That would be an extraordinarily well tuned biological process.
I sincerely hope that no such organism exist or ever does exist.
Such an organism might be adapted to enrich uranium cheaply and efficiently.
Nuclear terrorism would be come a daily occurrence.
It definitely happens in nature, but gets WAY more difficult as atomic number goes up. Easiest is hydrogen/deuterium, as the mass difference is quite large. For U235/U238, the mass difference is a LOT smaller.
Regarding your statement “That would be an extraordinarily well tuned biological process,” referring to biological differential handling of isotopes:
You, assuming you are a eukaryote like the rest of us, have such “well-tuned” biological processes running inside you. While isotopes the same element are chemically identical to a first approximation, its only true to that first approximation. The reality is that eukaryotic organisms become sterile when on a diet of isotopically “heavy” (but not radioactive) water, and die if that diet of “heavy” water is continued. In other words: we all have a “well tuned biological process” within us for which isotopic differences have significant effects.
Deuterium is one isotope which definitely has a biological effect. Heavy water is toxic. It disrupts fundamental biological processes. The reason appears to be that the presence of that extra neutron slightly increases the strength of the hydrogen bond, which in turn upsets reactions essential to cellular activity.
However, the bond energy effect of slight atomic weight differences decreases as you go up in atomic weight, to the point to undetectability. So, centrifuge are far more likely to be hacked or blown up by the Mossad than to be displaced by vats of bugs genetically engineered to concentrate U235.