Adenosine nucleotide “energy charge” ratios as an ecophysiological index for microplankton communities

Energy charge (EC) ratios of microplankton samples have been measured from their adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) contents, according to a method based on enzymatic tranformations of ADP and AMP into ATP, and the subsequent quantitative analysis of the latter by the bioluminescent reaction of a firefly lantern extract (Photinus pyralis). Interference caused by other non-adenylic nucleotides and bioluminescence inhibition by various compounds were reduced, or estimated, by the use of internal standards for each sample. This was accomplished both by the injection of a small volume of a non-commercial extract of P. pyralis and by measurement of the bioluminescent flash at its maximal value. This standard method for preparation and analysis afforded good reproducibility and permitted the calibration of individual samples, thus allowing the comparison and treatment of data in the large series necessary in oceanographic studies. Studies were made on natural populations taken from a polluted marine area where phytoplankton communities live under the stressful influence of an urban sewer outlet, near Marseille (France). Three sampling strategies in surface waters (transects from the outlet, sampling during a 24-h buoydrift, and regular sampling for a day-night period) conducted at a point in front of the outlet revealed a decrease of EC in relation to urban activity or approaching proximity of the sewer outlet. These low EC values (0.2 to 0.5) were caused by a high AMP content, perhaps originating from degraded biogenic particles and dead or metabolically impeded bacteria of terrestrial orgin stressed by ecologic factors such as increased salinity and decreased temperature. Further away from the sewage outlet, the EC increased as the bacterial population decreased, and the phytoplanktonic communities reappeared parallel to the dilution of the effluent. Although in mature and unperturbed ecosystems EC values of 0.70 to 0.80 were recorded, the values were generally lower than those measured in growing bacterial or phytoplankton cultures. This fact may be related to differences in the various metabolic states of multispecific populations. Some EC measurements from deeper microplankton samples presented in this paper were difficult to interpret, reflecting perhaps unsolved problems concerning the treatment of samples. However, it was possible to associate the range of variation in EC values to differences in composition and structure of the ecosystem of microplankton populations in neritic surface waters.