Red tides are harmful, but how are dinoflagellates beneficial to marine ecosystems?

The opening of this chapter discussed harmful red tides, or blooms of dinoflagellates. But not all dinoflagellate blooms produce problems for other species. Dinoflagellates are important components of many ecosystems, as we have seen throughout this chapter. Photosynthetic dinoflagellates also produce much of the atmospheric oxygen that most animals need to survive.

As we saw in Investigating Life: Can Corals Reacquire Dinoflagellate Endosymbionts Lost to Bleaching?, corals and many other species depend on symbiotic dinoflagellates for food. In addition, as photosynthetic organisms, free-living planktonic dinoflagellates are among the most important primary producers in aquatic food webs. They are a major component of the phytoplankton and provide an important food source for many species (see Key Concept 26.4).

Future directions

Some dinoflagellates produce a beautiful bioluminescence (Figure 26.21). Unlike the bioluminescent bacteria described at the start of Chapter 26, however, dinoflagellates cannot generate a steady bioluminescence, but produce bright flashes of light when disturbed, as people who swim in the ocean at night in certain regions often observe. What function do these flashes serve? Many light-emitting dinoflagellates are preyed on by other species, such as small crustaceans. When the dinoflagellates produce a bright flash, it functions like a “burglar alarm,” and attracts secondary predators of the crustaceans. Experiments have shown that crustaceans reduce their feeding on dinoflagellates when they flash this signal. However, recent research shows another function of bioluminescence in toxic species of dinoflagellates. These toxic species produce a much lower intensity bioluminescence which may function as a warning of their toxicity to potential predators. So depending on the intensity and other details of the flash in different species, the bioluminescence can serve either to attract secondary predators or to warn off primary predators. Research on communication by bioluminescence continues as a rich field of investigation.