Online Resource 2.Ranges, means, and and standard deviations (SD)of morphological characteristics of cyanobacterialendosymbionts obtained from the morphometric analysis of 120 diatom frustules belonging to each Epithemiaand Rhopalodiaspecies. The ranges shown were extracted from the data set as whole (i.e., by lumping all12 populations together for each species).

Cell features of
cyanobacterialendosymbionts / E. adnata / E. sorex / E. turgida / R. gibba
range / mean/SD / range / mean/SD / range / mean/SD / range / mean/SD
Number per host cell / 2-6 / 3.4+0.8 / 1-2 / 1.3+0.1 / 2-12 / 5.5+1.1 / 1-4 / 2+0.5
Cell diameter (µm) / 3.2-7.6 / 4.5+0.6 / 3-7.1 / 4.2+0.4 / 4.1-8.4 / 5.8+0.7 / 4.4-6.3 / 5.1+0.2
Cell lenght (µm) / 5.5-7.7 / 5.8+0.5 / 3.9-7.9 / 5.4+0.5 / 5.6-8.7 / 6.8+0.6 / 5.5-13 / 8.1+0.5
Single cell biovolume (µm3) / 17-229 / 53+27 / 14-187 / 47+13 / 33-307 / 114+44 / 65-245 / 115+17
Totalbiovolumeper host cell (µm3) / 34-1150 / 194+133 / 15-188 / 61+19 / 126-1950 / 637+276 / 89-723 / 238+84

Online Resource4.Correlation coefficient between environmental variables and the first two CCA axes.

Variables / Axis 1 / Axis 2
N:P / 0.82 / 0.07
NO3-N / 0.39 / -0.01
NH4-N / 0.22 / 0.31
TN, filtrable / -0.03 / 0.55
Canopy / -0.30 / 0.06
Sand/Silt / -0.07 / -0.40

Online Resource5. Results of one-way ANOVA, means, and standard deviations (SD) for environmental variables that differed significantly between stream sites where the N2-fixing algae recorded were only heterocystouscyanobacteria vs. those with only diatoms Epithemia/Rhopalodia.

Variables / Cyanobacteria (n=29) / Diatoms (n=20) / F / p
Mean / SD / Mean / SD
Canopy / 48.71 / 36.01 / 73.91 / 29.60 / 7.06 / 0.01
Sand/Silt / 17.55 / 18.33 / 30.45 / 19.63 / 5.53 / 0.02
Bedrock / 6.897 / 11.81 / 0.909 / 2.798 / 4.91 / 0.03
Silicate / 7.775 / 3.937 / 10.38 / 4.952 / 4.22 / 0.04