The toxic dinoflagellates Karlodinium veneficum and Pfiesteria piscicida sometimes are associated with harmful algal blooms known as “red tides,” where they feed on smaller, nonpoisonous microbes commonly found in the blooms. Although scientists know that dinoflagellates typically move in a helical trajectory, little is known about how their behavior differs based on species or environment. Understanding this behavior could lead to new ways to prevent the fish kills that algal blooms can cause.Holographic microscopy produced the trajectories of individual K. veneficum microbes shown in A (scale bars: 50 μm). Part of a 3-mm-deep volume shows P. piscicida before introduction of prey (B). The color scales in both images represent velocity in microns per second. Reprinted with permission of PNAS.The shallow depth of field of conventional microscopy poses a challenge to studying dinoflagellates because the containers holding the microbes likely affect their behavior as they bump into the walls. They also tend to clump together, making it hard to study individuals.To study these organisms in a more natural setting, researchers from The Johns Hopkins University and from the University of Maryland Biotechnology Institute, both in Baltimore, turned to cinematic digital holographic microscopy. They placed the organisms in cuvettes measuring 3 × 3 × 40 mm, a large swimming area in comparison with the size of individual organisms. A pulsed Crystalaser Nd:YLF laser at 660 nm illuminated the cuvette. The interference pattern that was in a plane 100 μm from the cuvette’s inner wall was recorded at 2000 fps by a 1024 × 1024-pixel Photron CMOS camera that was synchronized with the laser pulses.As detailed in the Oct. 30 issue of PNAS, analysis of the holographic data produced 3-D swimming patterns of thousands of individual organisms in a dense suspension, revealing a variety of previously un-known behaviors. For example, in the presence of prey, the predators stopped random swimming patterns to cluster around the prey. In addition, K. veneficum appeared to slow down and wait to “ambush” its prey, while the faster P. piscicida increased its speed and the radius of helical trajectories when in pursuit.