Sulfate respiration (sulfate reduction)

　Oxygen and even nitric acid are gone, and without oxidized minerals such as manganese oxide and iron hydroxide, organisms that use sulfuric acid as an oxidant will dominate. The reaction equation for sulfate respiration is written as follows.

　When sulfate reduction occurs, hydrogen sulfide (H₂S) is produced as a reaction product. Hydrogen sulfide is what smells like ditch river (often described as the smell of rotten eggs). Sulfuric acid reduction occurs in places where large amounts of organic matter accumulate for long periods of time. However, it must be without sulfate ions, which are the source of the sulfuric acid.

　The photograph below shows a core (cylindrical) sample of marine sediment from 300 m off the coast of Hokkaido, Japan, sealed in a glass tube for storage. The surface layer of the sediment, about 1 cm thick, is greenish brown in color. This is due to the abundance of phytoplankton-derived organic particles produced in the surface layer. A black layer can be seen directly below this layer. When this black sediment was collected and hydrochloric acid was added, bubbles were generated. When this gas was passed through a lead chloride (PbCl2) hydrogen sulfide detector tube, high concentrations of hydrogen sulfide were observed.

　Why is it that the black sedimentary layer, which generates hydrogen sulfide, does not reach the surface? Bottom seawater contains oxygen, which is supplied to the sediment surface. In the sediment surface layer, respiring oxygen microorganisms decompose fresh organic particles by respiration. This oxygenated sediment surface layer blocks the leaching of hydrogen sulfide from below. This is because hydrogen sulfide is relatively quickly oxidized back to sulfate ions when it encounters oxygen. (This sulfide oxidation is also believed to proceed quickly by bacterial action.)

　Since the Hokkaido coast faces the open ocean, the bottom seawater is supplied with oxygen on a steady basis, so hydrogen sulfide does not leach to the sediment surface. In inland bays such as Tokyo Bay, where the supply of organic particles is significant, hydrogen sulfide may leach to the sediment surface.

　Hydrogen sulfide is highly toxic to many organisms, so exudation of hydrogen sulfide onto sediment surfaces and into bottom waters can be very damaging to ecosystems.

　Again, the picture shows how oxygen respiration occurs at the sediment surface and sulfate respiration just below it. In the previous course, we explained that the order of respiration forms by organisms is oxygen, nitric acid, manganese oxide, iron hydroxide, and sulfuric acid. Before sulfuric acid respiration occurs, iron hydroxide is used as an oxidant. The reducing half-reaction of iron is described below.

　　Fe(OH)₃ → Fe²⁺ + 3OH^-

　In other words, where sulfate respiration occurs, divalent iron ions (Fe²⁺) are abundant in the surroundings. Hydrogen sulfide (H₂S) reacts with metal ions to produce black sulfides.

　　H₂S ＋  Fe²⁺  →   FeS + 2H⁺

　I referred to the black-colored areas in the sediments as "hydrogen sulfide generated layers" because iron sulfide (FeS), which is formed by the reaction of hydrogen sulfide and iron ions, is black.

Causes of the blue tide

　Hydrogen sulfide exudes from bottom sediments into the water immediately above the surface and reacts quickly when it meets oxygen to form S and water (H₂O). S is solid and colloidal. When bottom waters rise to the surface for some reason, the S colloids give the ocean surface a bluish-white color. This is called a blue tide. The solid S also reacts directly with oxygen to form SO₄^2-. Water in which blue tides occur is hypoxic, which is a matter of life and death for surface organisms. By the way, blue tides do not occur in eutrophic lakes (not to be confused with blue-green algae). This is because freshwater contains few sulfate ions.