Absorptions simultanées des ions NO3− et NH4+ par trois Diatomées de claires à huîtres, en culture axénique

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Abstract
Three diatoms from the oyster-ponds of the bay of Bourgneuf (Vendee, France) were isolated in axenic cultures and then grown with both NH4+ and NO3- ions as nitrogen sources. The simultaneous uptake of both ions begins only below a threshold concentration of NH4+ which varies with the algal species, but which always greatly exceeds the 1 .mu.M l-1 upper limit considered up to now as the upper limit for all the algae. The average values of the recorded limits are: 40.7 (s = 12.10) .mu.M l-1 NH4+ for Amphora coffeaeformis var. perpusilla, 16.2 (s = 8.02) .mu.M l-1 for Navicula ostrearia and 18.2 (s = 10.19) .mu.M 1-1 for Nitzschia sp. cf. ovalis. At sub-inhibiting concentrations of NH4+, the NO3- uptake mechanism acts at a reduced rate, which was demonstrated by the use of a preference index to amount roughly to 20-25% of the total nitrogen uptake. Below a threshold value of 6 .mu.M 1-1 NH4+, the full NO3- uptake rate resumes. So far as the isolated strains mimicked the behaviour of the natural populations, it is concluded that the oyster-pond micro-algae do not show ammonium preference within the same range of the threshold values as the oceanic phytoplankton species.