Long-term culture system for deep-sea mussels Gigantidas childressi

Abstract

The simulation of deep-sea conditions in laboratories is technically challenging but necessary for experiments that aim at a deeper understanding of physiological mechanisms or host-symbiont interactions of deep-sea organisms. In a proof-of-concept study, we designed a recirculating system for long-term culture (>2 years) of deep-sea mussels Gigantidas childressi (previously Bathymodiolus childressi). Mussels were automatically (and safely) supplied with a maximum stable level of ~60 µM methane in seawater using a novel methane-air mixing system. Experimental animals also received daily doses of live microalgae. Condition indices of cultured G. childressi remained high over years, and low shell thickness growth could be detected, which is indicative of positive energy budgets. Using stable isotope data, we demonstrate that G. childressi in our culture system gained energy, both, from digestion of methane oxidizing endosymbionts and from digesting particulate food (microalgae). Limitations of the system, as well as opportunities for future experimental approaches involving deep-sea mussels are discussed.

Phenotypic Plasticity of Symbiotic Organ Highlight Deep-sea Mussel as Model Species in Monitoring Exploitation of Deep-sea Methane Hydrate

“…4E). Consistent with this finding, researchers have reported the successful reestablishment of symbiotic associations in decolonized deep-sea mussels after resupplying the mussels with methane or sulfide 31 , confirming the speculation in the present study that the mussels could regain bacteriocytes after losing them.…”

“…Therefore, phenotypic changes in symbiotic cells and organs can occur frequently in part or in whole for deep-sea mussels regarding to the intensity of environmental change and serve as key indicators of deep-sea mining or other anthropogenic activities. Besides, the deep-sea mussel could survive even much longer than one year in natural condition since our experimental condition is much harsh 31,43 . While the phenotypic changes of the digestion system remain unexplored, the long-term survival of deep-sea mussels under methane deprivation might also endow them with transgenerational effects, allowing for a permanent transition from chemoautotrophy (a symbiotic association with thiotrophs) or chemoheterotrophy (a symbiotic association with methanotrophs) to strict heterotrophy (filter-feeding).…”

“…It is noteworthy that the lysosome-mediated digestion of symbionts is a common choice for almost all hosts under nutritional limitation, although this process can compromise and even destroy the symbiotic associations 41,42 . Nevertheless, symbiotic associations can recover gradually once methane or sulfide becomes available 15,31 , making the sacrifice of symbionts affordable. It was also noted that the mussel host reallocated energy to sustain the proliferation of ciliated cells and enhance the filter-feeding function of the gill.…”

“…For example, while the global metabolism and gene expression of endosymbionts could be influenced by depressurization stress (Chen et al, 2021), it is still challenging to cultivate deep-sea mussels in pressurized laboratory systems and to conduct simulations in deep-sea, which complicated the interpretation of the results. In addition, the maintenance of symbiotic associations under laboratory conditions is also challenging for deep-sea holobionts (Hiebenthal et al, 2021; Sun et al, 2022), which therefore complicates the choice of a proper control group during long-term laboratory cultivation. Alternatively, deep-sea mussels collected from the seepage region in an isobaric and isothermal way were employed as the control group to represent the natural state of symbiotic association.…”

“…We then examined the proteomics data of the G3M group to screen possible genes and processes guiding these developmental changes. Because the long-term maintenance of symbiotic associations under laboratory conditions remains a common challenge for research in Bathymodiolinae mussels (Hiebenthal et al, 2021), mussels from the InS group and G7D group served as control groups here to exclude the potential influence of the laboratory cultivation system and characterize the key molecules in detail. Consequently, two caspase proteins (caspase 2 and caspase 7) were found to be significantly up-regulated in the G3M group in comparison with mussels during the early stage of methane deprivation (G7D group).…”