Scientists demystify the polarization of nitrogen cycles in tubular worms in extreme environments

Recently, the team of Professor Feng Dong of Shanghai Ocean University has made a breakthrough in the geochemical imprinting of the life activities of tubular worms in deep sea cold springs, and related research has been published in Geology.

Tubular worms that develop in cold springs of the hippocampus. (Photo taken by “Seahorse”, photo provided by Tao Jun)

Extreme deep-sea ecosystems such as cold springs are potential candidates for exploring the origin and evolution of life on Earth. These ecosystems often rely on the symbiotic relationship between chemoautotrophic microorganisms and their hosts, forming an “oasis” of life in the deep-sea “desert”, of which tubular worms are one of the most representative macroorganisms. Adult tubular worms lack a functional digestive system, and their energy and material requirements are completely dependent on the symbiotic sulfur-oxidizing bacteria in the nutrient body. Previous studies have suggested that the appearance of tubular worms in the cold seep system seems to date back to the Devonian period, but the timing of the first appearance of tubular worms based on morphology, molecular biology, ecology, and fossil evidence is inconsistent.

In view of the above problems, the research team took Paraescarpia echinospica, a tubular worm developed in the cold spring area of the South China Sea “hippocampus” activity, as the research object, starting from its physiological characteristics, and exploring the unique geochemical imprint left by its life activities.

Using the nitrogen cycle of tubular worms as an entry point, the researchers found that organisms obtain nitrogen sources needed for their life activities through nitrogen fixation or nitrate reduction. The use of nitrogen by organisms often leads to a negative bias in the nitrogen isotopic composition in their bodies, which has been confirmed by the nitrogen isotopic composition of the nutrient body of tubular worms. The sulfur-oxidizing bacteria in tubular worm nutrition do not have nitrogen fixing genes, and their bodies contain 100 times that of environmental nitrate and the nitrate reduction rate is extremely fast. The nitrate reduction process utilizes nitrate reductase, a molybdenum-containing metalloenzyme.

Studies have shown that the molybdenum isotopic composition of the chitin shell of tubular worms in the cold spring area of the South China Sea “hippocampus” activity can reach a maximum negative value of -4.59‰, which is the lowest molybdenum isotope value reported by all natural substances so far. The extremely negative molybdenum isotopic composition may result from the preferential utilization of the isotope light molybdenum in the nitrate reduction process by tubular helminth symbionts.

By building a bridge between modern processes and geological records, this study provides a unique geochemical perspective for the identification of ancient tubular worms in geological history.

Collaborators from Qingdao National Laboratory for Marine Science and Technology, Hong Kong University of Science and Technology, University of Hamburg in Germany, French Institute for Marine Development, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Temple University in the United States and Guangzhou Marine Geological Survey participated in the work. (Source: Zhang Shuanghu, China Science News)

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