Nycthemeral variation of the alkaline phosphatase activity of a Diatom: Synedra capitata and two Cyanobacteria: Microcystis aeruginosa and Aphanizomenon flos aquae

The results we present here were on three quasi monospecific settlements : Synedra capitata (Diatom),Microcystis aeruginosa and Aphanizomenon flos aqu ae (Cyanobacteria). The alkaline phosphatase activity (APA) (EC : 3.1.3.1) exhibited a nycthemeral variation far Diatoms as well as Cyanobacteria, tho ugh every species had its own characteristic, Under similar experimental co nditions, the enzymatic activity was, in the average, 32 times higher (per unit of biomass) for Microcystis aeruginosa than for Synedra capitata and 3 times higher for Microcystis aeruginosa than for Aphanizomenon flos aquae. Under LD (16 : 8), Synedra capitata and Aphanizomenon flos aquae had a max imal enzymatic activity during the day time (maximal APA around 425 mu mol pNPP/l/h at 5 a.m., 11 a.m. and 5 p.m. for the Diatom and around 850 mu mol pNPP/l/h at II a.m., 3 p.m. and 7 p.m. for the Cyanobacteria) while the en zymatic activity of Microcystis aeruginosa was maximal at the beginnique of the night time (maximal APA around 1500 mu mol pNPP/l/h at 9 p.m.). The ob served daily rythm disappeared when the samples were placed in the darkness (DD) or under permanent light (LL). In the last case (LL), the mean enzyma tic activity of Microcystis aeruginosa was maximal and, on the contrary, th e mean APA of both Synedra capitata and Aphanizomenon flos aquae was minima l. There was a difference between the phytoplanktonic species in the orthop hosphate assimilation and in the capacity for mineralizing organic phosphor us because of the phosphatases. This difference in the enzymatic behaviour might explain ecological successions within the planktonic settlements.