[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-P04] Microbial association to diatoms in subseafloor sedimntary environments: potential of microbial survival mechanism in subseafloor?

*Tomoya Nishimura^1,2, Fumiaki Mori¹, Takeshi Trada³, Minoru Ikehara⁴, Yuki Morono¹ (1.Kochi Institute for Core Sample Research, Japan Agency for Earth-Marine Science and Technology, 2.kochi university, 3.Marine Works Japan, 4.Center for Advanced Marine Core Research, Kochi University)

Keywords:Subseafloor, microorganisms, diatom, guaymas, Southern Ocean, Microbial community analysis

Diatoms are unicellular algae surrounded by siliceous capsules and are found in diverse hydrosphere including marine ecosystems. They are estimated to constitute 444-582 Tg - C in global biomass and be a major primary producer accounting for about 1/3 of primary production. The role of diatoms in the ocean includes CO₂ assimilation, carbon cycling, and biological pumps. Recent report documented that the potential role specific microorganisms attaching to diatoms in water column to increase the formation of marine snow and increase the efficiency of biological pumps. Although the fossil of diatoms is also the global research target for paleoclimate especially in deeper marine environments below CCD, not many things are known for relationship of diatom fossils and microorganisms. In the sediment samples collected by the 2019 International Ocean Discovery Program Expedition 385 (Guaymas Basin) we have observed many microbes attaching to the surface of diatom shells in higher abundance of surrounding sediments. Based on the hypothesis that this attachment of microbes should have ecophysiological significance such as survival in extremely energy-limited subseafloor biosphere, we characterized the abundance and microbial community structure of diatom-attaching microbes in comparison to the sedimentary counterparts.
Samples were sediment cores containing diatoms from the drillied cores of Guaymas Basin and from the Atlantic sector of the Southern Ocean (Weddell Sea) of the Hakuho Maru KH-19-6 voyage and the Indian Ocean sector of the Southern Ocean of the Hakuho Maru KH-20-1 voyage. Comparison of the microorganism abundances in each sediment and the attached to diatom fossils revealed that the rate of attachment of microbial cells varied greatly depending on the collection site. Although we did not find diatom-specific sequences, microbial community analysis revealed compositional differences between diatom-surface and surrounding sediments.
In this presentation, considerations and perspectives regarding the differences between microbial communities in sediments and those attached to diatom fossils will be discussed both from an oceanographic and microbiological perspective at the collection site.