World Aquaculture September 2018

32 SEP TEMBER 2018 • WORLD AQUACULTURE • WWW.WA S.ORG features of photosynthesis. Plant pigments capture photons of light and use this energy to lyse water and provide electrons and hydrogen ions for the reduction of carbon dioxide to carbohydrate (CH 2 O) n . Energy captured by photosynthetic pigments also is transferred to ATP in a process called photophosphorylation and the energy trapped in ATP is transferred to carbohydrate when carbon dioxide is reduced to organic carbon. Despite being quite different physiological processes, photosynthesis and respiration can, for ecological purposes, be considered the reverse of each other. Photosynthesis requires light energy and minerals, removes carbon dioxide and releases oxygen, and reduces inorganic carbon to organic carbon. Respiration does not require light and minerals and it releases energy, it uses oxygen and releases carbon dioxide, and it oxi- dizes organic carbon to inorganic carbon. Dissolved oxygen concen- tration tends to be high and carbon dioxide concentration lowwhen photosynthesis is proceeding faster than respiration, and vice versa . Aerobic respiration can be explained without going through a detailed biochemical explanation of glycolysis and the Krebs cycle, although these processes should be referred to. The simple explanation in Table 1 can be used to illustrate oxygen use in aerobic respiration. Anaerobic respiration may be explained with simple summary equations such as the one for oxidation of acetic acid by iron reducing bacteria. It is helpful to explain the role of the redox potential in the se- quence of the various possible microbial processes, and to illustrate the way that the reactions tend to be stratified with sediment depth (Fig. 3). Simple explanations of significance of most biological processes on water quality are possible. Those teaching water science should use their ingenuity to prepare diagrams to explain the basic features of processes involved in controlling concentrations of water quality variables. Background of Students The general water quality course requires the use of principles and concepts from physics and chemistry as well as the application of mathematics, especially algebra. Many students who enter the water science course have taken only basic chemistry (introductory inorganic and sometimes introductory organic chemistry), introductory physics, but most have taken at least pre-calculus and some have taken calculus. Handouts with background information on chemistry and physics FIGURE 1. Use of bicarbonate by aquatic plants as an inorganic carbon source for photosynthesis in absence of free carbon dioxide (above pH 8.3). FIGURE 2. Illustration of the basic reactions occurring in photosynthesis.  FIGURE 3. Zonation of water and sediment based on electron acceptors in respiration. TABLE 1. Simplification of aerobic respiration and stoichiometry of organic carbon oxidized to molecular oxygen used.