Temperature and sulphide tolerance of hydrothermal vent fauna

Hydrothermal vent communities have very high biomass, which is dominated by relatively few endemic species. Considering the stark contrast between the amount of organic material available at the vents to that in most of the surrounding deep-sea, one might expect that more of the ambient deep-sea fauna would colonize these environments. However, the toxicity of the hydrothermal fluids and the variability of the chemical and thermal regime the vent fauna experience require specific adaptations before an organism can survive there. The aspects of the environment which are most different from that of the surrounding deep-sea are those induced by the spatially and temporally variable input of hydrothermal fluid into the environment, which results in habitats where temperature, pH, and chemical content (including dissolved gasses) vary greatly over short times and distances. At the highest levels of exposure to vent fluids where metazoans have been documented, the water is anoxic, laden with sulphide and heavy metals, and the average temperature is at least 35°C. In some habitats it is likely that the animals are occasionally in brief contact with water in the 100°C range. Overlaying this extreme, and mixing with it to create a range of intermediate chemical and physical conditions, is the cold, oxic and relatively pristine ambient deep-sea water. The hydrothermal vent fauna have not only adapted to live in these conditions, but some of the species are among the fastest growing invertebrates on the planet. To survive in the hydrothermal vent environment, the fauna have adaptations which allow exposure to a range of temperatures, while maintaining a supply of oxygen to their tissues and avoiding the toxic effects of their environment. Simple exclusion of the dissolved toxins would seem to be untenable when one considers the fact that these animals are aerobic and must utilise respiratory surfaces for gas exchange. This is especially true for the chemoautotrophic symbiont-containing animals, because they have the added requirement of supplying reduced sulphur compounds, usually the toxic gas hydrogen sulphide, to their symbionts. Understanding how the vent fauna tolerate and thrive in this hostile environment has been the focus of a majority of the physiological investigations of these animals. In parallel with these studies, biologists have long recognized the importance of characterizing the microhabitats occupied by the vent fauna. However, we have also recognized the difficulty in doing so because not only are the microhabitats temporally variable, but some of the spatial gradients of temperature and dissolved gasses are arguably the greatest of any habitat on the planet. For example, temperature gradients of well over 100°C per cm are not uncommon in chimney environments, and can exceed 300°C per cm on flanges.