Marine microbial eukaryotic diversity, with particular reference to fungi: Lessons from prokaryotes

388-398Novel molecular, analytical and culturing techniques have resulted in dramatic changes in our approaches towards marine eukaryotic diversity in recent years. This article reviews marine fungal diversity in the light of current knowledge, citing examples of how progress in understanding marine prokaryotes has often contributed to this new approach. Both ‘true fungi’ (termed mycenaean fungi in this review) and straminipilan fungi are considered. Molecular phylogenetic studies of prokaryotes has resulted in their redefinition as belonging to the Kingdoms Bacteria and Archaea. Likewise, major refinements have taken place in the phylogenetic classification of eukaryotes. In the case of fungi, it has now been realized that they are polyphyletic, belonging to the Kingdom Mycenae (Fungi), as well as the Kingdom Straminipila or Chromista. Although the total number of fungi on earth is estimated to be about 1.5 million, only a meagre number of obligate marine fungi , about 450 mycenaean and 50 straminipilan fungi have been described so far. It is likely that most of the true marine fungi have not yet been discovered. These are likely to have evolved between 1,500 million years ago (Ma) when fungi probably evolved in the sea and 900 Ma when they conquered land together with green plants. It now appears that most of the true marine fungi have not been cultured so far, similar to the ‘great plate count anomaly’ of bacteria. Thraustochytrids, which are abundant in the water column, but not easily culturable from that source is an example. Intelligent and novel culture methods might bring forth unusual and new marine fungi, as happened in the case of Pelagibacter ubique belonging to the SAR 11 group of bacteria. Molecular techniques might bring to light novel marine fungi, as is happening with bacteria. Such fungi may defy our conventional wisdom regarding these organisms in terms of morphology. Thus, several recent studies using 18S rRNA gene community profiles have discovered picoplanktonic marine fungi in the water column. Studies such as those on molecular diversity of eukaryotes in permanently anoxic habitats have also indicated that fungi may be abundant in exotic habitats and possess unusual physiology. A search for fungi in biodiversity-rich habitats, such as the coral reefs and the deep-sea, using a combination of molecular and novel culture methods is likely to reveal a fascinating diversity of marine fungi