Impact of cocultivation on the aggregation and sedimentation trends of cyanobacteria with native and modified clay minerals

Abstract

• Diazotrophic cyanobacteria were cocultivated with clays and clay composites. • Ca-Montmorillonite and kaolinite slightly hindered cyanobacteria growth. • Incubation with chitosan and iron oxide coated clay caused cell death. • Humic acid coated clay delayed sedimentation of cyanobacteria. • Chitosan- and iron-oxide clays were bactericidal and efficient flocculants. Removal and separation of cyanobacteria is of interest from both environmental and industrial standpoints. Flocculation and sedimentation using clay minerals can be advantageous, however, long-term effects such as resuspension of live bacteria or membrane damage, can be detrimental to these processes. In this respect, the effect of cocultivation of clays with cyanobacteria (CB) on aggregation trends and viability and consequently on separation processes is still largely unknown. In this study, Anabaena PCC 7120 was cultivated in the presence of relevant types of clays and clay composites, to observe their impact on growth, activity, and clay-CB complexation over time. Growth and activity of CB were monitored by looking at chlorophyll content and ammonia production. Clay-CB aggregation trends, extent, and morphology were assessed by measuring the decline of optical density over time, monitoring changes in zeta potential, and characterization using light and electron microscopy. The results revealed that CB grown in the presence of native Ca-montmorillonite and kaolinite suffer only a minor hindrance in growth rate and activity but undergo rapid sedimentation. Humic acid coated clay only slightly hindered growth yet inhibited sedimentation significantly. In contrast, chitosan and iron oxide coating caused damage to the cell membrane and CB death after several days. In terms of sedimentation, chitosan coated clays produced large flocs (up to 400 µm) and achieved high sedimentation rates of 46% after 3 min. Iron-oxide coated clays created the largest apparent aggregates (∼600 µm floc size) and exhibited the fastest sedimentation rates, 73% after 3 min and 87% after 30 min. The results stress the importance of looking at cocultivation with the different particles, since viability is not a priori hindered by aggregation, as in the case of native and humic-acid coated clays. On the other hand, chitosan and iron coated clays were both bactericidal and excellent flocculants. Thus, these findings can advance and improve the design of CB separation and removal strategies in natural and engineered systems.