Since the culture contains an autotroph with a decomposer as well as remineralizer, the best bet would be that the photosynthetic as well as fermentative byproducts are observed here as bubbles. A simple estimation of carbon di-oxide or H2S will corroborate the fact.
Firstly, since you have a mixotroph in the medium, it is very possible that in the absence of constant shaking, you are observing gas evolution due to photosynthetic processes. Also, I am curious as to how well your cells performed photosynthetically. Did you measure the photosynthetic activity by any means?