Bioresponsce of microalgae Oscillatoria limnetica to organophosphorous pesticide Glyphosate Jasim M. Salman* 1 , Esraa Abdul-Adel 1 1 Department of Biology, College of Science, University of Babylon,Iraq. Abstract: The microalga species, Oscillatoria limnetica, isolated from the artificial canal around University of Babylon in Al-Hilla city was cultured in the laboratory using BG-11 growth medium for biomass production and to test the effect of organophosphorous pesticide glyphosate on gowth rate, doubling time and photosynthesis pigments (chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, phycoerythrin, phycocyanin, allophycocyanin and phycobilliproteins). The presence of glyphosate caused an inhibitory effect on the growth rate of O.limnetica and increase doubling time. Comparisons with a control, which supported 0.0361 growth rate and 8.337days doubling time, showed that the highest reduction of the growth rate was 0.0285 and the top rise of the doubling time was 10.561 days at 15mg/l of glyphosate. In addition to, glyphosate caused inhibitory effects on photosynthetic pigments of the isolated algae.Maximum reduction of chlorophyll a, chlorophyll b, total chlorophyll and carotenoids was 0.6224, 0.1138, 0.736 and 0.143 mg/1, respectively in the presence of 15mg/1 of glyphosate. Moreover, maximum reduction of phycocyanin, allophycocynin, phycoerythrin and phycobiliproteins was 0.00207, 0.00361, 0.00165 and 0.0072mg/1 respectively, in 20mg/1 glyphosate. Keywords: Cyanophytae, bioresponce, photosynthesis pigments, orgnophosphorous pesticide, glyphosate. Introduction Over the years, pesticides have resulted in problems caused by their interactions with the biological systems in the environment and have harmful effects on algae, especially nitrogen fixing cyanophyta by influencing growth, photosynthesis, nitrogen fixation, biochemical and molecular composition, and metabolic activities 1 . Glyphosate-based herbicides are the world’s leading post-emergent, broad spectrum and non-selective herbicides for the control of annual and perennial weeds in agricultural lands, ornamental and residential gardens and in aquatic systems 2 . While the physicochemical and acute toxicological properties of glyphosate are well known 3 , with numerous studies focusing on aquatic animals (invertebrates and fish), limited information is available on the responses of photosynthetic microorganisms to the herbicide 4-5 . Glyphosate is a competitive inhibitor of the enzyme 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS), which is involved in the pathway of synthesis of shikimic acid 6 - 7 . It, will also affect photosynthesis 8 by forming complexes with cations which are co-factors of important photosynthetic components: magnesium as a co-factor in chlorophyll synthesis 9 , manganese and calcium as co-factors of the Mn-cluster 10 and iron as an essential component of the ferredoxin protein 11 . International Journal of PharmTech Research CODEN (USA): IJPRIF, ISSN: 0974-4304, ISSN(Online): 2455-9563 Vol.9, No.6, pp 446-454, 2016