Bacteria? Nope. I know what a bad case of black fungus growing in a fuel tank looks like as that's what I dealt with in my Mercedes just after I bought it. After a dosing of a biocide the fungus died and stopped clinging to the filter mesh screens. It turned into black specks which eventually made its way to the fuel filter. My truck's fuel tanks (VO and B20) don't have this issue. What I saw in the FPR looked like VO colored hardened silicone with a rubbery texture. VO polymerizes in the presence of heat and oxygen. Put a few ounces of VO in a glass dish in the summer sun and after a few months the surface will form a rubbery skin then after time all the VO will get rubbery.
