Cyanobacterial blooms, iron, and environmental pollutants

Abstract

Iron determines the abundance and diversity of life and controls primary production in numerous aqueous environments. Over the past decades, the availability of this metal in natural waters has decreased. Iron deficiency can apply a selective pressure on microbial aquatic communities. Each aquatic organism has their individual requirements for iron and pathways for metal acquisition, despite all having access to the common pool of iron. Cyanobacteria, a photosynthesizing bacterium that can accumulate and form so-called 'algal blooms', have evolved strategies to thrive in such iron-deficient aqueous environments where they can outcompete other organisms in iron acquisition in diverse microbial communities. Metabolic pathways for iron acquisition employed by cyanobacteria allow it to compete successfully for this essential nutrient. By competing more effectively for requisite iron, cyanobacteria can displace other species and grow to dominate the microbial population in a bloom. Aquatic resources are damaged by a diverse number of environmental pollutants that can further decrease metal availability and result in a functional deficiency of available iron. Pollutants can also increase iron demand. A pollutant-exposed microbe is compelled to acquire further metal critical to its survival. Even in pollutant-impacted waters, cyanobacteria enjoy a competitive advantage and cyanobacterial dominance can be the result. We propose that cyanobacteria have a distinct competitive advantage over many other aquatic microbes in polluted, iron-poor environments.

Keywords: Aqueous environments and iron; Environmental pollution; Harmful algal blooms; Iron; Polysaccharides; Siderophores; Toxins.

Figure 1.

Iron acquisition systems and cyanobacteria. With iron deficiency, cyanobacteria can demonstrate increased generation of superoxide which can function to reduce Fe³⁺ to Fe²⁺ (A). Following such ferrireduction, ferrous ion can be transported across a. Decreased availability of iron also impacts pathways for utilization of siderophores/receptors (e.g., synechobactin and anachelin) and xenosiderophores/receptors (e.g. aerobactin and desferrioxamine) (B). Similarly, metal deficiency will impact an increased production of polysaccharide with both release of exopolysaccharides and encapsulation which increase availability of iron (C). Finally, there is an augmented production of cyanotoxins with metal stress which facilitates accumulation of iron. The molecular formulas of these toxins contain numerous functional groups with a capacity to complex iron (e.g. carboxylates and amides) (D). A round, blue-green circle in the figure designates a cyanobacterium while any other shape designates a microbe other than a cyanobacterium.

Figure 2.

Microbial population in an aqueous environment before and after iron deficiency associated with an exposure to an environmental pollutant. The pollutant complexes iron decreasing concentrations of the metal available to microbes. After exposure to an environmental pollutant, cyanobacteria are selected by the low metal concentrations to proliferate. Diminished biodiversity and a dominance of the algae result. Round, blue-green circles in the figure designate cyanobacteria while all other shapes and colored circles designate microbes other than cyanobacteria.