Competition between cyanobacteria and green algae at low versus elevated CO2: who will win, and why?

Xing Ji,Maayke Stomp,Jolanda M H Verspagen,Jef Huisman

doi:10.1093/jxb/erx027

Abstract

Traditionally, it has often been hypothesized that cyanobacteria are superior competitors at low CO2 and high pH in comparison with eukaryotic algae, owing to their effective CO2-concentrating mechanism (CCM). However, recent work indicates that green algae can also have a sophisticated CCM tuned to low CO2 levels. Conversely, cyanobacteria with the high-flux bicarbonate uptake system BicA appear well adapted to high inorganic carbon concentrations. To investigate these ideas we studied competition between three species of green algae and a bicA strain of the harmful cyanobacterium Microcystis aeruginosa at low (100 ppm) and high (2000 ppm) CO2. Two of the green algae were competitively superior to the cyanobacterium at low CO2, whereas the cyanobacterium increased its competitive ability with respect to the green algae at high CO2. The experiments were supported by a resource competition model linking the population dynamics of the phytoplankton species with dynamic changes in carbon speciation, pH and light. Our results show (i) that competition between phytoplankton species at different CO2 levels can be predicted from species traits in monoculture, (ii) that green algae can be strong competitors under CO2-depleted conditions, and (iii) that bloom-forming cyanobacteria with high-flux bicarbonate uptake systems will benefit from elevated CO2 concentrations.

Highlights

Cyanobacterial blooms have become a major water quality problem in many eutrophic lakes worldwide (Chorus and Bartram, 1999; Verspagen et al, 2006; Guo, 2007; Michalak et al, 2013)
Our results show (i) that competition between phytoplankton species at different CO2 levels can be predicted from species traits in monoculture, (ii) that green algae can be strong competitors under CO2-depleted conditions, and (iii) that bloom-forming cyanobacteria with high-flux bicarbonate uptake systems will benefit from elevated CO2 concentrations
Our results indicate that the C. vulgaris strain we used can utilize bicarbonate: if this green alga could only use CO2, it would not be able to grow in monoculture in our chemostats at a dilution rate of 0.125 d–1 and dissolved CO2 concentrations lower than 0.01 μmol l–1 (Fig. 1)

Summary

Introduction

Cyanobacterial blooms have become a major water quality problem in many eutrophic lakes worldwide (Chorus and Bartram, 1999; Verspagen et al, 2006; Guo, 2007; Michalak et al, 2013). Dense cyanobacterial blooms often deplete the dissolved CO2 concentration in surface waters, sometimes down to less than 0.1 μmol l–1 corresponding to pCO2 less than 3 parts per million (ppm) (Lazzarino et al, 2009; Balmer and Downing, 2011). The gas was dispersed as fine bubbles supplied from the bottom of the chemostat vessels at a constant flow rate of a=25 l h–1, which ensured homogeneous mixing of the phytoplankton populations. The light intensity transmitted through the chemostat (Iout) was measured at the back surface of the chemostat vessel (Huisman et al, 2002)

Methods

Results

Discussion

Conclusion