Thiamine pyrophosphate (TPP), the active form of thiamine is a cofactor for enzymes involved in central metabolism pathways. However, it is also known to have a role as a stress signaling molecule in response to environmental changes. Anabaena sp. and N. oculata are microorganisms which are abundantly found in Malaysia's freshwater and marine ecosystem. However, not much studies have been done especially in regards to thiamine biosynthesis. This work aimed to amplify of gene transcripts coding for thiamine biosynthesis enzymes besides looking at the expression of thiamine biosynthesis genes upon stress application. Various stress inducers were applied to the cultures and RNA was extracted at different time points. The first two genes, ThiC and ThiG/Thi4 encoding enzymes of the pyrimidine and thiazole branch respectively in the thiamine biosynthesis pathway were identified and amplified. The expression of the genes were analysed via RT-PCR and the intensity of bands were analysed using ImageJ software. The results showed up to 4-fold increase in the expression of ThiC and ThiG gene transcript as compared to control sample in Anabaena sp. ThiC gene in N. oculata showed an expression of 6-fold higher as compared to control sample. In conclusion, stresses induced the expression of the gene coding for one of the most important enzymes in thiamine biosynthesis pathway. This is an agreement with the hypothesis that overexpression of thiamine is crucial in assisting plants to combat abiotic stresses.Keywords Thiamine, Vitamin B1, Stress, Microalgae, Cyanobacteria, Gene expression IntroductionIn recent years, oxygenic phototrophic microorganism such as cyanobacteria and microalgae have been the center of interest as they are ideal models for many applications. They bloom rapidly with minimal amount of nutrients such as light, sugar, carbon dioxide, nitrogen, potassium and phosphorus (Behera, 2015). As compared to plants, they do not require land to grow and they are fast growing with simple genetic background to manipulate. Prokaryotic cyanobacteria do not have membrane-bound cell organelles and a defined nucleus whereas eukaryotic microalgae are more complex in structure and possess nucleus (Seckbach, 2007).Thiamine and its active form, thiamine pyrophosphate (TPP) play important roles in human nutrition and central metabolism (Tunc-Ozdemir et al. 2009, Guan et al., 2014. Besides that, in recent studies thiamine and TPP have been reported as crucial stress-response molecules in plant adaptations to counteract different abiotic stress conditions (Tunc-Ozdemir et al. 2009;Rapala-Kozik et al. 2012). It is hypothesized that thiamine enhances tolerance to oxidative stress via salicylic acid (SA) signaling pathway (Ahn et al. 2007).Presently, cyanobacteria and microalgae are not fully utilised as a source for thiamine production. Knowledge of thiamine biosynthesis pathway will pave the way for manipulation of thiamine biosynthesis for various purposes. Thiamine that are available in the market today are mainly synthe...