Identification and exopolysaccharide synthesis by Antarctic yeasts Evaluation of the antineoplastic activity of Antarctic yeast Sporobolomyces salmonicolor grown at different culture conditions

Antarctic is among the most scarcely investigated extreme niches in relation to microorganisms living there and synthesis of unusual molecules for adaptation to extremely low temperatures, lack of water availability and precipitation, numerous freeze–thaw cycles, strong wind levels and high sublimation, evaporation and ultraviolet radiation. Several yeast genera such as Cryptococcus, Mrakia, Candida, Rhodotorula, Leucosporidium, Debaryomyces have been described as part of the biodiversity of the continent.

The aim of the present research was to studying yeast variety in the Livingston Island, Antarctica, and yeast potential to synthesize extracellular polysaccharides. Thanks to the cooperation of the Bulgarian Antarctic Expedition, three different strains were isolated and identified. Based on a genetic analysis of ITS1-5.8S-ITS4 regions of rRNA they were related to two genera – Cystobasidium and Vishniacozima. One of the strains belonged to the species Cystobasidium ongulense and two to Vishniacozima victoriae. The morphological differences of the two species included the color of the colonies - red and cream, respectively, while both species were glossy and with a smooth edge. Cystobasidium ongulense cells were larger and elongated. The differences between the species also included the different number of assimilated carbon sources. The ITS domains of the rRNA gene were amplified using the universal primers ITS1 and ITS4.  The process of extracellular polymer biosynthesis was performed at a cultivation temperature of 21°C±1°C. The cell growth over 5.5gL^-1 and exopolysaccharide production from 1.6 to 3.6gL^-1 were registered at 120^th h of the fermentation process.

Based on sampling of soil, moss or penguin feathers in Bulgarian base, the Livingston Island, new results for yeast diversity were accumulated and they can contribute to the fundamental worldwide knowledge on the biodiversity of the Antarctic continent. Unusual origin of exopolysaccharides synthesized by the isolates and their valuable preliminary characterized properties suggest a possibility for their biotechnological exploration.

Nowadays, deaths from neoplasms are at the top of the World Health Organization list. The interest of scientists is focused on the search for natural substrates perspective for the targeted anticancer chemotherapy. Antarctic yeasts represent an unexplored object regarding their antineoplastic potential. The studies in the present work demonstrated the capacity of Sporobolomyces salmonicolor AL₃₆ for cell growth in different cultivation processes. During the transfer of small-scale flasks cultivation to the bioreactor system, optimal biomass quantities of approximately 6.0 g/L were recorded. A comparative examination of the metabolic profiles of the two extracts tested reveals differences in the synthesized molecules corresponding to different cytotoxicity (antineoplastic activity) in vitro on malignant cell lines. The median inhibitory concentration (IC₅₀) of each extract determined by the MTT test was used as a parameter for evaluating the antiproliferative effects. Most sensitive to the in vitro effect of the extract of Sp. salmonicolor AL₃₆ cultivated in flasks was the cell line SKW-3 (T cell leukemia, derivative of KE-37) - IC₅₀ = 35.3 µg/ml, while the bioreactor biomass extract was most cytotoxic for RPMI-8226 (multiple myeloma) cells - IC₅₀ = 28.27µg/ml. The proteome analysis of treated and untreated malignant cells showed that both yeast extracts have a strong potential to inhibit anti-apoptotic proteins which reveals a mode of action related to induction of apoptosis and proliferation inhibition.