Cyanobacteria harmful algal blooms (cHABs) are found in many waterbodies across the world, occurring in freshwater, brackish, and coastal marine systems. They accumulate significant amounts of biomass which reduce water clarity for other aquatic organisms and can potentially produce toxins which are harmful for humans, livestock, and wildlife. Because these blooms can occur in areas used as potable water resources, many industries have an invested interest in the monitoring and control of cHABs (1). A plethora of research has established a link between water pollution and bloom growth and persistence. Emerging work is addressing drivers of toxicity, toxin remediation, and bloom community structure. Despite the diverse research currently being done on cHABs worldwide, there are many questions that still need to be answered, such as why aren’t blooms significantly controlled by top-down trophic interactions, what drives seasonal succession of blooms, and how harmful algal bloom communities work together to promote bloom establishment and longevity.

The North American Great Lakes are a vital freshwater resource, containing ~18% of the world’s available surface fresh water. Lake Erie is the shallowest and warmest of the Great Lakes and receives nutrients from urban, industrial and agricultural sources. In the 1960s and early 1970s, dense phytoplankton blooms, including the nitrogen-fixing cyanobacteria Dolichospermum spp. (then named Anabaena spp.) and Aphanizomenon flos-aquae, were common. The intensity and frequency of these blooms decreased during the 1970s and 1980s with the implementation of phosphorus loading reductions. However, by the mid-1990s, cyanobacterial blooms returned to Lake Erie, now dominated by the non-nitrogen-fixing cyanobacteria Microcystis spp. and Planktothrix spp., which produce potent cyanotoxins (microcystins).

We are expanding our understanding of how harmful algal blooms form, persist, and decline, through the use of environmental isolates that are maintained in the lab. This way, we can test individual effects of abiotic and biotic interactions, generate synthetic communities, and do precision molecular characterizations. We are also expanding our scope beyond Lake Erie to include reservoirs here in Texas. 

Viruses play a role the life cycle of their hosts, and the detection of phages within HABs has led to an interest in understanding the implications of a viral infection for cyanobacterial dynamics of the bloom. Lytic infections (infections which cause the rupture and death of the host) release dissolved organic matter into the system, likely stimulating heterotrophic bacterial activity and offering energy to the microbial loop. The viral lysis of the Microcystis bloom behind the Lake Erie 2014 Water Crisis converted a portion of the microcystin pool into the dissolved phase, some of which bypassed treatment and entered the finished drinking water, shuting down the water supply for for >400,000 residents. 

Besides lytic infections, viruses may reproduce using lysogeny, a process in which the viral genome lies dormant in the host cell, replicating when the host does. There is evidence to suggest that lysogeny is not uncommon in harmful algal blooms as lysogenic genes have been found during Microcystis dominated blooms, and the shift between these genes and lytic infections was tied to environmental cues. 

Part of the issue of identification of cyanophages is the lack of available reference sequences due to a low success rate in cyanophage isolation. Characterizing and documenting the activity of cyanophages in cHABs is important in assessing risks associated with toxin-producing cHABs.

Chytridiomycota, informally chytrid, is the classification of a group of fungal organisms based on their ability to produce zoospores encased in sporangia as part of their life cycle. Within this classification is the order Rhizophydiales, which encompasses parasitic chytrids that are known to infect a number of phytoplankton species, including the cyanobacteria Planktothrix. 

Parasitic chytrid infection in filamentous cyanobacteria such as Planktothrix can increase the edibility of low-quality food sources (like cyanobacteria). Chytrid infections can fragment the host, making them smaller in size and easier to consume and digest. Infections can also promote the release of cellular material (organic carbon and nitrogen), which can act as alternative food sources for heterotrophic bacteria. Additionally, the zoospores themselves can act as a food source for grazers.

Despite the massive mortality seen in infections done in the lab, chytrid infections on cyanobacteria are rare in the wild. While infections can be diminished via high surface water temperatures and temporary spikes in conductivity, the greatest limitation for widespread chytrid infections may be the constant surface mixing of shallow waters where the cyanobacteria dominate, occasional sediment-water interface mixing, and sediment resuspension from high flow and very high wind events.

Free-living planktonic amoebae use colonies of planktonic cyanobacteria as an appropriate habitat where they can feed on associated microorganisms or on the cyanobacteria themselves. Whitton (1973) was the first to report the existence of Microcystis-grazing amoebae (MGA) but until recently, only a few other amoeboid organisms ingesting Microcystis in natural samples were observed. Here we present ongoing work that is looking at MGA in the Vannella (planktonica and simplex) and Korotnevella genus. 

We know that heterotrophic bacteria can form symbiotic relationships with cyanobacteria, either as epibionts on colonies, known as "the phycosphere," or as co-existing, free-living taxa. These phytoplankton–bacteria interactions revolve around resource exchange, which can be reciprocal or exploitative. For example, associated bacteria can complement missing genomic functions, such as vitamin synthesis and nitrogen cycling and form mutualistic relationships with cyanobacteria. Aquatic heterotrophic bacteria obtain a significant, though variable, portion of their carbon from phytoplankton, with up to 50% of phytoplankton-fixed carbon consumed by bacteria. Bacteria primarily utilize the large quantities of labile dissolved organic carbon released by phytoplankton into the water column but also consume more complex algal products, such as mucilage and polysaccharides, as well as senescent or dead phytoplankton biomass.

Research on phytoplankton aggregates, particularly those dominated by Microcystis, indicates that the bacterial communities associated with Microcystis colonies differ from those found in other cyanobacteria and free-living communities in the surrounding water. In fact, bacteria associated with HABs may influence the fitness and relative abundance of different Microcystis strains, which in turn affects toxin concentrations in Microcystis blooms. Despite their close relationships, the role of bacteria in HABs and their interactions with bloom-forming cyanobacteria are often overlooked.

Competition among cyanobacterial species, including planktonic forms like Microcystis, Dolichospermum, and Planktothrix, as well as benthic genera such as Nostoc, is shaped by environmental gradients in light, temperature, nutrient availability, and habitat structure. Planktothrix can dominate in turbid, low-light, nitrogen-rich systems due to its buoyancy control and tolerance for poor light conditions, while Dolichospermum and Nostoc can gain a competitive edge in nitrogen-limited environments through nitrogen fixation. Benthic species like Nostoc also benefit from substrate attachment, allowing persistence under hydrodynamic stress and reduced grazing pressure. These species compete not only for resources but also through allelopathic interactions, influencing bloom composition, toxin production, and ecosystem function.