Select all that apply.
cyanobacteria
serve as a food source for other organisms
are used for medicinal purposes
produce oxygen
perform nitrogen fixation
cause disease

Cyanobacteria performs all the function that given in the Question.

Explanation:

Cyanobacteria serve as a food for other organism, They are photosynthetic bacteria. Cyanobacteria and plants are capable of using light energy which trapped by the chlorophyll to convert carbon dioxide and water into oxygen and its food as a glucose, this process is called photosynthesis.

Cyanobacteria are used for medical purpose because they have rich source of biologically active compound with antibacterial, antifungal, antiviral activities.

Cyanobacteria produce oxygen because they oxygenic photosynthetic  bacteria. They trap sun's energy absorbs carbon dioxide and emit oxygen.

Cyanobacteria performs nitrogen fixation because they contain additional fixed nitrogen which absorbs by the plants from their roots. Nitrification cannot occurs in the presence of oxygen. So, nitrogen is fixed iin the specialised cells called heterocysts.

i believe it would be the lungs.