I have extracted and sequenced 16S-rDNA from suspended biomass collected on filters. Now I want to divide the OTU counts by the volume of water filtered for each sample, in order to abtain semi-quantitative abundance estimates. Does anyone know a reasonalble way to do it? I'm using the mothur and phyloseq. Thus far I only found the option to normalize all samples to one value. In my case, however, I have different values (volumes of filtered water) for each sample.
Thanks for any suggestions,