A huge bloom of toxic algae that took over Florida's largest freshwater lake has been captured in stunning images taken from space.

The NASA images show an expanse of blue-green algae that covered Lake Okeechobee in Florida this summer. The toxic bloom appeared in May and expanded to 33 square miles (85 square kilometers) in area, covering a good chunk of the 730-square-mile (1,913 square km) body of water - the second-largest lake entirely within the contiguous United States (second only to Lake Michigan).

The invasion by the single-celled organisms, also called cyanobacteria, was still present on July 2, 2016, when the images were captured by NASA's Operational Land Imager, aboard the Landsat 8 satellite.

RELATED: Florida Declares State of Emergency Over Algae Bloom

Algal blooms occur for a variety of reasons. Pollution, nitrogen-fertilizer-laden runoff from farms and even warm lake water can fuel the growth of these single-celled creatures. The cyanobacteria - often blue-green algae or other phytoplankton - use sunlight to make food and thrive when concentrations of nitrogen and phosphorous rise.

Algal overgrowth can choke off other life in lakes and rivers, and some algae also release toxins that can sicken people, causing liver failure in the most serious cases.

WATCH:How Can Something Be A Plant And An Animal?