ANAEROBIC SEDIMENT. “One factor contributing to these high light requirements is the reducing sediments to which seagrasses are rooted. These sediments challenge seagrass root tissues with anaerobic conditions since marine sediments are often oxygen deficient. When the internal transport of oxygen from shoot to root tissues is not sufficient, seagrasses can be forced to resort to fermentative metabolism.”
http://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-11-8