http://www.spaceref.com/news/viewpr.html?pid=26...

[link] ?pid=26778

This article may have implications for analysis of OM images. It says that a type of magnetactic bacteria on Earth produced magnetite crystals 4 microns in size. I presume this means the bacteria were even larger. The OM is supposed to have 4 micron resolution, so if similar bacteria were present, they might perhaps be barely visible in OM images.