Interaction between CO2-consuming autotrophy and CO2-producing heterotrophy in non-axenic phototrophic biofilms

Abstract

As the effects of climate change become increasingly evident, the need for effective CO2 management is clear. Microalgae are well-suited for CO2 sequestration, given their ability to rapidly uptake and fix CO2. They also readily assimilate inorganic nutrients and produce a biomass with inherent commercial value, leading to a paradigm in which CO2-sequestration, enhanced wastewater treatment, and biomass generation could be effectively combined. Natural non-axenic phototrophic cultures comprising both autotrophic and heterotrophic fractions are particularly attractive in this endeavour, given their increased robustness and innate O2-CO2 exchange. In this study, the interplay between CO2-consuming autotrophy and CO2-producing heterotrophy in a non-axenic phototrophic biofilm was examined. When the biofilm was cultivated under autotrophic conditions (i.e. no organic carbon), it grew autotrophically and exhibited CO2 uptake. After amending its growth medium with organic carbon (0.25 g/L glucose and 0.28 g/L sodium acetate), the biofilm rapidly toggled from net-autotrophic to net-heterotrophic growth, reaching a CO2 production rate of 60 μmol/h after 31 hours. When the organic carbon sources were provided at a lower concentration (0.125 g/L glucose and 0.14 g/L sodium acetate), the biofilm exhibited distinct, longitudinally discrete regions of heterotrophic and autotrophic metabolism in the proximal and distal halves of the biofilm respectively, within 4 hours of carbon amendment. Interestingly, this upstream and downstream partitioning of heterotrophic and autotrophic metabolism appeared to be reversible, as the position of these regions began to flip once the direction of medium flow (and hence nutrient availability) was reversed. The insight generated here can inform new and important research questions and contribute to efforts aimed at scaling and industrializing algal growth systems, where the ability to understand, predict, and optimize biofilm growth and activity is critical.

Investigation and Optimization of CO2 Uptake

“…Ronan P., Kroukamp O., Liss S.N., Wolfaardt G. (2021). Interaction between CO2 consuming autotrophy and CO2-producing heterotrophy in non-axenic phototrophic biofilms.…”

“…Ronan P., Kroukamp O., Liss S.N., Wolfaardt G. (2021). Investigating the impact of acute nitrogen starvation on CO2 uptake in phototrophic biofilms.…”

“…Diversity in phototrophic biofilms promotes biomass productivity and enhances robustness compared to monocultures (Gonçalves et al, 2017;Mattsson et al, 2021;Kazamia et al, 2014), enabling these biofilms to persist within a wider range of environmental conditions while being less susceptible to collapse caused by contamination (Sharp et al, 2017). In microalgal cultivation, therefore, it will be beneficial to explore an approach focussed on creating and managing an environment to select for desired species interactions and whole-biofilm functions (Kazamia et al, 2014;Ronan et al, 2021). Interestingly, Sharp et al (2017) showed that when two different lakederived phototrophic cultures with very different initial community compositions were both cultivated in a biofilm photobioreactor under identical conditions, the communities became remarkably similar based on 16S rRNA analysis.…”

“…The phototrophic culture used in this study was derived from a wastewater lagoon in Dundalk, Ontario, Canada. The culture was previously found to comprise both a photosynthetic and non-photosynthetic fraction (Ronan et al, 2021), which is common in naturally occurring phototrophic cultures (Roeselers et al, 2008). A modified Bold's Basal Medium (M-BBM) was used as growth medium for the phototrophic culture and contained 0.22 g/L (NH4)2SO4, 0.025 g/L NaCl, 0.025 g/L CaCl2 • 2H2O, 0.075 g/L MgSO4 • 7H2O, 0.175 g/L KH2PO4, 0.075 g/L K2HPO4, 8.34 mg/L FeSO4.…”

Phototrophic Biofilms Transform Soil-Dissolved Organic Matter Similarly Despite Compositional and Environmental Differences

“…Despite high species diversity, complex intra- and inter-species interactions, and diverse metabolic pathways, phototrophic biofilms tend to have high functional similarity. ,, Photoautotrophs fix CO 2 and convert it into carbohydrates, proteins, and lipids via photosynthesis, while heterotrophs can produce extracellular enzymes such as hydrolases and dehydrogenases to decompose macromolecules into smaller compounds. , Phototrophic biofilms can therefore similarly transform DOM even if grown on different soils with different community compositions because of their high similarity in EPS production, carbon metabolism, and chemical composition of biomass. , However, the coordinated effects of growth and decomposition of phototrophic biofilms on soil DOM transformation remain largely unknown.…”