I am not aware of synthetic omega-3. All EPA and DHA are naturally produced by living organisms.

The fish from which the fish body oils are extracted get them from the fish they consume in their food chain, which begins with the algae which makes them in the first place.

So the algae which is used to make the DHA is natural and just bypasses the fish who eat it.

Krill oil is also a natural source.

The extraction of the oils, and/concentration of them, is a man-made artificial process, but not "unnatural".

DHA made commercially in vats, is referred to as Vegetarian sourced. This is important for pregnant and nursing women, to avoid heavy metal or dioxin contamination for their infants.

Most fish body oils, do not have mercury or heavy metals in them, but some are showing up now with dioxins. The process of removing them from fish bodies, tends to remove the toxins as well...it is called distillation.