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Foraminifera are unicellular eukaryotes known to have a shell, called a test, generally made of secreted calcite (CaCO3). For the first time, we report a foraminifera with a composite calcite–opal test in the cosmopolitan and well-studied benthic species Bolivina spissa (Rotaliida), sampled from Sagami Bay in Japan at 1410 m depth. Based on comprehensive investigations including scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and Fourier-transform infrared (FTIR) spectroscopy, we inspect the morphology and composition of the novel opaline layer coating the inside part of the calcitic test. Using scanning transmission electron microscopy (STEM) and EDS analyses, we detected probable silica deposition vesicles (SDVs), organelles involved in opal precipitation in other silicifying organisms, confirming that the foraminifera itself secretes the opal layer. The layer was systematically found in all studied individuals and had no apparent substructure. Its thickness showed a growth pattern analogous to the calcitic shell of B. spissa, being the thickest in the oldest chamber (proloculus) and becoming thinner toward the younger chambers (apertural side). Its absence in the youngest chambers indicates that silicification occurs subsequent to calcification, probably discontinuously. We further discuss the potential function(s) of this composite test and propose that the opal layer may serve as a protective barrier against predators using either mechanical drilling or chemical etching of the calcitic test. Isotopic composition measurements performed separately on the proloculus part and the apertural side of B. spissa suggest that the presence of an opal layer may alter the calcitic isotopic signal and impact palaeoenvironmental proxies using foraminiferal test composition. If silicification in Foraminifera were found to be more widespread than previously thought, it could possibly have important implications for foraminiferal evolution, palaeoceanographic reconstructions, and the silica cycle at global scale. [ABSTRACT FROM AUTHOR]

Plant remains from land, particularly sunken wood, represent significant input of organic matter into the generally oligotrophic deep sea, leading to the formation of macrofaunal assemblages around them. Following the onset of decomposition by wood-boring bivalves, microbes and fungi the wood eventually becomes an anoxic and reducing environment attracting chemosynthetic fauna. Wood falls have primarily been studied by either opportunistic sampling by trawls or artificial deployments, while in situ observations of natural systems remain scarce. Here, we report a natural late-stage wood-fall community at 5505 m depth in the Northwestern Pacific abyssal plain by a manned submersible. Our observations revealed details in faunal distribution that cannot be obtained by trawled material, including the presence of associated xenophyophores. We identified a total of 26 animal species and one xenophyophore from the wood fall including both wood-associated and chemosymbiotic taxa, although cocculiniform limpets were conspicuously missing. Living xylophagaid bivalves were also not observed, but the wood was heavily burrowed and they were clearly once present. This rare finding improves our understanding of deep abyssal wood falls at the senescence stage, and highlights the importance of direct seafloor observations. [ABSTRACT FROM AUTHOR]

Benthic foraminifera, members of Rhizaria, inhabit a broad range of marine environments and are particularly common in hypoxic sediments. The biology of benthic foraminifera is key to understanding benthic ecosystems and relevant biogeochemical cycles, especially in hypoxic environments. Chilostomella is a foraminiferal genus commonly found in hypoxic deep‐sea sediments and has poorly understood ecological characteristics. For example, the carbon isotopic compositions of their lipids are substantially different from other co‐occurring genera, probably reflecting unique features of its metabolism. Here, we investigated the cytoplasmic and ultrastructural features of Chilostomellaovoidea from bathyal sediments of Sagami Bay, Japan, based on serial semi‐thin sections examined using an optical microscope followed by a three‐dimensional reconstruction, combined with TEM observations of ultra‐thin sections. Observations by TEM revealed the presence of abundant electron‐dense structures dividing the cytoplasm. Based on histochemical staining, these structures are shown to be composed of chitin. Our 3D reconstruction revealed chitinous structures in the final seven chambers. These exhibited a plate‐like morphology in the final chambers but became rolled up in earlier chambers (toward the proloculus). These chitinous, plate‐like structures may function to partition the cytoplasm in a chamber to increase the surface/volume ratio and/or act as a reactive site for some metabolic functions. [ABSTRACT FROM AUTHOR]

Highlights • Stiffness of cellulose hydrogel is independent on crystallinity of skeletal matrix. • Poroelasticity of cellulose hydrogel stands out upon fast compression. • Sub-micrometer-scale morphology generates poroelasticity. Abstract The poroelasticity of cellulose hydrogels was studied by a series of compression tests. The stiffness of the cellulose hydrogel was found to be independent on the crystallinity of a skeletal gel matrix, suggesting that the poroelasticity made a significant contribution to mechanical properties of the hydrogel. Indeed, typical poroelastic responses were observed when the cellulose hydrogels were subjected to the compression tests with varied compression strain rates (i.e. softness upon slow compression but stiffness upon fast compression). The fluid mechanics theory showed that sub-micrometer-scale morphological difference of the gel matrix, which was observable by scanning electron microscopy, affected the flow behavior of water inside porous structure, leading to the increased stiffness upon fast compression. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]