Production of phosphatase and extracellular stalks as adaptations to phosphorus limitation in Didymosphenia geminata (Bacillariophyceae)
Didymosphenia geminata is a benthic bloom-forming diatom that is invasive in many temperate, oligotrophic freshwater ecosystems. D. geminata blooms are unusual, resulting from prolific basal stalk production stimulated by phosphorus limitation. The adaptive value of stalk production and bloom development is disputed. We examined blooms in relation to stalk biomass, biovolume and phosphatase activity. An austral summer survey of 15 sites within the Waitaki River of New Zealand compared reference communities (no detectable D. geminata), with those impacted by high and low D. geminata biomass. Sites were compared for differences in phosphatase location and activity using chromogenic substrates, community composition using morphological identifications, and overlying water and pore-water chemistry. Experimental microcosms subjected live proliferations to varied phosphate concentrations, and phosphatase rates and location were examined. Survey results identified phosphatase activity increased with D. geminata biomass, with lowest rates in reference communities. Pools of labile nutrients were detected in D. geminata mats, and in vitro hydrolysis rates were rapid in replete phosphoester conditions (~0.2 mmol l−1 h−1 cm−2 at 16°C), with activity concentrated on stalks. Our results suggest D. geminata bloom development is an adaptation to maximise supply of phosphate under chronic phosphorus limitation.
KeywordsPhosphatase Nutrient cycling Didymosphenia geminata Periphyton Phosphorus
Ash free dry mass
5-Bromo-4-chloro-3-indolyl phosphate–nitroblue tetrazolium
Dissolved reactive phosphorus
Filterable organic phosphorus
Filterable hydrolyzable phosphorus
4-Nitrophenyl phosphate disodium salt hexahydrate
The Department of Conservation, the Ministry for Primary Industries and the Miss E L Hellaby Indigenous Grassland Research Trust funded the research. The samples were transported live following permissions obtained from the Ministry for Primary Industries under S.52 of the Biosecurity Act 1993. We thank Hayley Stoddart for water chemistry analysis and anonymous reviewers, Elizabeth Graham, Catherine Febria and particularly Paul Broady and David Hamilton for their helpful comments on drafts.
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