Answer:
Algae is a group of a unicellular or multicellular organism which can produce the food on their own through a process called photosynthesis. and are thus called autotrophs.
The bacteria are prokaryotic organism which obtain nutrition wither by producing food on their own like photosynthetic bacteria or they derive nutrition from another organism called heterotrophic bacteria.
Since in bacteria domain, both the autotrophic and heterotrophic species are known in which the heterotrophic bacteria can cause the disease as they have the adaptive features to derive nutrition from the other organism while the algae species are only autotrophic in nature so they do not cause disease.