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

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 km(2)) 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 (NO3-)-N-15 uptake, PO43- uptake and N-15(2) 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.