Editor's note: The South China Sea, vast and vast, with its vast islands and reefs, is rich in resources, beautiful and rich. In order to better understand the South China Sea and recognize the South China Sea, the Voice of the Chinese Academy of Sciences and the South China Sea Institute of Oceanography of the Chinese Academy of Sciences jointly set up a column on "Beautiful South China Sea" to interpret the relationship between the "monsoon" of the South China Sea and daily life, unveil the 10,000-year mark formed by the "lithosphere" resources of the South China Sea, open the magical door of the interaction of "biodiversity" in the South China Sea, and travel happily in the ocean of knowledge.
Deep sea and its benthic biomes
Internationally, the area with a water depth of 1000m and greater than 1000m is generally defined as a deep sea. Oceans account for more than 70% of the Earth's surface area, of which about three-quarters are deep seas with a depth of more than 1000m. The deep sea has the unique extreme conditions of the physical and chemical environment - dark, high pressure, low oxygen, often rich in heavy metals and other substances, long considered unsuitable for biological survival, also known as the "forbidden area of life", until 1977 "Alvin" in the eastern Pacific Ocean around the 2500m hydrothermal vent around the 2500m hydrothermal vents found a large number of worms, clams and mussels and other benthic communities.
Subsequently, scientists discovered deep-sea cold spring ecosystems. The discovery of deep-sea hydrothermal fluids, cold springs and their biomes became one of the most remarkable scientific discoveries of the late 20th century, redefining the margins of life. Due to the influence of geological structure, hydrodynamics, complex terrain and special physical and chemical environment, the deep-sea environment and the shallow sea area of the light zone have a completely different biome composition and breed unique biomes, for example, compared with the primary productivity generated by the shallow sea relying on photosynthesis, the deep-sea chemical energy microorganisms can oxidize methane, hydrogen sulfide and other reducing substances in the dark environment to release energy to synthesize organic matter, and provide food sources for deep-sea mussels, clams, etc., also known as the "black food chain". Therefore, deep-sea biomes are also important places for studying the origin of life and adaptive evolution.
Figure 1 Mussel beds in cold spring ecosystems in the South China Sea
Seahorse Cold Spring
Cold spring refers to the distribution of deep-sea activities and passive continental edge slopes, mainly containing water, methane-based hydrocarbons, hydrogen sulfide and other substances, at a specific temperature and pressure, the natural gas hydrate and natural gas stored on the seabed will reach a balanced state, so that natural gas from the depths of the seabed along a certain channel to the seabed surface of the process.
Haima Cold Spring is a large-scale active cold spring first discovered by the Guangzhou Marine And Sea Geological Survey in 2015 in the southeastern waters of Qiongdongnan in the South China Sea, and is named "Haima Cold Spring". The area of Haima Cold Spring is about 618km2, of which the area where cold spring activity has been found is about 350km2, and the water depth is 1350~1430m. The hippocampal cold spring is the largest deep-sea cold spring ecosystem found on the mainland, which provides an important place for studying scientific issues such as the biodiversity and evolution of the deep-sea in the South China Sea.
Recently, the research team of He Maoxian of the South China Sea Institute of Oceanography of the Chinese Academy of Sciences published the research results on the parasitic relationship mechanism of the Benthic biodiversity of the cold spring of the South China Seahorse cold spring and its typical benthic organisms - Gigantidas haimaensis and the Phosphatebrabratii of the Gillib phosphate plantii in Marine Science. and frontiers in microbiology journal Frontiers in Microbiology.
Figure 2 Geographical map of cold springs in the seahorse (Feng, 2021)
The hippocampal cold spring has a highly abundant macrobenthic biome
Using remote control unmanned submersibles in the hippocampal cold spring area 4 stations for large benthic samples, through morphology and DNA identification, these benthic organisms mainly include deep-sea mussels, accompaniment clams, cap shells, snails, sea cucumbers, sea snake tails, gill scales, tube worms, submarine shrimp, Alvin shrimp, spider crabs, anemones, etc., belonging to mollusks, echinoderms, link animals, arthropods and spiny animals. The four stations can be clearly divided into two community structures: mussel beds (mainly inhabited by deep-sea mussels) and concomitant clam beds (mainly inhabited by consound clams), of which the number of benthic species in the mussel bed community structure is significantly higher than that of the consequent clam beds, and the mussel beds and the consorted clam beds provide habitat and food for other organisms. The same species with a distant habitat usually have some differences in genetic background due to geographical isolation and other reasons, and some even have phenotypic differences; Station 1 and station 2 are relatively close apart, there is no geographical isolation phenomenon, inhabiting a large number of deep-sea mussels (hippocampal clams), researchers using nuclear DNA single nucleotide variation analysis, found that there are differences in the genetic background of deep-sea mussels between these 2 stations, there is a more obvious genetic differentiation, showing genetic patchiness (CGP) phenomenon, whether this is related to the special deep-sea environment in which they inhabit, there is no more evidence.
Figure 3 Large benthic organisms in cold springs of the hippocampus
The "thief" in the deep-sea mussels , the Gill Phosphine Perch
Phosphateum perceptii is a polychaetacean that is usually observed to live in the body of the hippocampal clam of the deep-sea mussel seahorse (Figure 4) and is thought to be parasitically adapted to the extreme environment of the deep sea, but the mechanism of its parasitization on the host mussel is not well understood. The researchers found that gill phosphorus parasitism led to a significant decrease in the number of co-organism species in the gills of the host deep-sea mussels, but a significant increase in the number of chemical energy symbiotic bacteria, and caused a significant upregulation of anabolic genes of host proteins and lipids, but a significant downregulation of immunity and growth-related genes. It shows that gill scales cause the host mussel to change its physiological metabolic state to adapt to its parasitism: that is, the host reduces the rejection of gill scale parasitism by reducing the expression level of immune genes, which is beneficial to its parasitism in the mussel; Chemical energy symbiotic bacteria can use chemicals such as hydrogen sulfide in water as food, and ultimately supply the nutrients needed by heterotrophic organisms such as mussels, and the relative number of chemical symbiotic bacteria in the mussel gills can convert more nutrients for the mussels to need, at the same time, the mussels also increase their own protein and lipid synthesis, these increased nutrients not only meet the needs of the mussels themselves to grow and survive, but also meet the needs of parasitic gills to obtain nutrients from the mussels for their own needs, that is, Pei's gills scales as "thieves" Identity stimulates the host to produce more nutrients and steal some for itself.
Fig. 4 Effects of gill phosphorus parasitism on gene expression and symbiotic bacteria of host mussels
Source: South China Sea Institute of Oceanology, Chinese Academy of Sciences