AME 74:73-83 (2015)  -  DOI: https://doi.org/10.3354/ame01728

Role of salinity, nitrogen fixation and nutrient assimilation in prolonged bloom persistence of Cyanothece sp. in Lake St Lucia, South Africa

S. J. du Plooy1,*, R. Perissinotto1, A. J. Smit2, D. G. Muir3

1DST/NRF Research Chair in Shallow Water Ecosystems, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
2Department for Biodiversity & Conservation Biology, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
3Department of Biology, Medgar Evers College, The City University of New York, 1650 Bedford Avenue, Brooklyn, New York 11225, USA
*Corresponding author:

ABSTRACT: Worldwide, cyanobacterial blooms are becoming more frequent, exacerbated by eutrophication and other anthropogenic actions and also associated with global climate change. In June 2009, a widespread bloom of the unicellular cyanobacterium Cyanothece sp. appeared in North Lake and False Bay of Lake St Lucia, a large (360 km2) estuarine lake system in KwaZulu-Natal, South Africa, and persisted for 18 mo. It remains unclear how the bloom status was maintained for so long. This study investigated aspects of the nutrient (N and P) assimilation of Cyanothece sp. and how these may relate to maintaining a persistent bloom state during hypersaline conditions. The effects of salinity and nutrient limitation on the nutrient uptake dynamics of Cyanothece sp. were evaluated with 15NO3- uptake, PO43- uptake and 15N2 fixation experiments. Nitrogen fixation was observed in this Cyanothece sp. isolate from St Lucia. Highest nutrient assimilation rates in all experiments were recorded at the lowest salinities, decreasing progressively up to a salinity of 120, with very little activity observed above this level. No 15N2 fixation was measured above this salinity. Results indicate that Cyanothece sp. was well suited to take advantage of the conditions present during the onset of the bloom at salinities <100. However, once salinity increased above 120, nutrient uptake abilities would have been drastically reduced.  Regardless, cells still survived under these extreme saline conditions, as most of their potential grazers and autotrophic competitors disappeared from the St Lucia Estuary.


KEY WORDS: 15NO3- · 15N2 · Cyanobacteria · Cyanothece · Lake St Lucia · Hypersalinity · Nutrient uptake · Nitrogen fixation


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Cite this article as: du Plooy SJ, Perissinotto R, Smit AJ, Muir DG (2015) Role of salinity, nitrogen fixation and nutrient assimilation in prolonged bloom persistence of Cyanothece sp. in Lake St Lucia, South Africa. Aquat Microb Ecol 74:73-83. https://doi.org/10.3354/ame01728

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