The solar energy absorbed and fixed by various phototrophs is stored as biomass and eventually dissipated by a variety of small and large heterotrophs. The laws of thermodynamics constrain the rates and efficiencies of energy transfer and transformation, but the requirements of living organisms impose further losses due to respiration, egestion of unassimilated matter, and mortality. Nutrients, on the other hand, particularly those that are scarce and essential are cycled much more efficiently, as most organisms have evolved physiological mechanisms for conservation of such elements. The role of microbes and meiobenthos in marine sediments is now often equated to the role their pelagic counterparts play in the microbial loop. The role of sediment bacteria in benthic food chains and nutrient cycles can best be simplified as aerobic trophic links and anaerobic nutrient sinks. A notable exception is the oxidized lining of burrows and tubes of larger metazoans.