Milky sea – an unusual phenomenon in the ocean

The milk sea is a phenomenon that has long amazed scientists and sailors. When it occurs, the water resembles a huge spot of milk or a surface of pure snow, especially visible at night. Until recently, there was no certainty that the milky sea was not just a sailor’s legend by chance. Thanks to the cooperation of scientists and the technology made available by NASA, the nature of this phenomenon has been better understood. Today, not only do we know how it is formed, but we can also predict with high probability where it will occur.

What is the mysterious milk sea?

Milky sea is a bioluminescence effect caused by the bacterium Vibrio harveyi, which is commonly found in the oceans. This bacterium exhibits a special property known as quorum sensing or density signaling. When a population multiplies and exceeds a certain number of individuals per unit volume of water, it begins to emit light.

The milky sea was for years difficult to capture with any research equipment. As a result, its occurrence was recorded so rarely that the very existence of the phenomenon was questioned in the scientific world. It was only confirmed by the collection of eyewitness records, which were compared with satellite images. We now know that it occurs primarily in the Indian Ocean, near the coast of the Horn of Africa, Indonesian Java and the Banda Sea. It most often appears during the summer and winter seasons.

From the legend of the milk sea to regular checkups

The phenomenon appeared in the accounts of sailors as early as the 19th century. It also found its way into literature. Its description can be found, among others, in the book Twenty Thousand Miles of Undersea Navigation by Julius Verne. The author even diagnosed the causes of its formation quite accurately.

However, the first attempt to systematize knowledge about the milky sea was the work of Steven Miller, a researcher at the Naval Research Laboratory in California. Miller began collecting case descriptions recorded by eyewitnesses, trying to match them on the basis of date with satellite images. An entry from the logbook of the British merchant ship Lima proved to be a breakthrough. Its crew spotted the phenomenon while sailing along the coast of the Horn of Africa on January 25, 1995. A careful analysis of satellite images from that day confirmed the presence of the white spot. However, the images were very blurry, making it impossible to regularly track the phenomenon by satellite.

New opportunities have arisen thanks to NASA’s collaboration with the US National Oceanic and Atmospheric Administration (NOAA). With the launch of the new SUOMI NPP satellites in 2011, more accurate tracking of marine luminescence became possible. Equipped with much more accurate sensors, they could easily detect light on the surface of the water.

Bioluminescence, or an ocean full of light

Bioluminescence is common in oceanic organisms. On land, it occurs less frequently, but can be observed, for example, in skylights and some fungi. According to evolution researcher Dr. Peter Vršanský of the Slovak Academy of Sciences, the reason for the rare occurrence of this phenomenon on land and in fresh inland waters may be the toxicity of the components. Cold ocean water washes them out more easily.

The source of bioluminescence is a chemical reaction taking place in cells. A group of enzymes – luciferases, which catalyze the corresponding pigments (called luciferins) – are responsible for the activation.

About 76 percent of marine organisms have this ability. Most are deep-sea dwellers. It originally developed in marine bacteria and cyanobacteria. In animals, such as mollusks, parasitoids and even fish, it can occur through symbiosis with appropriate bacteria or through specialization of selected body cells.

The development of this ability may have had many evolutionary reasons. Based on observations of today’s behavior, we can guess what role light plays in the life of the ocean. Some of its inhabitants use it to attract prey, others to deter or redirect the attention of a predator. Still others, like the aforementioned Vibrio harveyi bacteria, use it to communicate with members of their species.

Unusual bioluminescence – why does the milky sea amaze researchers?

Most of the phenomena associated with bioluminescence and observed on the surface of the oceans are caused by phytoplankton. In particular, by the species Lingulodinium polyedrum. Plankton is deposited near the coast. This gives the impression of filling the water with small luminous dots, usually blue, which off the coast of the Maldives is called the Sea of Stars. Other similar phenomena can be observed off the coast of China or on the beaches of California, among others.

The milky sea, unlike typical and well-known cases of bioluminescence, does not consist of glowing dots, but is a uniform white surface. It also has a much larger range – up to 100,000 ^km2. In addition, the glowing effect lasts longer – even for several weeks.

According to recent hypotheses, the cause is the appearance of algae on which Vibrio harveyi bacteria multiply in selected areas. Under normal conditions, the number of microbes per milliliter of seawater does not exceed 10 individuals. During multiplication, in summer and winter, there can be as many as 100 million per milliliter.

The research work carried out by Steven Miller and collaborating doctoral student Justin Hudson has made it possible to systematize when and where the milky sea appears. Hudson also took into account the warming of waters in the Pacific during El Ninio and the austral monsoon occurring in summer in the southern hemisphere. Thanks to an algorithm developed on this basis, the scientists knew when and where to begin direct surveys. Water samples taken on site helped to better understand the nature of the phenomenon.

As Steven Haddock of the Monterey Bay Research Institute in California points out, we actually know very little when it comes to the causes of the milky sea. This makes one realize how many secrets ocean waters still hold.

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