Nitrogen uptake dynamics of a persistent cyanobacterium Cyanothece sp. bloom in Lake St Lucia, South Africa

Abstract

Worldwide, persistent cyanobacterial blooms are becoming more frequent and are often associated with effects of 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 months. It remains unclear how the bloom status was maintained for so long. This study investigates aspects of the nutrient uptake dynamics of Cyanothece sp. and how this process may relate to maintaining a persistent bloom state during hypersaline conditions. ¹⁵N nutrient uptake experiments were performed to evaluate the effect of the most important environmental variables on the nutrient uptake rate of Cyanothece sp. Highest 15N uptake rates were during salinities of 35 and temperatures of 30 °C for both N sources (43.9 [SE 18.3] and 85.5 [SE 42.9] μg atom N l^–1 h^–1 for ¹⁵NH₄+ and 15NO3 − respectively). In contrast, at high salinities of 160 and low temperatures of 10 °C, 15NH4 + and ¹⁵NO₃− uptake rates were low (3.76 [SE 0.23] and 14.5 [SE 6.40] μg atom N l^–1 h^–1 respectively). Hence nutrient uptake was significantly influenced by environmental variables, particularly salinity, which suppressed uptake rates, and temperature, which facilitated them. The long-term bloom maintenance appears to be attributed to efficient nutrient uptake rates, absence of grazers during the hypersaline phase, and the ability of Cyanothece sp. to outcompete other microalgae at temperatures >25 °C.

Keywords: ¹⁵N isotope pairing technique, cyanobacteria, estuarine lake, hypersalinity, iSimangaliso, unicellular

African Journal of Marine Science 2014, 36(2): 155–161