Decoupling a novel Trichormus variabilis-Synechocystis sp. interaction to boost phycoremediation

Abstract

To conserve freshwater resources, domestic and industrial wastewater is recycled. Algal systems have emerged as an efficient, low-cost option for treatment (phycoremediation) of nutrient-rich wastewater and environmental protection. However, industrial wastewater may contain growth inhibitory compounds precluding algal use in phycoremediation. Therefore, extremophyte strains, which thrive in hostile environments, are sought-after. Here, we isolated such an alga - a strain of Synechocystis sp. we found to be capable of switching from commensal exploitation of the nitrogen-fixing Trichormus variabilis, for survival in nitrogen-deficient environments, to free-living growth in nitrate abundance. In nitrogen depletion, the cells are tethered to polysaccharide capsules of T. variabilis using nanotubular structures, presumably for nitrate acquisition. The composite culture failed to establish in industrial/domestic waste effluent. However, gradual exposure to increasing wastewater strength over time untethered Synechocystis cells and killed off T. variabilis. This switched the culture to a stress-acclimated monoculture of Synechocystis sp., which rapidly grew and flourished in wastewater, with ammonium and phosphate removal efficiencies of 99.4% and 97.5%, respectively. Therefore, this strain of Synechocystis sp. shows great promise for use in phycoremediation, with potential to rapidly generate biomass that can find use as a green feedstock for valuable bio-products in industrial applications.

Figure 1

Acclimation of algal culture for growth in wastewater. Flasks with BG-11 medium or wastewater were mock-inoculated or inoculated with algal cells. Samples were photographed after a week of growth. Plus sign (+) denotes addition of the indicated inoculum; minus sign (−) denotes mock-inoculation.

Figure 2

Microscopic images of cells from acclimated and non-acclimated algal cultures. (a) Filamentous cells not exposed to wastewater. Heterocysts and akinetes are indicated by red and blue arrows, respectively. (b) Vegetative cells in a filament from BG-11 cultures. (c) Higher magnification of vegetative cells in a filament. (d) Single cells in algal cultures acclimated for growth in wastewater. (e) Higher magnification of a single cell from the wastewater-acclimated culture. (f) Wastewater-acclimated single cell captured in the process of cell division.

Figure 3

Interaction between cells of T. variabilis and Synechocystis sp. The micrographs show different sections with large T. variabilis and/or small Synechocystis sp. cells. (a,b) Show the same section, except that artificial colouring has been introduced in b to clarify different structural components. (c) Shows another section while (d) shows a magnified cell of Synechocystis sp. Nanotubular structures grow out of Synechocystis sp. cells and disappear into the polysaccharide capsule of T. variabilis cells. T. var, T. variabilis; Syn, Synechocystis sp.; PC, polysaccharide capsule. Arrows indicate the nanotubular structures. The samples analysed by TEM were harvested from exponential growth phase cultures grown in the N-deficient BG-11₀ medium.

Figure 4

External source of nitrate is required for viability and growth of Synechocystis sp. (a) Growth curves of Synechocystis sp. in N-deficient BG-11₀ and N-replete BG-11 growth media. T. variabilis growing in N-deficient BG-11₀ medium was included as a positive control. (b) Gradual cell death of Synechocystis sp. grown in BG-11 after transfer to BG-11₀. (c) T. variabilis culture grown in BG-11 and transferred to BG-11₀ medium continues to grow.