Ultrastructure and function of stalks of the diatom Didymosphenia geminata
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One of the most striking features of the diatom Didymosphenia geminata, which has increased markedly in abundance in a number of countries in recent years, is the very large branched stalks. In order to help understanding their role, an ultrastructural study was carried out on two populations, one from a stream in northern England and the other from a river on Vancouver Island, Canada. In both cases, the main part of the stalk had a central reticulate core surrounded by an outer region with dense fibres. A longitudinal structure in the uppermost part of the stalk just under the collar surrounding the base of the cell may perhaps correspond to a tube. The structure of the septa formed where branches divide is also described. Phosphomonoesterase activity known to be present in the stalks was shown to occur in the inner peripheral layers of the stalks and especially in the collar area. The results show that stalks have a complex structure suggesting their importance for their phosphatase activity to overcome low inorganic phosphate concentrations. Their large surface may function in herbivory avoidance, a better exposure of cells to turbulent conditions to increase nutrient uptake, adsorption of limiting elements and gas exchange.
KeywordsDidymosphenia geminata Ecology Monitoring Phosphomonoesterase activity Stalks Ultrastructure
We are indebted to Prof. M. Bothwell (National Water Research Institute, Vancouver Island) for sending material from Canada and Prof. B.A. Whitton (University of Durham) for sending the UK material and several comments on the manuscript. Grants CGL2006-09864 and CERT07-10321 from Spanish Ministry of Education and Science and Séneca Foundation from Murcia Autonomous Community partially financed the study.
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