A novel model for cyanobacteria bloom formation: the critical role of anoxia and ferrous iron

Abstract

SUMMARY1. A novel conceptual model linking anoxia, phosphorus (P), nitrogen (N), iron (Fe) and sulphate to the formation of noxious filamentous and colonial cyanobacteria blooms is presented that reconciles seemingly contradictory ideas about the roles of P, N and Fe in bloom formation. 2. The model has several critical concepts: (i) P regulates biomass and productivity in fresh waters until excessive loading renders a system N-limited or light-limited, but it is the availability of ferrous ions (Fe 2+ ) that regulates the ability of cyanobacteria to compete with its eukaryotic competitors; (ii) Fe 2+ diffusing from anoxic sediments is a major Fe source for cyanobacteria, which acquire it by migrating downwards into Fe 2+-rich anoxic waters from oxygenated waters; and (iii) subsequent cyanobacterial siderophore production provides a supply of Fe 3+ for reduction at cyanobacteria cell membranes that leads to very low Fe 3+ concentrations in the mixing zone.3. When light and temperature are physiologically suitable for cyanobacteria growth, bloom onset is regulated by the onset of internal Fe 2+ loading which in turn is controlled by anoxia, reducible Fe content of surface sediments and sulphate reduction rate.4. This conceptual model provides the basis for improving the success of approaches to eutrophication management because of its far-reaching explanatory power over the wide range of conditions where noxious cyanobacteria blooms have been observed.

Blooming algae: a Canadian perspective on the rise of toxic cyanobacteria

“…The naked eye can typically detect algal accumulations when chlorophyll concentrations reach ϳ20 g·L −1 , which corresponds roughly to the total phosphorus (TP) guidelines of 30 g·L −1 set by many provinces in Canada for the protection of aquatic life. Other definitions include dominance of cyanobacteria above 50% of total phytoplankton biomass regardless of the development of visible biomass (Molot et al 2014). Operational definitions for harmful cyanobacterial blooms include the triggers set by the World Health Organization (WHO) based on cell counts (>20 000 cyanobacterial cells·mL −1 , excluding the ubiquitous picocyanobacteria of 0.2-2 m in diameter), which several jurisdictions in Canada have adopted (Watson et al 2008).…”

“…Several review articles have been published outlining the reasons for the dominance (Pick and Lean 1987;Paerl 1988); this continues to be an active area of research, with Canadian researchers contributing novel explanations and interpretation (Molot et al 2014;Taranu et al 2012), as well as novel approaches from paleolimnology (Taranu et al 2015) and molecular biology (Fortin et al 2010). In many ecological studies, cyanobacteria tend to be examined as one functional group when there is considerable functional diversity among bloom-forming taxa.…”

“…Cyanobacteria may outcompete eukaryotes for this essential element through the production of unique siderophores that serve as Fe chelators (Rueter and Petersen 1987). Molot et al (2014) suggested that the availability of ferrous ion regulates the ability of cyanobacteria to compete with eukaryotic algae; because cyanobacteria have higher Fe requirements and there is a tight linkage between Fe and P cycling, levels of bioavailable Fe (ferrous) may explain seasonal blooms and their development even in oligotrophic to mesotrophic lakes. With a better understanding of the nutritional requirements of the most common toxigenic taxa, it should be possible to influence phytoplank-ton composition just as total algal biomass was controlled in the late 1970s through P abatement.…”

Evaluation of a Modified Monod Model for Predicting Algal Dynamics in Lake Tai

“…Besides temperature, TN, and TP, other factors, such as nitrogen-to-phosphorus ratio [31], anoxia and ferrous iron [32], and climate change [33,34], have been found to affect cyanobacterial dynamics/blooms in lakes. However, limited by data availability, our study could not evaluate prediction performance of the modified Monod model when either of those other factors is used as independent variable s in Equation (4).…”

Phosphorus Forms in Sediments as Indicators of Anthropic Pressures in an Agricultural Catchment in Southern Brazil

“…Consequently, in these anoxic conditions, reductive dissolution of ferric iron minerals occurs, releasing the bound P stored in sediments into the water column, which increases the algal available P and may promote eutrophication. Molot et al (2014) argue that internal Fe 2+ loading from anoxic sediments is also an important factor that regulates the ability of cyanobacteria to compete with eukaryotic phytoplankton. Linking this to our results, even if the sediment coming from the undisturbed site reaches a lentic environment, such as Guaíba Lake, the probability of P release promoted by the dissolution of Fe oxides in a reduced environment will be very small.…”