[B-BG02] Geo-Bio Interactions and its Applications

[BBG02-02] Microorganisms attaching to diatom fossils and its implication to microbial survival in the subseafloor environment

*Tomoya Nishimura^1,2, Fumiaki Mori^2,3, Takeshi Terada⁴, Minoru Ikehara³, Yuki Morono² (1.Graduate School of Integrated Arts and Sciences, kochi university, 2.Kochi Institute for Core Sample Research, Japan Agency for Earth-Marine Science and Technology (JAMSTEC), 3.Marine Core Research Institute, Kochi University, 4.Marine Works Japan)

Keywords:Subseafloor, microorganisms, diatom, IODP, guaymas, Southern Ocean

The subseafloor is known as a highly nutrient-poor environment. Despite such extreme conditions, a diverse range of life remains active beneath the subseafloor. In recent years, research on the deep biosphere has been actively conducted to enhance our understanding of global biogeochemical cycles and the origins of life. In the sediment samples collected by the 2019 International Ocean Discovery Program Expedition 385 (Guaymas Basin), we observed many microbes attaching to the surface of diatoms in higher abundance than surrounding sediments. These observations suggested the potential that microorganisms in subseafloor sediments favorably attach to diatom fossils in extremely nutrient-poor subseafloor environments. Diatoms are one of the primary producers in the marine ecosystem, contributing to approximately one-third of oceanic primary production. Recent studies have suggested that microbes specifically associated with living diatoms in the water column promote marine snow formation, thereby enhancing the efficiency of the biological pump. While various studies have investigated microbial communities and interactions associated with living diatoms, much less is known about the relationship between diatom fossils—deposited as marine snow—and subseafloor microorganisms.
In this study, we hypothesized that the attachment of subseafloor microorganisms to diatom fossils contributes to survival in nutrient-depleted environments. We analyzed the microbial communities associated with diatom fossils to test this hypothesis, focusing on their cell densities and community compositions. Samples were taken from sediment cores taken from various sites, including the Guaymas Basin (IODP Exp. 385), Southern Atlantic Ocean (Weddell Sea, KH-19-6), and Indian sector of the Southern Ocean (KH-20-1). The sediment samples collected from the shallow subseafloor were observed with fluorescence microscopy and scanning electron microscopy. Additionally, microbial community structure was analyzed by sequencing 16S rRNA gene amplicons. The fluorescence microscopy observations revealed that roughly 5% of the microbial cells present in the surface sediments attached to diatom fossils. Also, we observed increased microbial attachements onto diatom fossils along the subseafloor depth of observed sediments. Scanning electron microscopy observations showed that the microorganisms are attached to the diatoms by producing thread-like fibers. Microbial community structure analysis demonstrated a distinct community structure in diatom-attached microbes from those in bulk sediments. These results indicate that the surface of diatom fossils acts as a niche hosting certain microbial species.
The presentation will describe the differences in microbial communities attaching to diatom fossils from those in bulk sediments and discuss it with the oceanographic and microbiological characteristics at the collection site.