Catalyzes the activation of long-chain fatty acids as acyl-adenylates (acyl-AMP), which are then transferred to the multifunctional polyketide synthase (PKS) type III for further chain extension. Involved in the biosynthesis of sulfolipid 1 (SL-1). ATP + hexadecanoate + holo-[(hydroxy)phthioceranic acid synthase] = AMP + diphosphate + hexadecanoyl-[(hydroxy)phthioceranic acid synthase] ATP + holo-[(hydroxy)phthioceranic acid synthase] + octadecanoate = AMP + diphosphate + octadecanoyl-[(hydroxy)phthioceranic acid synthase] Lipid metabolism; fatty acid biosynthesis. Belongs to the ATP-dependent AMP-binding enzyme family.