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| CORRECTION OF THE FORMULA TO CONVERT FLUORESCENCE MEASUREMENT TO PHAEOPHORBIDE a CONCENTRATION IN ACIDIFICATION METHOD |
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| In the acidification method for measuring chlorophyll a and phaeophorbide a the fluorescence readings are converted to pigments concentration using the formulae originally given by Lorenzen (1966), which were rewritten in more simple and convenient form by Strickland and Parsons (1972). They are as follows......These two formulae have been widely used for two decades in biological oceanography studies. As we know now that phaeophorbide a is almost the only degradation product present, which is abundant in water column and in herbivores fecal pellets. Recently we found, by rederivation, that Lorenzen’s second formula gives not the weight concentration of phaeophorbide a but its equivalent (on molar basis) weight concentration of chlorophyll a. So the equation must be corrected by a factor of 0.6633, the ratio of molecular weight of phaeophorbide a to that of chlorophyll a. This means that all the phaeopigment data obtained earlier by this method are 50.8% higher than their real value. Though the error is systematic, it is too big that some wrong conception and wrong conclusion can be derived. Based on their seriously deviated data Shuman and Lorenzen (1975) concluded that the conversion of chlorophyll a to phaeophorbide a after the digestion of herbivorous copepod was 100% on a molar basis. In fact their experiments showed a pigments loss as high as 34%. Furthermore, based on this wrong conclusion, plant pigments were used as tracers to study assimilation efficiency in copepods by some zooplanktologist. Our grazing experiments showed a pigments loss ranged from 6% to 80%, which is closely related with the digestion efficiency. When food was abundant in environment, ingestion rate and gut contents were high, the pigments loss would be small, and vice versa. It seems true that some of the pigments can be destroyed or absorbed during the passage through the gut of herbivores. |
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