الفهرس 
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Abstract
Lichens are promising sources for many valuable metabolites such as antibiotic, anticancer and antioxidant activities. However, their extremely slow growth rate poses a great obstacle in their way of industrial application. In this thesis, lichen samples were collected from the Egyptian habitat and were identified as Xanthoria parietina upon examination of their external morphology and performance of spot tests that confirmed the lichen’s identification. Afterwards, the lichen associated microorganisms were isolates which resulted in the isolation of 7 different microorganisms: 1 alga, 3 fungi and 3 bacteria. The lichen associated bacteria were identified as Bacillus licheniformis, Bacillus aryabhattai and Bacillus altitudinis by means of 16s RNA sequencing and screened for different characteristics, bacteria were screened for their ability to withstand abiotic stress, B. aryabhattai was the best isolate to tolerate drought stress while B. licheniformis was the best to tolerate salinity stress. All bacteria were able to produce IAA, GA-3 and ammonia where B. licheniformis was the best to produce both hormones while B. altitudinis produced highest ammonia content. The isolated alga was identified morphologically as Trebouxia sp. and its biochemical composition was studied when grown on BBM medium supplemented with different nitrate concentrations. Highest protein content was obtained when the alga was grown on 5N BBM while highest carbohydrates content was obtained when the alga was grown on 9N BBM and lipids content was very low in all treatments. In an attempt to study the interaction between the alga and the associated bacteria, the alga was co-cultured with each bacterial isolate individually and combined in ratio of 1:1 and 1:1:1 respectively. Dry biomass of the alga increased significantly in each treatment while the highest protein content was obtained when alga was co-cultured with B. altitudinis. For protein content optimization, the alga was co-cultured with B. altitudinis in different volumes on BBM medium supplemented with different nitrate concentrations as suggested by CCD design in Minitab, the highest protein content was obtained when the alga was co-cultured with 200 ml B. altitudinis on 5N BBM medium. This implies that lichen associated bacteria do play a valuable role in the lichen symbiosis.