Researchers report deep-sea clams adjust bacterial partnerships when energy drops

A research team led by Qian Peiyuan, chair professor in the Department of Ocean Science at the Hong Kong University of Science and Technology, reported findings on how deep-sea chemosynthetic symbioses — partnerships between animals and microbes that run on chemical energy — cope with environmental change, according to Phys.org. The work was carried out with international partners and published in the journal Science Advances. The paper is titled "In situ evidence of tiered adaptations buffering a chemosynthetic clam holobiont against environmental sulfide fluctuations."

According to the report, direct evidence had long been lacking on how deep-sea symbiotic organisms adapt to fluctuating environments and how hosts and symbionts work together to maintain system stability. The team focused on Archivesica marissinica, a dominant deep-sea clam at the Haima cold seep, and its sulfur-oxidizing bacterial symbionts — bacteria living with the clams that get energy from sulfur compounds.

The Haima cold seep contains seep sites at different developmental stages, which the researchers described as a "natural laboratory" for studying biological responses to environmental change. Clams were moved from their native sediments into transplantation cages positioned about 0.5 metres (1.6 feet) above the seafloor, preventing them from reaching hydrogen sulfide-rich sediment. According to the report, this setup simulated decreased hydrogen sulfide availability, leading to reduced energy and nutrient acquisition.

The researchers conducted transplantation studies at two sites, HM-2 and HM-3, with HM-2 representing severe hydrogen sulfide limitation and HM-3 representing relatively moderate limitation. They combined deep-sea in situ transplant experiments with in situ sample fixation, and used techniques that analyse genes, proteins and cell structures, including metagenomics, transcriptomics, proteomics, quantitative PCR, in situ hybridization, transmission electron microscopy and protein structure prediction. According to the report, reduced hydrogen sulfide availability first triggered pronounced metabolic reprogramming in the bacterial symbionts.

The researchers described a tiered adaptation strategy in the holobiont — the host animal together with its microbial partners — covering symbiont metabolism, host regulation of symbiosis and resource transport. They reported that metabolic flexibility in the symbionts, coupled with the host's finely regulated population of bacterial symbionts, helps sustain the host's energy stability. The report says the findings provide in situ evidence of how chemosynthetic holobionts at deep-sea cold seeps remain resilient when energy supplies fluctuate.

Besides HKUST, the research involved the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Ocean University of China, the Institute of Oceanology, Chinese Academy of Sciences, and the University of Calgary in Canada, according to the report. It represents an outcome within the framework of two U.N. Decade programs, "CliMetS" and "MOCSI."