Every summer since 2018, the Virginia Department of Health has posted swimming advisories on parts of Lake Anna. The trouble starts in the upper North Anna and Pamunkey branches, where the water is shallow, warm and fed by runoff carrying nitrogen and phosphorus. In 2024 the advisory went up in June and did not come down until November 7, after two clean samples taken at least ten days apart. The signs warn about harmful algae. The cells turning the water green are not algae, and they are not plants.

Cyanobacteria are bacteria

The green comes from cyanobacteria, long called blue-green algae. A common bloom former, Microcystis, is a sphere about 2 to 4 micrometers across, glued to thousands of others in a slimy colony. Look inside one cell and there is no nucleus; the DNA lies loose in the cytoplasm. There are no membrane-bound organelles at all, so no mitochondria and no chloroplasts. There are ribosomes, a cell membrane and a cell wall. That list makes a cyanobacterium a prokaryote, the same kind of cell as the bacteria on your skin.

Photosynthesis with no chloroplast

It still makes its own food. In a plant, chlorophyll sits on stacked membranes called thylakoids inside each chloroplast. In a cyanobacterium the thylakoids run loose through the cytoplasm, and the whole cell does the work a chloroplast does for a plant: it captures light, builds sugar from carbon dioxide and water, and gives off oxygen. Bloom strains also carry gas vesicles, tiny protein floats that lift the colonies to the sunlit surface, which is why a bloom gathers as scum.

Put an Elodea leaf from a classroom aquarium under the same microscope and the difference is plain. Each leaf cell is a eukaryote: its DNA is wrapped inside a nucleus, and it holds 50 to 120 chloroplasts, each one about the size of an entire Microcystis cell. Green on the outside says nothing about the kind of cell inside.

From counting cells to measuring toxins

Some cyanobacteria release toxins. Microcystin damages the liver and can sicken swimmers and kill dogs that drink bloom water. Under Virginia's 2021 guidance, an advisory went up when a sample held at least 40,000 Microcystis cells per milliliter, at least 100,000 cells per milliliter of the kinds that can make toxins, or at least 8 micrograms of microcystin per liter. Lake Anna crossed a cell-count line every year from 2018 on, yet none of its samples held toxin above the advisory level. A cell count tells you how many cells are present, not what those cells are making. In April 2025 Virginia began issuing advisories on measured toxins alone.