Direct measurement CO2 fixation and O2 liberation

Photosynthesis can be measured in terms of either CO2 fixed or O2 released. For terrestrial plants the method of choice is direct measurement of CO2 removal from the gas stream by monitoring the infrared absorption by CO2, but for obvious reasons this will not work in an aquatic medium. Because of the stoichiometry of the overall photosynthetic process (ยง 8.5), approximately one O2 molecule is liberated for every molecule of CO2 fixed. However, since the average composition of plant biomass differs somewhat from CH2O due to the presence of protein, lipid and nucleic acid as well as carbohydrate, the O2:CO2 ratio (known as the photosynthetic quotient) is usually in the range 1.1 to 1.2, rather than exactly 1.0. In the case of the more active photosynthetic systems, it is convenient to measure O2 liberation - using chemical analysis, an oxygen electrode or manometry. For field measurements of phytoplankton photosynthesis, however, in all except highly productive waters, the much more sensitive procedure of measuring fixation of 14CO2 is used: this method was introduced by Steemann Nielsen in 1952. Bottles containing water samples with the natural phytoplankton population present and with small amounts of [14C]-bicarbonate (hydrogen carbonate, HCO3~) added, are suspended at a series of depths throughout the euphotic zone, generally for periods of a few hours in the middle of the day. The amount of radioactivity fixed in cells, collected on a filter and treated with acid to remove excess [14C]-bicarbonate, is determined.

Alternatively, the incubations of the phytoplankton samples with [14C]-bicarbonate can be carried out in the laboratory at the same temperature as that in the water body, and at a series of irradiance values designed to correspond to different depths. For marine phytoplankton, Jitts (1963) introduced the technique of carrying out the laboratory incubations under a series of thicknesses of blue glass, to simulate the variation of spectral distribution, as well as total irradiance, with depth. Failure to reproduce, in the laboratory, spectral distributions similar to those found underwater can lead to substantial errors in estimates of primary production.778

The rate may be expressed as either gross or net photosynthesis. Gross photosynthesis is the total rate of carbon dioxide fixation, making no allowance for the fact that some CO2 is simultaneously lost in respiration. Net photosynthesis is the total rate of photosynthetic CO2 fixation minus the rate of loss of CO2 in respiration. The rate of increase in oxygen concentration within an illuminated bottle containing phytoplankton is a measure of net photosynthesis; a value for gross photosynthesis may be obtained by adding to this the rate of respiratory oxygen consumption measured in a darkened bottle incubated in parallel. Whether the 14CO2 fixation method measures net or gross photosynthesis, or something between the two, remains a matter of controversy. In short-term incubations, such as are possible in the more productive waters, there are a priori as well as experimental grounds for considering 14CO2 fixation as an approximate measure of gross photosynthesis.

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