Strains of the Harmful Cyanobacterium Microcystis aeruginosa Differ in Gene Expression and Activity of Inorganic Carbon Uptake Systems at Elevated CO ₂ Levels

Abstract

Cyanobacteria are generally assumed to be effective competitors at low CO 2 levels because of their efficient CO 2 -concentrating mechanism (CCM), and yet how bloom-forming cyanobacteria respond to rising CO 2 concentrations is less clear. Here, we investigate changes in CCM gene expression at ambient CO 2 (400 ppm) and elevated CO 2 (1,100 ppm) in six strains of the harmful cyanobacterium Microcystis. All strains downregulated cmpA encoding the high-affinity bicarbonate uptake system BCT1, whereas both the low-and high-affinity CO 2 uptake genes were expressed constitutively. Four strains downregulated the bicarbonate uptake genes bicA and/or sbtA, whereas two strains showed constitutive expression of the bicA-sbtA operon. In one of the latter strains, a transposon insert in bicA caused low bicA and sbtA transcript levels, which made this strain solely dependent on BCT1 for bicarbonate uptake. Activity measurements of the inorganic carbon (C i ) uptake systems confirmed the CCM gene expression results. Interestingly, genes encoding the RuBisCO enzyme, structural carboxysome components, and carbonic anhydrases were not regulated. Hence, Microcystis mainly regulates the initial uptake of inorganic carbon, which might be an effective strategy for a species experiencing strongly fluctuating C i concentrations. Our results show that CCM gene regulation of Microcystis varies among strains. The observed genetic and phenotypic variation in CCM responses may offer an important template for natural selection, leading to major changes in the genetic composition of harmful cyanobacterial blooms at elevated CO 2 .C O 2 concentrations in the atmosphere may double during this century (1). In marine ecosystems, enhanced dissolution of atmospheric CO 2 causes ocean acidification (2, 3). In freshwaters, however, CO 2 concentrations may vary widely. In many lakes, the dissolved CO 2 concentration exceeds the concentration expected from equilibrium with the atmosphere due to the input of large amounts of organic carbon from terrestrial systems (4, 5). Conversely, the photosynthetic activity of dense phytoplankton blooms can deplete the CO 2 concentration far below atmospheric levels, which increases pH and makes bicarbonate the most abundant inorganic carbon species (6-8). Cyanobacteria are often considered to be very successful competitors at low CO 2 levels (9, 10), and global warming is predicted to favor an expansion of cyanobacterial blooms in eutrophic waters (11-13). However, the response of bloom-forming cyanobacteria to elevated CO 2 levels is not yet well understood.Cyanobacteria typically use a CO 2 -concentrating mechanism (CCM) with up to five different uptake systems for inorganic carbon (C i ): three for bicarbonate and two for CO 2 uptake (14). The two sodium-dependent bicarbonate uptake systems, BicA and SbtA, are present in some but not all freshwater cyanobacteria (15-17). BicA combines a low affinity for bicarbonate with a high flux rate, whereas SbtA usually has a high affinity for bicarbonate but a low flux r...

Rapid adaptation of harmful cyanobacteria to rising CO 2

“…In all Microcystis strains investigated so far, both CO 2 uptake systems were present (23) and the genes encoding these two CO 2 uptake systems were constitutively expressed (31)(32)(33). A possible explanation might be that the sustained activity of both intracellular CO 2 uptake systems helps to maintain a low CO 2 (aq) concentration in the cytoplasm, thus maximizing the diffusive influx of CO 2 .…”

“…3D). Laboratory studies have shown that the high-affinity bicarbonate uptake system BCT ceases its activity, whereas the BicA enzyme remains active at elevated CO 2 levels (31,32), and that bicarbonate still accounts for most of the C i uptake even at elevated CO 2 levels (34-36). Hence, in alkaline waters, adaptation of the bicarbonate uptake systems to changes in C i availability is indeed likely to have major fitness consequences.…”

Expression of holo-proteorhodopsin in Synechocystis sp. PCC 6803

“…Growth was monitored via cell density by measuring the OD 750 of a small volume (150 μl) sample from each culture in a Multiskan FC Microplate Photometer (Thermo Scientific, Finland). For selected experiments, instead of OD 750 , the number of cells per ml and average cell size was analyzed in triplicate with a Casy 1 Model TTC cell counter (Schärfe System GmbH, Reutlingen, Germany) with a 60 μm capillary (52).…”

Strong effects of elevated CO2 on freshwater microalgae and ecosystem chemistry

“…Microalgal growth response included stimulations to bloom‐forming species when consumers were excluded (Fig. S6), with observed increases in biomass, cell abundance, population densities, and chlorophyll a in bloom‐forming Microcystis aeruginosa (Qiu and Gao ; Sandrini et al , b ), and increases in cell division rate, biovolume, growth rate, and chlorophyll a in bloom‐forming Cylindrospermopsis raciborskii under elevated CO 2 conditions (Wu et al ; Pierangelini et al ). Elevated CO 2 may thus worsen the impacts of algal blooms, although it is difficult to extend the results from the lab to the field.…”