Extracellular Polymeric Substance Production by Benthic Pennate Diatoms

Benthic pennate diatoms are found in a wide variety of marine, freshwater and terrestrial environments. The majority of species produce extracellular polymeric substances (EPS, primarily polysaccharides) that coat the outside of the cell, form complex external structures such as stalks and tubes, biofilm matrices, and are involved in gliding motility. Diatom EPS is a significant carbon source in aquatic food webs, influencing, and being influenced by, closely-associated heterotrophic bacterial assemblages. Diatoms produce many different forms of EPS, varying in chemistry, properties and rates of production. EPS production is linked to photosynthesis, but diatoms can also produce EPS in darkness, utilising the storage product chrysolaminarin as a source of glucose. Environmental factors, cell growth rates and nutrient stoichiometry all influence EPS production and composition, but the mechanisms by which diatoms regulate and alter their EPS production pathways are not known. Reprogramming of metabolic pathways involved in carbohydrate synthesis, determined by RNA transcriptomics, results in changed EPS composition and production in response to environmental cues in some benthic diatoms. Some key enzymes involved in carbohydrate cycling and EPS synthesis have been identified, but the molecular machinery for polysaccharide assembly and intracellular translocation is not understood in detail. Rhythms of diatom behaviour in responses to light intensity, circadian controls and stress-responses are linked to EPS production, but the controlling mechanisms and metabolic pathways and feedbacks are not known. The development of molecular biology approaches are providing tools that will enable researchers to understand how diatoms regulate the production of a range of complex EPS types, the production of which are fundamental to life of benthic diatoms.