Microbial mats in the mangrove forests of Sandspit coast area, south of Karachi, Pakistan were studied between Jan 2012-Jan 2014. Six major filamentous cyanobacteria belonging to three genera Oscillatoria (3), Spirulina (2) and Phormidium (1) were identified in the backwaters mat samples. One of the cyanobacterium species Spirulina labyrinthiformis is reported from this site for the first time in Pakistan. The most dominant species was Oscillatoria brevis (22% abundance), closely followed by Phormidium tenue (21%). These filamentous forms were present in all seasons and tolerated varying physico-chemical ranges such as temperature (15-28ºC), pH (6.8-7.5), salinity (36-42‰) and dissolved oxygen (0.201-0.543ppm). Chlorophyll a levels in mat area sediments were ranged between 0.039 up to 5.050 mg/g. The minimum and maximum biovolume was 0.174 and 1.649mm3/l respectively. We observed a strong positive correlation (p<0.05) between observed filamentous form of cyanobacteria and field parameters such as water-soil-air temperatures, pH and dissolved oxygen. The outcome suggests a potential for detailed molecular microbial mat study. Further studies are required to understand the interactions of microbial mat with soil and water components.

Key words: Mangrove, Cyanobacteria, Microbial mat, Taxonomic assessment.