Visualization of interactions between chemoautotrophic symbionts and gill epithelial cells of Thyasira cf. gouldi using confocal microscopy and transmission electron microscopy

Many species of marine clams from the family Thyasiridae form symbioses with sulfur oxidizing bacteria, which are maintained externally on gill epithelial cells. Transmission electron microscope images suggest that host cells phagocytose and lyse symbionts to obtain nutrients. However, as this technique can only reveal two-dimensional sections of cells, interactions between host and symbionts, particularly with regards to phagocytosis and lysis, remain incompletely characterized. Here, fluorescent labelling and confocal microscopy of Thyasira cf. gouldi gills are used to visualize host nuclei, actin, acidic organelles, and symbiotic bacteria. I found that DAPI stained host nuclei but not symbionts. Actin labelling using phalloidin labeled microvilli but did not show obvious phagocytotic structures. The combined use of LysoTracker and CTC to label acidic organelles and symbionts, respectively, proved problematic due to colocalization, but labelling suggested low numbers of acidic organelles (lysosomes) and active symbionts. Results obtained, along with a review of transmission electron microscope images of T. cf. gouldi gill cells, suggest that nutrient transfer pathways may involve more morphologically complex structures and potentially different processes than previously thought. In future work, single gill cells should be reconstructed using FIB-SEM to visualize host cellular structures and symbiont cells in three dimensions