FROZEN HEAT | Volume 1

Box 2.3 What is chemosynthesis?

methane seeps) imply that such sea-floor ecosystems have been occurring for millions of years (Goedert and Benham 2003; Peckmann and Thiel 2004). 2.4.1 Overview of chemosynthetic communities at methane seeps The term chemosynthesis refers to metabolisms that obtain energy to create biomass by using chemical energy through reduction-oxidation reactions. If the carbon source used to form the biomass is inorganic (carbon dioxide), the organisms are called chemoautotrophs. Chemoautotrophs are different from photoautotrophs, organisms that use light energy through photosynthesis to obtain energy for growth from carbon dioxide. Chemoautotrophs are bacteria, exclusively. Animals that live symbiotically with chemoautotrophs are called chemosymbiotic organisms. Animals (including humans) that feed on algae, plants, or other animals are called heterotrophs. Famous examples of chemosymbiotic animals are the giant tubeworms, clams, and mussels thriving on chemical energy in the permanent darkness of hydrothermal vent systems and methane seeps (Corliss et al. 1979; Felbeck 1981; Levin 2005). These ecosystems are often described as functioning completely independent of sunlight, especially when located in the deep sea. At first glance, this statement appears correct, because primary producers at the basis of these ecosystems gain energy and carbon from inorganic compounds, even in the absence of light. However, the chemicals needed to oxidize energy-rich molecules such as hydrogen sulphide, are oxygen and nitrate. Both were rare in ancient oceans. Only through photosynthesis did oxygen accumulate and react with reduced nitrogen compounds to form nitrate. Hence, modern complex chemosynthetic ecosystems, especially those with higher organisms, are not completely independent of sunlight. They depend on compounds produced directly or indirectly through photosynthesis. This dependence is even

stronger at seeps, where methane, and sometimes petroleum, form the basis for chemosynthetic food chains. Both are often fossil-transformation products of photosynthetically produced organic matter.

Figure TB-2.3: Near the sea floor above active methane seeps, gas hydrate can form mounds such as that pictured above in the Gulf of Mexico. The gas hydrate mound is tinted orange by small amounts of oil, and is partially covered by a thin sediment drape (grey material). (Courtesy of Ian MacDonald).

hydrogen sulphide, which is a by-product of AOM (Paull et al. 1984; Sibuet and Olu 1998; Levin 2005), as well as ani- mals that directly consume methane (Childress et al. 1986; Schmaljohann and Flugel 1987). Free-living chemosynthetic bacteria include sulphur bacteria such as Beggiatoa , Thiop- loca , or Thiomargarita (Jørgensen and Nelson 2004), as well as aerobic methane-oxidizing bacteria. Filamentous sulphur bacteria can sometimes form extensive white or orange mats on the sediment surface (Fig. 2.4).

Chemosynthetic communities found at methane seeps in- clude both microorganisms and animals that depend on

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 43