Here, we evaluate the bacterial communities associated with the growth of tank-raised Posidonia angustifolia seedlings grown in autoclave treated and untreated propagating 'play sand' and a natural meadow mix (comprising beach sand mixed with meadow sediment) over a 12-week period. Comparisons are made with the samples collected from native meadows (comprising P. angustifolia fruit and roots) to determine bacterial taxa that may support improved vigor.