The ecology and taxonomy of Synechococcus from saltwater lakes in the Vestfold Hills, Antarctica

In 1992, a marine Synechococcus was discovered in a meromictic lake in the Vestfold Hills, Antarctica. This thesis describes the ecology and taxonomy of this organism. Ace Lake is a saltwater, meromictic that was isolated from the marine environment approximately 6000 years ago. In 1992, The lake was 25 m deep, the top 12 m was oxygenated and the lake had a salinity range of 16 to 40 g kg-1 salt. The recent discovery of Synechococcus in Ace Lake was aided by flow cytometric methods. In Ace Lake, Synechococcus occured in the highest densities below the pycnocline with maximum numbers occurring just above the oxic/anoxic interface. Synechococcus bloomed in spring with numbers declining again in early January. At the peak of the bloom in 1992, a density of 8 x 106 cells mr1 was recorded at 11 m in the lake. No diel periodicity in the growth of Synechococcus was detected. Synechococcus was also present in two of ten other meromictic lakes and basins. The organism occured throughout the aerobic zone in Pendant Lake, in densities of approximately 10-7 cells m1-1, and below the pycnocline in Lake Abraxas in densities of 1.4 x 10 7 cells m1-1. It is possible that salinity restricts the distribution of Synechococcus in the meromictic lakes of the Vestfold Hills. Synechococcus strains were isolated from Ace Lake, Pendant Lake and Lake Abraxas for further characterisation. The three strains were similar in size and had the same lipid soluble pigment signature, with two unknown carotenoid pigments present in addition to the chlorophyll a, zeaxanthin and bb carotene. The three strains had phycoerythrin as their principle accessory light harvesting pigment. They were genetically similar (99. 7 % similarity in the 16S rRNA sequence) and had a G + C content of between 57 and 58 mol %. They were also genetically similar (95. 7 % similarity in the 16S rRNA sequence) to another marine picocyano bacteria, Prochlorococcus marinus. Based on the square root temperature dependence model, the minimum and maximum theoretical growth temperatures of the Ace Lake Synechococcus strain was -8° C, and 29.8° C. The optimal theoretical growth temperature was 19.7° C. ln-situ growth rates of the Ace Lake Synechococcus strain at 6 m, 8 m and 10 m in Ace Lake were determined. These rates were -0.118 d-1, 0.072 d-1 and 0.341 d-1 respectively. An increase in water temperature and a re~~on in light intensity increased the in-situ growth rate of the Ace Lake Synechococcus population. The grazing pressure on Synechococcus in Ace Lake was not determined. It is probable, however, that the distribution and abundance of Synechococcus in Ace Lake, Pendant Lake and Lake Abraxas is controlled by grazing. Chapter 1 summarises and reviews the current ecological and taxonomic research that has been undertaken on Ace Lake. Chapter 2 describes the flow cytometric techniques that were developed to study Synechococcus in Antarctic Lakes. Chapter 3 discusses the ecology of Synechococcus in Ace Lake and chapter 4 the distribution of Synechococcus in meromictic lakes in the Vestfold Hills. Chapter 5 describes the taxonomic characteristics of three Antarctic Synechococcus strains and chapter 6 discusses controls of Synechococcus growth in Ace Lake.