(eng): An observer for the reconstruction of the state of the phenomenological two timescale model of the microalgae growth, so-called photosynthetic factory model formally described as a bilinear system, is designed. Three states of the model form the probability vector and while one of the states is measurable in real time the second is available only in integrated quantities. The observer designed here uses both information channels to increase precision of the reconstruction. The results are demonstrated on simulations