Electron microscopy and the deep sea: Discovery and exploration of symbioses between invertebrates and bacteria

Abstract

Electron microscopy has played an important role in the discovery of symbioses between sulfur-oxidizing chemoautotrophic bacteria and marine invertebrates. Initially proposed for deep-sea hydrothermal vent tubeworms, associations between chemoautotrophs and invertebrate hosts appear to be widespread in nature (see review). In these proposed mutualistic symbioses, bacterial endosymbionts are thought to provide the host animal with an internal source of nutrition via the autotrophic fixation of CO2, analogous to the role of chloroplasts in plants. Reduced inorganic compounds, e.g., sulfide, serve as the symbionts' source of energy and reducing power. The host, in turn, provides its symbionts with the necessary inorganic nutrients for chemosynthesis.Transmission and scanning electron microscopy have indicated that numerous procaryotic-like cells, resembling Gram-negative bacteria in ultrastructure, comprise the bulk of the trophosome tissue in Riftia pachyptila, the vent vestimentiferan tubeworm. Further transmission electron microscopy (TEM) studies show that the symbionts are contained within membrane-bound vacuoles and appear intracellular. Activities of enzymes associated with sulfur metabolism and with carbon fixation, notably ribulose-1,5-bisphosphate carboxylase (RuBPCase), the key enzyme of the Calvin Benson cycle, have been detected in the trophosome tissue, indicating that the symbionts are chemoautotrophs.