Ecology of the marine protists, the Labyrinthulomycetes (Thraustochytrids and Labyrinthulids)

Abstract

The Labyrinthulomycetes, comprising Thraustochytrids and Labyrinthulids, are marine osmoheterotrophic straminipilan protists which have been isolated from a variety of habitats all over the world. The limited knowledge on their ecological role(s) is brought together in this review. Labyrinthulids are prevalent on or in living marine algae and sea grasses as parasites, commensals or mutualists. Thraustochytrids, on the contrary, are rarely found on these living plants and appear to be inhibited by plant antimicrobials. Members of both groups often abound on dead autochthonous, as well as allochthonous, plant material such as macroalgae and submerged mangrove leaves, and probably play an important role as saprobes by virtue of extracellular enzyme production and chemical alteration of detritus. Labyrinthulomycetes re-portedly cause diseases in animals and there are indications that they live as commensals or mutualists within the guts and tissues of marine invertebrates, as well as being saprobic on such animal materials as faeces and mollusc shells. Thraustochytrids are common in the neritic and oceanic water column and sediments, including the deep sea. Seasonal studies indicate poor correlation with phytoplankton blooms, but often reveal Thraustochytrid biomass equivalent to that of bacteria during times of phytoplankton decay. It is concluded that they may play an important role in mineralisation of phytoplankton and zooplankton detritus in the sea. Their high content of Omega-3 polyunsaturated fatty acids suggests that they may form an important link in the food-web. More knowledge about their ability to degrade various forms and concentration levels of organic carbon in the sea, as well as possible phagotrophy will help to resolve their exact ecological niche vis-a-vis the bacteria in the marine ecosystem.