Quantitative analysis of the effects of morphological changes on extracellular electron transfer rates in cyanobacteria

Abstract

Background: Understanding the extracellular electron transport pathways in cyanobacteria is a major factor towards developing biophotovoltaics. Stressing cyanobacteria cells environmentally and then probing changes in physiology or metabolism following a significant change in electron transfer rates is a common approach for investigating the electron path from cell to electrode. However, such studies have not explored how the cells' concurrent morphological adaptations to the applied stresses affect electron transfer rates. In this paper, we establish a ratio to quantify this effect in mediated systems and apply it to Synechococcus elongatus sp. PCC7942 cells grown under different nutritional regimes. Results: The results provide evidence that wider and longer cells with larger surface areas have faster mediated electron transfer rates. For rod-shaped cells, increase in cell area as a result of cell elongation more than compensates for the associated decline in mass transfer coefficients, resulting in faster electron transfer. In addition, the results demonstrate that the extent to which morphological adaptations account for the changes in electron transfer rates changes over the bacterial growth cycle, such that investigations probing physiological and metabolic changes are meaningful only at certain time periods. Conclusion: A simple ratio for quantitatively evaluating the effects of cell morphology adaptations on electron transfer rates has been defined. Furthermore, the study points to engineering cell shape, either via environmental conditioning or genetic engineering, as a potential strategy for improving the performance of biophotovoltaic devices.

Green notes: The rhythms of cyanobacteria exoelectrogenesis as de-composed by the Hilbert-Huang transform

“…Fe (-)|20% CO 2 cells, were found to be elongated by up to 1.6 µm relative to Fe (-)|Air cells (Fig. S5) and by ≈0.5-0.8 µm relative to cells grown in standard conditions (reported in previous work) 16 . In S. elongatus, this phenotype indicates elevated levels of RpaA-P, which implies a reduction/interruption in normal CikA activity 19,22 .…”

“…This is particularly true for the Fe (+)|Air condition, where redox active ferric ions are assimilated by the cells. We previously estimated that the concentration of ferric ions in fresh BG11 medium reduces by half within four days of culturing and reduces to almost zero within 11 days under similar growth conditions 16 . Similar rates of ferric ion depletion rates in S. elongatus cultures have been independently reported from atomic absorption spectroscopy concentration measurements 17 .…”

“…(ii) the erratic increase in current density is due to biofilms in exponential phase, where there is rapid growth in electrochemically active biomass in the biofilm; and (iii) the stable currents are achieved when the biofilm reaches stationary phase where concentration of active biomass in the biofilm remains constant, and gradual changes in cellular morphology contribute to changes in the mean current level by influencing electron export rates 16 . Patterns previously reported when operating under a diel (24h) light-dark period were observed, namely a dip and recovery in current density at dawn (inset, Fig.…”

A biophotoelectrochemical approach to unravelling the role of cyanobacterial cell structures in exoelectrogenesis

“…A possible explanation is the different shapes of the cells, as Synechocystis and Synechococcus are spherical and rod-shaped, respectively, and cell morphology has been predicted to have a role in exoelectrogenesis. 42 But the exact reason for the difference in their photocurrent profiles remains to be determined and will be the subject of future studies examining other model organisms. 43 We observed that spheroplasts, without an integrous periplasmic space, 37 yielded a monophasic photocurrent profile distinct from the complex profile of whole cells.…”

Time series analysis and long short-term memory (LSTM) network prediction of BPV current density

“…A stock culture of the cyanobacterium Synechococcus elongatus sp. PCC7942 (Pasteur Culture Collection), PCC7942 henceforth, was grown in liquid Blue-green medium (BG11) and cell concentration (cells ml −1 ) determined as previously described in previous work 23 . BPV devices were constructed as a galvanic electrochemical cell with a membrane electrode assembly (MEA).…”