Eight new antimicrobial natural products named chrysophaentins A-H belonging to a new structural class have been isolated from the marine chrysophyte alga Chrysophaeum taylori. Their structures were determined by extensive 2D NMR and MS techniques and are characterized by the presence of two polyhalogenated, polyoxygenated omega,omega'-diarylbutene units connected by two ether bonds to form the suite of macrocyclic natural products. Chrysophaentin A, the most potent of these antibiotics, inhibited the growth of clinically relevant Gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MIC(50) 1.5 +/- 0.7 microg/mL), multidrug-resistant S. aureus (1.3 +/- 0.4 microg/mL), and vancomycin-resistant Enterococcus faecium (MIC(50) 2.9 +/- 0.8 microg/mL). In vitro enzyme assays and transmission electron microscopy showed chrysophaentin A to inhibit the GTPase activity of the bacterial cytoskeletal protein FtsZ with an IC(50) value of 6.7 +/- 1.7 microg/mL, as well as GTP-induced formation of FtsZ protofilaments. Saturation Transfer Difference (STD) NMR experiments further confirmed chrysophaentin A binds to FtsZ, and NMR competition experiments with GTPgammaS showed chrysophaentin A and GTP to bind competitively to FtsZ. Last, molecular docking simulations provided a low energy model in which chrysophaentin A binds in and occludes a large portion of the GTP binding site of FtsZ in a manner that is consistent with the binding epitope determined by STD NMR.