Overall planktonic oxygen and carbon dioxide metabolisms: the problem of reconciling observations and calculations of photosynthetic quotients

Although the photosynthetic quotient (P0) of micro-algae in culture has been studied and characterized, that for natural plankton is poorly understood Paired measurements of gross oxygen photosynthetic production and 14CO2 incorporation have been abstracted from a number of sources and examined to see if they could provide a representative value for planktonic populations The analysis provided a range of quotients from 0.5 to 3.5, the distribution was bimodal and the modes higher than those normally attributed to micro-algae A detailed analysis of the biochemical basis of the relationship between the photosynthetic products and reactants led to the conclusion that one could only accommodate the data set by postulating the production of ‘unconventional’ photosynthetic products, such as methane or hydrogen or extensive overassimilation of nitrate When the potential systematic errors in the 14CO2 and O2 techniques were examined, it was found that they did not cancel but produced a bias. It was concluded that these systematic errors precluded the use of paired O2 and 14CO2 measurements as a means of obtaining reliable estimates of the PQ of natural plankton It was argued that paired measurements of chemically determined gross O2 and CO2 production and consumption were a more reliable basis for the calculation of PQ, on the grounds that they share the major systematic errors, which therefore cancel to give a less biased quotient Earlier observations of chemically determined CO2 and O2 flux have given discrepant PQ values: it was concluded that these could be attributed to analytical error. Data were presented from in vitro and in situ studies of O2 and CO2 metabolism obtained using high precision analytical methods. The results illustrated that, providing an error analysis was made, essentially all the PQ values derived from these observations could be contained in the range 1 0—1 36: the spread of P0 values calculated for a ‘typical’ algal cell producing conventional’ metabolism