Monday, 16 February 2015 14:16

Persistent bloom of Cyanothece sp. in the St Lucia estuarine lake

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Cyanothece sp. bloom at the mouth of the Mkhuze River in North Lake, St Lucia, on 20 July 2009 (Photo courtesy of Ricky Taylor).The St Lucia Estuary is part of the iSimangaliso Wetland Park, South Africa’s first UNESCO World Heritage Site, and is the largest estuarine system in Africa. This three-lake complex is particularly vulnerable to droughts and hypersaline conditions due to its large surface area (350 km2) and shallow water depth (average depth 0.9 m). A widespread bloom of the unicellular cyanobacterium Cyanothece sp. appeared in June 2009 and persisted for 18 months within the two northern basins of St Lucia (False Bay and North Lake). This was the first recorded bloom of this genus globally.

Cyanothece sp. cells show great resistance to changing environmental variables and are capable of surviving conditions that other competitors and grazers cannot withstand. Our study so far shows that Cyanothece sp. cells maximise their nutrient acquisitions when conditions are favourable and then utilise those nutrient reserves to survive the harsh conditions. Laboratory experiments indicate that Cyanothece sp. cells are able to maintain nutrient (N and P) ability across a full salinity range of 0 - 300. Additionally, this Cyanothece sp. isolated from St Lucia is able to fix atmospheric N2 up to a salinity of 120. In addition to being halotolerant, these nutrient assimilation abilities (particularly N2 fixation) provide cells with an added advantage, allowing them to persist uninterruptedly for 18 months during extreme hypersaline conditions.

Cells of the bloom-forming Cyanothece sp. isolated from St Lucia (Photo courtesy of  David Muir).At low salinities, in the normal estuarine range, Cyanothece sp. is a rare component of the phytoplankton. However, as salinities increase during drought conditions, this cyanobacterium remains able to assimilate nutrients. With the elimination of eukaryotic competitors and normal components of the plankton (particularly grazers), due to increasing salinities, Cyanothece sp. is able to continue blooming up to a range in salinity of > 60 and < 120, at which point all eukaryotic competitors are eliminated. Above a salinity of 120, Cyanothece sp. resorts to a survival mode, by maintaining a minimal cellular structure with minimal uptake and metabolic activity, unaffected by grazing. In Lake St Lucia, these Cyanothece-specific bloom conditions persisted until December 2011, when heavy rainfall and subsequent floods sealed the demise of the bloom. Salinities suddenly decreased (resulting in both the dilution of the bloom and probably in bursting of cells under hypo-osmotic conditions) and a diverse trophic community returned, including competitors and grazers. Despite the beach spillway that now connects the Mfolozi River to Lake St Lucia, providing some freshwater input, the estuary mouth remains largely closed to the ocean. Thus, if drought conditions were to reoccur, these may lead to a new Cyanothece bloom, as background concentrations of the cyanobacterium remain in the lake.

Prof. Renzo Perissinotto Authors:
Prof. Renzo Perissinotto (PhD, Pr Sci Nat)
DST/NRF Research Chair: Shallow Water Ecosystems
Nelson Mandela Metropolitan University
P.O. Box 77000, Port Elizabeth 6031
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Schalk du Plooy
PhD Botany Candidate
DST/NRF Research Chair in Shallow Water Ecosystems
Nelson Mandela Metropolitan University
P.O. Box 77000, Port Elizabeth 6031, SOUTH AFRICA
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Journal Reference:
Du Plooy, S. J., Perissinotto, R., Smit, A. J., & Muir, D. G. (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.

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