Physiological and Proteomic Responses of Continuous Cultures of Microcystis aeruginosa PCC 7806 to Changes in Iron Bioavailability and Growth Rate

Abstract

The hepatotoxin microcystin (MCYST) is produced by a variety of freshwater cyanobacterial species, including Microcystis aeruginosa. Interestingly, MCYST-producing M. aeruginosa strains have been shown to outcompete their nontoxic counterparts under iron-limiting conditions. However, the reasons for this are unclear. Here we examined the proteomic response of M. aeruginosa PCC 7806 continuous cultures under different iron and growth regimes. Iron limitation was correlated with a global reduction in levels of proteins associated with energy metabolism and photosynthesis. These proteomic changes were consistent with physiological observations, including reduced chlorophyll a content and reduced cell size. While levels of MCYST biosynthesis proteins did not fluctuate during the study period, both intra-and extracellular toxin quotas were significantly higher under iron-limiting conditions. Our results support the hypothesis that intracellular MCYST plays a role in protecting the cell against oxidative stress. Further, we propose that extracellular MCYST may act as a signaling molecule, stimulating MCYST production under conditions of iron limitation and enhancing the fitness of bloom populations. IMPORTANCEMicrocystin production in water supply reservoirs is a global public health problem. Understanding the ecophysiology of hepatotoxic cyanobacteria, including their responses to the presence of key micronutrient metals such as iron, is central to managing harmful blooms. To our knowledge, this was the first study to examine proteomic and physiological changes occurring in M. aeruginosa continuous cultures under conditions of iron limitation at different growth rates. C yanobacteria ("blue-green algae") proliferate in warm stratified water bodies rich in nitrogen and phosphorus (1, 2). Cyanobacterial blooms can have a negative impact on the appearance, taste, and odor of water, and their subsequent decay can lead to oxygen depletion and fish kills (3). However, of greatest concern to public health is their production of potent toxins. Microcystis aeruginosa (Kützing) Lemmermann is a common bloom-forming cyanobacterial species that produces hepatotoxic microcystins (MCYSTs) (1). These nonribosomally synthesized peptides inhibit eukaryotic protein phosphatases, leading to liver necrosis in acute doses and hepatocellular carcinoma in chronic low doses (4, 5).There is strong evidence that the production of MCYST is a direct function of cell division (6). It is therefore not surprising that parameters affecting cyanobacterial growth rate (e.g., nutrients, trace metals, temperature, pH, and light) have been correlated with fluctuating MCYST levels in batch culture experiments (7-9).Recent molecular studies suggest that iron (Fe) and nitrogen (N) also play a role in regulating the expression of MCYST biosynthesis (mcy) genes. In M. aeruginosa, for example, the mcy promoter region contains binding sites for the ferric uptake regulator (Fur) and the global nitrogen regulator (NtcA) (10, 11). Despite these genetic clu...

Still challenging: the ecological function of the cyanobacterial toxin microcystin – What we know so far

“…Under oxidative stress, more MCs are released to the surrounding environment and therefore might play a role as a signal molecule to synchronize the cell responses to the stress-inducing agents. Increased intracellular MC concentration in response to oxidative stress conditions such as iron limitation support the hypothesis of MCs involvement in protection against oxidative stress, while the enhancement in extracellular MCs suggests the possible role of MCs as a signal molecule to make a better environmental adaptation (Yeung et al 2016).…”

“…A recent proteomic study using continuous cultures of M. aeruginosa PCC 7806 showed that under iron depletion physiological changes such as reduction in chlorophyll a content besides enhancement in MC production. Increased intracellular and extracellular MCs in continuous culture of M. aeruginosa PCC 7806 under iron depletion provided more evidence of the assistance role of MCs in the better fitness of Microcystis under oxidative stress conditions (Yeung et al 2016). In addition, mcy gene transcripts were enhanced under oxidative stress in various quantities i.e.…”

“…• Increased extracellular MCs (Yeung et al 2016) Similarity between the sequence of the mcy gene cluster…”

Daphnia magna Exudates Impact Physiological and Metabolic Changes in Microcystis aeruginosa

“…Recent research raises the possibility that microcystins may play a role in enhancing the ability of surviving oxidative stress [1,31]. The increased production of intracellular MC under conditions of stress [32] allowed MC-producing strains to cope better with oxidative stress over non-toxic strain [21,30,33]. However, other findings show opposite results: The nontoxic M. aeruginosa PCC7806 mutant was favored over its toxic wild type when exposed to prolonged oxidative stress caused by hydrogen peroxide [34].…”

Relationship between Photosynthetic Capacity and Microcystin Production in Toxic Microcystis Aeruginosa under Different Iron Regimes

“…Consequently, it is deduced that the majority of MC–LR was kept inside the cells during our experiment and little MC–LR was released into external environment. Several studies have also reported that maximum MC concentrations were not recorded until the end of the growth cycle or during bloom collapse [ 73 ]. Therefore, iron addition could actually increase the intracellular toxin production but may not raise the extracellular MC–LR concentration during the blooms.…”