Understanding the factors which affect algal growth, and their biochemical composition, enables future scientists and engineers to efficiently integrate algal bioreactors into industrial waste streams, and maximize the usable products that can be created by algae. This project aimed to evaluate the differences in production and cellular respiration of mixotrophically grown treatments of the algae strain Monoraphidium KAC90 using cheese whey waste as a carbon source to autotrophically grown treatments  of the algae strain Monoraphidium KAC90 . Treatments were grown semi-continuously over 16 days. In vivo reduction of tetrazolium salt 2-para (iodophenyl)-3(nitrophenyl)-5(phenyl) tetrazolium chloride (INT), was used to measure the cellular respiration rates of samples, and this project marks the first application of this method on high-density microalgae in an open pond system in late autumn (SE Sweden). The autotrophic treatments experienced high rates of cellular respiration, low biomass yields, and static(non-increasing) production, resembling microalgae struggling to grow. Biochemical analysis showed that autotrophic treatments were prioritizing to energy storage and cellular integrity, supporting that the autotrophic treatments were under environmental stress. Meanwhile, the mixotrophic treatments had high production and exponential biomass growth in conjunction with very low cellular respiration rates, indicating reduced biomass loss. Mixotrophically grown Monoraphidium KAC90 showed to be an effective growth method under stressful environmental conditions, but due to high kill-control values and high standard deviations, the application of in vivo INT reduction on high density algal cultures such as the mixotrophic treatments should be further investigated.