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

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 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 15 NO 3 − uptake, PO 4 3− uptake and 15 N 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 15 N 2 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.

Biosynthesis system of Synechan, a sulfated exopolysaccharide, in the model cyanobacteriumSynechocystissp. PCC 6803

“…On the other hand, our study demonstrated that the gas-trapped EPS is sufficient for bloom formation of Synechocystis 6803, which does not produce gas vesicles, without addition of any divalent cations. This result is consistent with reports that cyanobacteria without gas vesicles form booms in natural environments, including freshwater lakes ( Casero et al, 2019, du Plooy et al, 2015, Steffen et al, 2012 ).…”

Biosynthesis of a sulfated exopolysaccharide, synechan, and bloom formation in the model cyanobacterium Synechocystis sp. strain PCC 6803

“…On the other hand, our study demonstrated that the gas-trapped EPS is sufficient for bloom formation of Synechocystis 6803, which does not produce gas vesicles, without addition of any divalent cations ( Figure 4B ). This result is consistent with reports that cyanobacteria without gas vesicles form booms in natural environments, including freshwater lakes ( Casero et al, 2019 ; du Plooy et al, 2015 ; Steffen et al, 2012 ).…”

Uptake rates of ammonium and nitrate by phytoplankton communities in two eutrophic tropical reservoirs

“…Incubations with trace amounts of 15 N isotopes added to water samples have been used for evaluating nitrate, ammonium, urea and mixed amino acid uptake by phytoplankton in environmental since the 1960s (Dugdale & Goering, 1967;Neess, Dugdale, Dugdale, & Goering, 1962). Given that historically N has been thought to limit phytoplankton growth in marine systems and phosphorus more so in freshwaters, most direct uptake studies on natural communities have focused on marine samples, with fewer examples carried out in lakes and reservoirs (Berman, Sherr, Sherr, Wynne, & McCarthy, 1984;du Plooy, Perissinotto, Smit, & Muir, 2015;Takamura, Iwakuma, & Yasuno, 1987;Toetz, Varga, & Loughran, 1973).…”