Beyond the target species - Implications of microalgal succession and associated microbiome in industrial-scale photobioreactors

Abstract

In the present study, four continuous industrial-scale microalgae reactors were evaluated regarding microalgal and microbial species composition and characteristics of the biomass produced. The results showed that complex patterns of species succession occurred in all reactors, being influenced by culture medium (synthetic medium or domestic wastewater), contamination, environmental variables and surface to volume ratio. Over time, the inoculated target species, Scenedesmus almeriensis, was gradually replaced by other green algae in raceway type reactors (RW) and by filamentous cyanobacteria in the thin layer cascade reactor (TL). The highest dry biomass was recorded in the TL, reaching 2.6 g L−1, and did not exceed 0.9 g L−1 in all RW. In general, the carbon, nitrogen, protein and lipid contents in the biomass were higher in outdoor RW, followed by TL and then by indoor RW. Carbohydrates and carotenoids did not show significant differences between the reactors and chlorophyll values followed the same pattern as dry mass. The microbiome which was analyzed only in the outdoor RW reactors sensitively reflected environmental and chemical variations, presenting a diverse composition and relevant quantitative importance in the biomass. Despite this, the chemical and biochemical characteristics of the biomass were similar to those obtained in typical algal cultures of higher purity, proving to be suitable for various uses such as biofertilizers or biofuels. The results reinforce that industrial microalgal cultures are microbial consortia and knowing their composition and succession as well as the possible manipulation of their composition, are strategic challenges for the technical and economic viability of industrial production of microalgae.