Imagine sailing through a pitch-black ocean night, only to find the waters around you glowing eerily like a vast field of snow. This enchanting phenomenon, known as milky seas or mareel, has both baffled and mesmerized sailors for centuries. These rare events, occurring roughly zero to two times a year globally, are a spectacular display of bioluminescence that turns the sea into a surreal, glowing expanse.
Before science offered potential explanations, milky seas were part of maritime folklore. From tales of ship-eating sea monsters to beautiful mermaids, one could wonder whether they are just the vivid imaginations of weary sailors or harbor a sprinkle of truth. Over time, sailors’ descriptions found their way into classic sea-adventure fiction novels, like Jules Verne’s "Twenty Thousand Leagues Under the Seas" and Herman Melville’s "Moby Dick." The milky sea was merely one of many unlikely stories told by imaginative sailors for centuries. The truth, as it turns out, is just as fascinating as the fiction.
Milky seas, as the name implies, are vast areas of water that appear white. These phenomena occur at night, especially in dark, moonless conditions, with the light coming not from reflected sources like moonlight, but from something within or on the water. This light is bioluminescence, a type of chemical luminescence produced by living organisms.
The leading hypothesis for milky seas involves the role of bioluminescent bacteria. These bacteria become luminous only after reaching a certain population density, a process known as “quorum sensing.” Quorum sensing is a type of cell-to-cell communication where bacteria detect and respond to the density of their population by altering gene expression. When the bacteria are present in sufficiently high concentrations, they begin to glow continuously in the presence of oxygen. This continuous glow, rather than the brief flashes typically associated with other bioluminescent organisms like dinoflagellates, creates the steady and widespread light observed in milky seas. The light produced by these bacteria is relatively faint per cell, but when billions of them gather over a vast area, the combined effect can result in a visually stunning, uniformly glowing sea.
One intriguing theory suggests that these bacteria might form large surface slicks, aided by organic material produced during a previous or concurrent phytoplankton bloom. This could provide the necessary conditions for the bacteria to reach the critical concentration required for quorum sensing, leading to the widespread bioluminescent display. Moreover, the role of bioluminescent bacteria in the ecosystem is rather interesting. Unlike dinoflagellates, which use their light to deter predators, these bacteria use their glow to attract larger creatures. The reason behind this is rather counterintuitive: the bacteria want to be eaten. The gut of a fish provides an ideal habitat for these bacteria to thrive, so they light up to attract fish, ensuring their colonies find a suitable environment.
Historically, most accounts of milky seas have come from ship logs, with the phenomenon predominantly observed in the northwestern Indian Ocean and the Maritime Continent region. This area is particularly conducive to such events due to its warm surface temperatures and high productivity from upwelling currents, which support microbial growth.
While all of these ideas are compelling, it's important to note that the exact conditions leading to milky seas are highly unpredictable, and much of what we understand is based on a few documented cases, including those observed via satellite. Future studies of this elusive phenomenon may provide further insights, especially with advancements in satellite technology that can monitor low-light emissions over vast oceanic areas. By focusing on regions where milky seas are more frequently reported, scientists hope to unlock the mysteries behind this captivating natural wonder.
Hosna Akhgary is a BSc candidate at ±«ÓăÖ±˛Ą, majoring in Pharmacology.
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