Achnanthes longipes cells were subjected to varying concentrations of methylmercury (6, 8, and 10 mM, and 6, 8and 10 µM) that were inoculated in the medium and allowed to incubate for seven days. Each set-up was prepared in three replicates and those without methylmercury added served as control. The amount of total carbohydrate produced from each setup was quantitated by phenol-sulfuric assay using spectrophotometric analysis. Varying absorbance values were observed in cultures treated with 6, 8, and 10 µM methylmercury but the differences were not statistically significant. Surprisingly, increasing the concentration of methylmercury even a thousand fold (6, 8, and 10mM) showed similar insignificantly varying results. To determine the effect on the stabilization of sediments, diatom cells were grown with sediment added as substrate. Overall, results indicated no significant difference between the control and the treatments. This implies that A.longipes is highly tolerant to methylmercury relative to the concentration used in the experiment. Mercury is one of the heavy metal toxicants in the aquatic environment which exists in the seawaters in its predominant organic complexes. This pollutant can have hazardous effects on the coastal environment including diatoms, which are important assemblage of algal community. Diatoms are a major component of marine and freshwater biofilms. Cells secrete extracellular polymeric substance composed of polysaccharides that vary in monosaccharide composition and serves a variety of functions; to include as main source of primary productivity, substratum adhesion, and its role in sediment stabilization. This research is conducted in an effort to understand the effect of methylmercury on the polysaccharide production by a benthic diatom Achnanthes longipes. Moreover, the consequent effect of the produced polysaccharide on sediment stabilization is also investigated as this may have important implications on coastal preservation and/or restoration. • Cells were grown in Erlenmeyer flasks with f/2 media at a salinity of ~30 psu in axenic condition. These were maintained in culture chamber at 18°C with an irradiance of 502.4 lux provided by fluorescent lamps. • Diatom cells were subjected to varying concentration of methylmercury (6, 8, and 10 µM and 6, 8 and 10 mM) that was inoculated in the medium and allowed to incubate for seven days. Each set-up was prepared in three replicates and the flasks without methylmercury added served as control. • The amount of total carbohydrate produced from each setup was quantitated by phenol-sulfuric assay at an absorbance of 485 nm using a spectrophotometer. A calibration curve was obtained with a regression value of 0.99 using glucose as standard. • To determine the effect on the stabilization of sediments, diatom cells were grown in the treatment conditions as previous but with sediment added as substrate. The sediment consisted of a mixture of sand (1%), silt (34%) and clay (65%), which were autoclaved to maintain the cultures in axenic condition. The cells were incubated for seven days. • Each culture was agitated using a mechanical stirrer at 160 rpm for 3 minutes. Sediment stability was measured according to the degree of re-suspension after agitation which was quantitated by spectrophotometric analysis. • Results showed highest carbohydrate concentration from the control (8.8µg/µL) with an average absorbance of 0.09AU. • Lower carbohydrate concentrations were obtained from the different treatments however, analysis of variance (ANOVA) showed that the difference was not statistically significant (p>0.05). Cultures treated with 6, 8, and 10 µM methylmercury showed carbohydrate concentrations of 6.5, 7.3, and 6.7 µg/µL, respectively with an average absorbance of ~0.07 AU. • Cultures treated with 6, 8, and 10 mM methylmercury produced 6.6, 6.9 and 6.7 µg/µL of carbohydrate, respectively with an average absorbance of ~0.07AU. Comparison between treatments was also analyzed using Tukey's test which showed no significant difference (p>0.05). Overall, results indicated no significant difference between the control and the treatments. • The absorbance values after re-suspension of the control and the sediments treated with 6, 8, and 10 µM, and 6, 8 and 10 mM did not vary significantly (p>0.05). Figure 1: Effect of methyl mercury on carbohydrate concentration and mean absorbance values of sediments with varying methyl mercury.