Fluorescent seas are a type of bioluminescence phenomenon.

This phenomenon refers to the process where organisms convert chemical energy into light energy through certain chemical reactions within their bodies. The glow of fireflies is the most well-known example of bioluminescence in nature.

There is a wide variety of bioluminescent organisms in the sea, ranging from single-celled algae like dinoflagellates to invertebrates such as glowing jellyfish, segmented worms like bioluminescent polychaetes, mollusks such as various glowing cephalopods, and even crustaceans like phosphorescent shrimp and dinoflagellates.

Considering the phenomenon of "fluorescent beaches," larger jellyfish and cephalopods can be excluded, as they do not exhibit such strong fluorescence. Additionally, crustaceans like phosphorescent shrimp are unlikely to produce such intense fluorescence. Therefore, the most probable cause is the proliferation of single-celled algae, particularly dinoflagellates.

Dinoflagellates are a primitive type of single-celled algae. Many of their characteristics are between those of prokaryotic bacteria and eukaryotic protozoans, and they are generally classified as dinoflagellates. Dinoflagellates typically have cellulose plates covering their bodies and 2-3 flagella in grooves between these plates, which they use for movement and feeding in water.

Many species of dinoflagellates can produce light, including organisms like Noctiluca scintillans (commonly known as sea sparkle) and others like Gonyaulax spp. Sea sparkle, in particular, is a distinctive type of dinoflagellate with large bodies, lacking cellulose plates, and often lacking chloroplasts, relying on its flagella to feed on prokaryotes or other algae.

For dinoflagellates, bioluminescence might serve as a defense mechanism. When disturbed by predators like zooplankton, the emitted fluorescence can attract higher-level predators like fish, which are more sensitive to light, effectively deterring their immediate predators. While the "fluorescent beaches" formed by dinoflagellates are aesthetically captivating for humans, the underlying message is one of caution.

The intense fluorescence, typically led by organisms like sea sparkle, indicates an increase in nutrients such as phosphorus, potassium, and iron in the seawater. This increase promotes the growth of bacteria, cyanobacteria, and single-celled algae, which are food sources for dinoflagellates.

This phenomenon can be an early sign of eutrophication in seawater. If left unchecked, it could lead to an explosive growth of dinoflagellates, resulting in red tides. Dinoflagellates are the main algae involved in coastal red tides. Their massive reproduction, death, and decomposition can deplete oxygen in the water, leading to the suffocation and death of other marine organisms, posing a serious threat to aquaculture industries.

Additionally, dinoflagellates can produce various toxins such as brevetoxins during their growth, which can accumulate in shellfish and other marine organisms, increasing food safety risks.

Luciferin is a type of small molecule that, when combined with luciferase, can emit light. Luciferase uses ATP to release light from luciferin, completing the conversion of chemical energy to light energy. Recent studies have shown that the bioluminescence mechanism of sea sparkles involves an electrochemical change similar to action potentials in animal nerve cell membranes on the vacuolar membrane of the algae.

This change opens voltage-sensitive proton channels, allowing protons from the vacuole to enter small sacs called scintillons on the vacuolar membrane. This activation of luciferase within the scintillons leads to the production of light. Other types of dinoflagellates may also have similar bioluminescence mechanisms.

In conclusion, the fluorescence of the seas is a fascinating yet intricate natural phenomenon, with dinoflagellates playing a central role. It serves as a warning sign of potential environmental imbalances and emphasizes the delicate balance of marine ecosystems.