There seems to be very little information about Markus Hartman. I can find no trace of him prior to 1652, in the sources that are available to me. His ship was lost in the great storm that struck on November 1, 1653. I suspect that he drowned.
We first read of him in a letter from Michiel De Ruyter to the Admiralty of Zeeland. On September 3, 1652, De Ruyter met Captain Hartman at sea, with some other ships, about 3 in the morning.
In April 1653, De Ruyter was on board Markus Hartman's ship, the Gekroonde Liefde, 36 guns (Middelburg Directors). In mid-May 1653, Captain Hartman was De Ruyter's Rear Admiral. De Ruyter commanded the fourth squadron of five in the Dutch fleet, prior to the Battle of the Gabbard.
In early August 1653, prior to the Battle of Scheveningen, Markus Hartman was still with De Ruyter. Captain Hartman's "squadron" (really, division) consisted of his ship, the Gekroonde Liefde of 36 guns, the ships of Jacob Wolphertsen (unknown name or guns), Jacob Swart (Vliegende Faam, 28 guns), and Captain de Bitter (the Mercurius of unknown guns).