another study has been conducted on the genus Sulfurimonas, an autotrophic mesophilic group, to understand the speciation of microorganisms at deep-sea vents. Through detailed analyses of multiple gene sequence data, we found that microbial dispersals are limited among deep-sea hydrothermal regions distributed around the world (e.g., Okinawa Trough, Mariana Trough, Mid-Atlantic Ridge, and Indian Ocean Central Ridge), and that allopatric speciation, similar to that observed in plants and animals, is occurring even in microorganisms living in deep-sea hydrothermal environments. We hope to unravel the evolution of Epsilonproteobacteria, including the adaptation from high to low temperatures, the transition from chemosynthetic autotrophy to chemosynthetic heterotrophy, and the factors driving their speciation.
MINO Sayaka・Faculty of Fisheries Sciences, Hokkaido University Assistant Professor
Photo: Tube worm on the Crab Spa hydrothermal vent site (Photo courtesy of Stefan Sievert, WHOI/NSF/HOV Alvin, © Woods Hole Oceanographic Institution)
References
Mino S et al (2023) Hydrogenimonas cancrithermarum sp. nov., a hydrogen- and thiosulfate-oxidizing mesophilic chemolithoautotroph isolated from diffuse-flow fluids on the East Pacific Rise, and an emended description of the genus Hydrogenimonas. Int J Syst Evol Microbiol. PR
29 July 2024 posted
Deep-Sea Fish
Generally, fish living at depths of 200 m or more are called deep-sea fish (Amaoka 2009). Of the 450 fish families, more than 100 include deep-sea fish. Deep-sea fish occur in many different habitats; some swim in the mid-water levels such as barbeled dragonfish and footballfish, some live on the deep sea floor such as grendiers and eelpouts, and others move from the deep sea during the day to shallower water at night such as lanternfish, in accordance with the movements of their prey. Deep-sea fish are extremely diverse morphologically and ecologically. Fish such as common fangtooth (Anoplogaster cornuta) and viperfish (Chauliodus sloani) possess powerful canines, which are thought to enable them to reliably prey on prey that are rarely encountered in the deep-sea environment.
Background photo Anoplogaster cornuta (Anoplogasteridae) Mouth large, with strong canines (photo and specimen deposited in Hokkaido University Museum)
In the case of deep-sea anglerfish, the females are larger than the males, and the males bite and attach themselves to the females. In deep-sea anglerfish such as the family Ceratiidae, the male is fused with the female's tissues and becomes part of her body, and parasitizes her. This is also thought to be a strategy to ensure reproduction, as there are few opportunities for males and females to meet each other in the deep sea.
Background photo Ceratias holboelli (Ceratiidae) Photographed specimen female, with parasite male on her abdomen (photo and specimen deposited in Hokkaido University Museum)
Fish such as stomiiform fish and myctophids have many photophores on the ventral side of their bodies. These fish swim in the mid-depths, but in an environment where light pours in from above, a shadow is prominent when viewed from below, making them easier to spot by predators. By emitting light themselves using their photophores, they blur their silhouette and make themselves less likely to be targeted by predators. This type of light-emitting is called "counter illumination." The deepest sea species that has been collected so far is thought to be the ophidiid "Yominoashiro" (Abyssobrotula galatheae), which was recorded at a depth of 8,370 m.
IMAMURA Hisashi・Faculty of Fisheries Sciences, Hokkaido University・Professor／the Fisheries Science Center, the Hokkaido University Museum・Director
Background photo Diaphus gigas (Myctophidae) Ventral side of body with many small photophores (photo and specimen deposited in Hokkaido University Museum)
References
Amaoka K (2009) Deep-sea fishes–monsters of underworld–. Bookman-sha Co. Ltd. Tokyo.
29 July 2024 posted
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