In mutualistic systems, the ability to associate with diverse symbionts of distinct physiological traits often facilitates broadening of the niche. In lichen symbiosis, this process remains understudied. While such flexibility has been demonstrated for some species, others associate only with symbionts of comparable characteristics. We investigated whether the broad niche of Hydropunctaria—a genus of seashore lichens inhabiting the supralittoral zone across a salinity gradient—is linked to algal symbiont turnover in response to the changing salinity. Using Sanger sequencing of lichen symbionts and Illumina metabarcoding of free-living algae, we assessed symbiont diversity, selectivity, and the presence of algal symbionts in the environment. Despite the presence of multiple potential algal partners in the surrounding environment, Hydropunctaria exhibited a highly specific and stable association with a single algal symbiont across a wide salinity range. This suggests that the ecological niche breadth in this symbiotic system is driven not by the ability to change partners in response to changing salinity, but rather by the association between generalist, euryhaline symbionts. Our findings also point to selective mechanisms beyond partner availability alone.