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Q19 - Electron Transport Between Oxic and Anoxic Marine Sediment Layers

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Ecology

In the sea, O2 diffuses from the free water down into the top oxic layer in the sediment and here becomes reduced to water. Anoxic layers are beneath this layer, and here bacteria-mediated processes take place. One process is the oxidation of H2S to SO42−, which is tightly coupled to the O2-reduction in the oxic layer above. This coupling between oxic and anoxic processes can only be explained by “electric currents” transporting electrons from H2S oxidation to O2 reduction. In experiments, attempts were made to identify the electron conductor (Fig.). A, physically cut apart by a thin “knife” (before: left diagram, and after: right diagram); B, the two layers were separated by filters with pores of two sizes.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. The pH peak in the oxic layer is due to production of water from oxygen
B. Physically interrupting the sediment by the "knife" did not affect O2 reduction
C. The filters demonstrated that any specific solutes were not transporting the electrons
D. Electrons for the O2-reduction most likely came from donors in the oxic layer
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Question reproduced from IBO 2015, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)