I still cannot workout which specific authority POTUS has over the Marines that Q is alluding to. The War Powers Act seems to apply equally to all branches. Posse Comitatus seems to apply equally to all branches. If either were revoked or limited, it does not seem to affect the Marines specifically.
By tradition the Marines have always protected the President, and Washington DC, but that seems to be just tradition. We know he has sole authority over the Secret Service, but that does not seem to be Marine specific.
I am digging into whether the Marines have some special role during Martial Law situations, but nothing is popping out.
Any Anons out there with other ideas?