Moderated log$_{2}$ fold change of OTUs proportions for labeled versus
control gradients (heavy fractions only, densities $>$1.725 g/mL). All OTUs passing the 
sparsity treshold (see methods) at a specific incubation day are
shown. Red color denotes a proportion fold change that has a corresponding
adjusted p-value below a false discovery rate of 10\% (ratio is significantly
greater than 0.25, black line.)
