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This report is the first investigation of yeast biodiversity from the oligotrophic hypersaline coastal waters of the Arabian Gulf surrounding Qatar. Yeasts and yeast-like fungi, were cultured from seawater sampled at 13 coastal areas surrounding Qatar over a period of 2 years (December 2013-September 2015). Eight hundred and forty-two isolates belonging to 82 species representing two phyla viz., Ascomycota (23 genera) and Basidiomycota (16 genera) were identified by molecular sequencing. The results indicated that the coastal waters of the Qatari oligotrophic marine environment harbor a diverse pool of yeast species, most of which have been reported from terrestrial, clinical and aquatic sources in various parts of the world. Five species, i.e., Candida albicans, C. parapsilosis, C. tropicalis, Pichia kudriavzevii and Meyerozyma guilliermondii (n = 252/842; 30% isolates) are known as major opportunistic human pathogens. Fifteen species belonging to nine genera (n = 498/842; 59%) and 12 species belonging to seven genera (n = 459/842; 55%) are hydrocarbon degrading yeast and pollution indicator yeast species, respectively. Ascomycetous yeasts were predominant (66.38%; 559/842) as compared to their basidiomycetous counterparts (33.6%; 283/842). The most isolated yeast genera were Candida (28%; 236/842) (e.g., C. aaseri, C. boidinii, C. glabrata, C. intermedia, C. oleophila, C. orthopsilosis, C. palmioleophila, C. parapsilosis, C. pseudointermedia, C. rugopelliculosa, C. sake, C. tropicalis and C. zeylanoides), Rhodotorula (12.7%; 107/842), Naganishia (8.4%; 71/842), Aureobasidium (7.4%; 62/842), Pichia (7.3%; 62/842), and Debaryomyces (6.4%; 54/842). A total of eleven yeast species ( n = 38) isolated in this study are reported for the first time from the marine environment. Chemical testing demonstrated that seven out of the 13 sites had levels of total petroleum hydrocarbons (TPH) ranging from 200 to 900 µg/L, whereas 6 sites showed higher TPH levels (> 1000-210

Reliable delineation of aquatic toxicity cut-offs for poorly soluble hydrocarbons is lacking. In this study, vapor and passive dosing methods were applied in limit tests with algae and daphnids to evaluate the presence or absence of chronic effects at exposures corresponding to the water solubility for representative hydrocarbons from five structural classes: branched alkanes, mono, di, and polynaphthenic (cyclic) alkanes and monoaromatic naphthenic hydrocarbons (MANHs). Algal growth rate and daphnid immobilization, growth and reproduction served as the chronic endpoints investigated. Results indicated that the dosing methods applied were effective for maintaining mean measured exposure concentrations within a factor of two or higher of the measured water solubility of the substances investigated. Chronic effects were not observed for hydrocarbons with an aqueous solubility below approximately 5 μg/L. This solubility cut-off corresponds to structures consisting of 13–14 carbons for branched and cyclic alkanes and 16–18 carbons for MANHs. These data support reliable hazard and risk evaluation of hydrocarbon classes that comprise petroleum substances and the methods described have broad applicability for establishing empirical solubility cut-offs for other classes of hydrophobic substances. Future work is needed to understand the role of biotransformation on the observed presence or absence of toxicity in chronic tests.