Although phosphorus typically limits the growth of freshwater phytoplankton populations, little is known about how the common toxic alga Microcystis aeruginosa responds to variations in phosphorus concentrations and sources. Our study on the effect of the less-available organic phosphorus on Microcystis blooms and has revealed that all forms of the polluting nutrient phosphorus must be managed to control toxic blooms in freshwater systems.

Why We Care
Toxic cyanobacteria, once called blue-green algae (such as Microcystis), have become a serious threat to human health in many freshwater ecosystems, including the Great Lakes. Toxic blooms also threaten the natural resources and local economies dependent on the Great Lakes. While recent studies indicate that phosphorus enrichment can specifically promote toxic strains of the harmful cyanobacteria, Microcystis, blooms are increasing despite the reduction of phosphorus in the Great Lakes in recent decades. Current nutrient management plans focus on improving the quality of freshwater systems and strive to reduce concentrations of total phosphorus but do not address the different types of total phosphorus, including particulate, dissolved, organic, and inorganic. We need to identify the mechanism that is allowing Microcystis to grow in low total phosphorus waters.

What We Are Doing
We began by identifying the genomes of two replicas of Microcystis. By examining these genomes and our own clones of Microcystis, we have determined that Microcystis possesses special genes that make enzymes capable of using the less-common organic phosphorus. These special genes are normally dormant but become expressed under conditions of low total phosphorus and relatively high organic phosphorus.

This work is part of the Ecology and Oceanography of Harmful Algal Blooms (ECOHAB) program. The project team includes partners from Stony Brook University and the New York Sea Grant Program.

What We Are Finding
Thus far, we have found that, as available inorganic phosphorus is used up in the spring and early summer, the normally unavailable organic phosphorus becomes more important to cyanobacteria. To utilize the normally unavailable organic form of phosphorus, Microcystis turns on special genes that allow it to bloom under conditions unfavorable to other phytoplankton.

Next Steps
Using lake-based enclosures, we will examine gene expression patterns of phosphorus transport and metabolism genes by adding organic and inorganic phosphorus from different locations in the Great Lakes and from sources such as farms, urban areas, and sewage outfalls. This approach will provide an indication of the role of organic and inorganic phosphorus in the occurrence of Microcystis in the Great Lakes. Our final objective is to create an outreach campaign based on our findings. The project will develop a fact sheet and host workshops focused on the role of phosphorus in the dynamics of Microcystis blooms. Recipients of the fact sheet and participants in the workshops will include state, local, and federal water treatment facilities, health departments, resource management agencies, and regional stakeholders who manage and regulate sources of phosphorus to the Great Lakes. In addition, educators and the news media will be a part of the outreach effort.