Dominance of diatoms over cyanobacterial species in nitrogen-limited biofilms

Published observations, mainly on phytoplankton, suggest that (N ₂-fixing) cyanobacteria can outgrow diatoms under nitrate limitation. This study focussed on the competition for limiting nitrogen between 3 diatom species and 2 cyanobacteria species in biofilms. The potential of the diatom species Nitzschia perminuta (GRUNOW) M PERGALLO, Navicula trivialis LANGE-BERTALOT and Achnanthes lanceolata (BRÉBISSON) GRUNOW and the cyanobacterial species Leptolyngbya foveolarum (RABENHORST ex GOMONT) ANAGNOSTIDIS et KOMÁREK and Cylindrospermum stagnate (KÜTZING) RIPPKA et al. to grow in monocultures and in mixed cultures of a) the three diatoms and b) all five species was tested at nominal nitrate concentrations of 50 and 500 μM. Algal growth in biofilms was measured using a PHYTO-PAM, which measured fluorescence of biofilm suspensions, allowing simultaneous observations of diatoms and cyanobacteria in mixed cultures. Growth of all diatom species and the cyanobacterium L. foveolarum was similar in monoculture, reaching a maximum of 180 μg chl-a l^-1 at 50 μM nitrogen and 950 μg chl-a l^-1 at 500 μM nitrogen. Only the heterocyst forming cyanobacterium C. stagnale reached significantly higher densities, with about 800 μg chl-a l^-1 at 50 μM nitrogen and 1500 μg chl-a l^-1 at 500 μM nitrogen. However, in the mixed cultures, N. perminuta outgrew all other algae under the low nitrogen treatment while L. foveolarum dominated at the higher nitrogen concentration. It is concluded that interaction between species may impair the capacity of individual species to exploit nitrogen.