Abundant Chitinous Structures in Chilostomella (Foraminifera, Rhizaria) and Their Potential Functions

Abstract

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 Chilostomella ovoidea from bathyal sediments of Sagami Bay, Japan, based on serial semithin 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.

New observations on test architecture and construction of Jullienella foetida Schlumberger, 1890, the largest shallow-water agglutinated foraminifer in modern oceans

“…7, 8 of Loeblich & Tappan, 1987 ). These may serve to direct the flow of the cytoplasm, and perhaps increase its surface to volume ratio, as suggested recently for the much smaller calcareous foraminifera Chilostomella ovoidea Reuss, 1850 ( Nomaki et al, 2020 ).…”

16S rRNA Gene Metabarcoding Indicates Species-Characteristic Microbiomes in Deep-Sea Benthic Foraminifera

“…Alternatively, this OTU could belong to an algal symbiont, which the foraminifera potentially transfer asexually, as suggested for some foraminiferal species (Tsuchiya et al, 2015). Recently, the ultrastructure of C. ovoidea was found to contain abundant chitinous structures that are not commonly observed in other foraminifera, highlighting that this particular species has unique ecological characteristics (Nomaki et al, 2020). The function and origin of these structures are still unknown.…”

“…The function and origin of these structures are still unknown. They may be synthetized either by the foraminifera itself or alternatively linked to specific type of feeding or symbiosis (Nomaki et al, 2020). If the origin of the chitinous structures is algal, the distinct chloroplast signal in C. ovoidea could be linked to the same phenomenon.…”