Qualitative Assessment of Microalgae–Bacteria Biofilm Development on K5 Carriers: Photoheterotrophic Growth in Wastewater

Wastewater (WW) treatment using biofilms harboring bacteria and microalgae is considered a promising polishing solution to improve current treatment technologies present in wastewater treatment plants (WWTPs), but their interaction in a sessile community remains to be understood. In this work, multi-species biofilms of <i>Chlorella vulgaris</i>, <i>Chlorella sorokiniana</i>, or <i>Scenedesmus obliquus</i> were selected as representative microalgae species of interest for WW bioremediation, and <i>Rhodococcus fascians</i>, <i>Acinetobacter calcoaceticus</i>, or <i>Leucobacter</i> sp. were selected as the bacteria for co-cultivation in a synthetic WW since they are normally found in WW treatment processes. The attached consortia were developed in specific carriers (K5 carriers) for 168 h, and their biofilm formation ability was evaluated in a profilometer and via scanning electron microscopy (SEM) imaging. From the selected microorganisms, <i>C. sorokiniana</i> was the microalga that adapted best to co-cultivation with <i>R. fascians</i> and <i>A. calcoaceticus</i>, developing a thicker biofilm in these two consortia (3.44 ± 0.5 and 4.51 ± 0.8 µm, respectively) in comparison to the respective axenic cultures (2.55 ± 0.7 µm). In contrast, <i>Leucobacter</i> sp. did not promote biofilm growth in association with <i>C. vulgaris</i> and <i>C. sorokiniana</i>, while <i>S. obliquus</i> was not disturbed by the presence of this bacterium. Some bacterial clusters were observed through SEM, especially in <i>A. calcoaceticus</i> cultures in the presence of microalgae. In some combinations (especially when <i>C. vulgaris</i> was co-cultivated with bacteria), the presence of bacteria was able to increase the number of microalga cells adhered to the K5 carrier. This study shows that biofilm development was distinctly dependent on the co-cultivated species, where synergy in biofilm formation was highly dependent on the microalgae and bacteria species. Moreover, profilometry appears to be a promising method for biofilm analyses.