In this new article series, we will look at how pollution impacts the sensory reception of various organisms, and in this particular article, we will examine how pollution inhibits communication and ecological relationships via bioluminescence.
Written by: Shreya
One of the first biology lessons we ever learned and the medium through which we consume the rest of the universe is our 5 senses: vision, hearing, touch, smell, and taste. Our engagement and interpretation of our surroundings is entirely dictated by these senses, and through them we can communicate with one another, surmise the edibility of a foul-smelling carton of milk, identify danger shifting in the shadows, and recognize those we love. They allow us to survive and experience our surroundings, which is why it is odd to consider the fact that we are only truly consuming fractions of reality. Our 5 senses, while pivotal to our existence, are only 5 of hundreds, maybe even thousands of “senses”. The range of senses that organisms possess is as diverse as life itself, with some species being able to detect the electrical fields generated by other organisms and others registering vibrations as slight as 0.1 microns. Even our experience with our own senses are limited: The average human possesses 3 types of color receptors – red, blue, and green, which makes us trichromats; however, the rare tetrachromat can see an entirely new color range, which brings with it 99 million new colors. Clearly, the universe is much more complex and abuzz with information than we can “sense” (1).
However, the ability of organisms to engage with their surroundings through their unique senses has been heavily compromised by industrialization, pollution, and anthropogenic changes. Pollutants such as greenhouse gases, heat, heavy metals, organic matter, antibiotics, and much more can inhibit or irregularly trigger sensory reception. Throughout this series, we will be taking a look at some of the ways pollution impacts sensory ecology, or how organisms gather and process information from stimuli in their environment. In this article, let’s examine how various sources of pollution impact one of the major forms of communication and defense in marine environments: bioluminescence.
Bioluminescence is the production of light by a living organism and is used for an assortment of purposes by 76% of all life in the ocean. The trait is so useful for survival that it is thought to have evolved at least 70 different times into separate, unrelated species. Common purposes for bioluminescence is to attract prey, startle predators, misdirect predators towards an alternate location, and counter-illuminate shadows. For instance, many squid and jellies flash intense bursts of light to stun their predators and give themselves time to flee, while angler fish attract prey to their bioluminescent organ– which happens to hang right above their open and heavily fanged jaw. The Hawaiian bobtail squid possesses a light organ with bioluminescent bacteria that produces the exact amount of light that is penetrating the water from above so that the squid does not cast a shadow, which prevents predators and prey from noticing its movement. In addition, many strains of marine bacteria and algae, such as dinoflagellates, are bioluminescent.
Bioluminescence is produced as a result of various stimuli, from mechanical to stress-related, but its production process is almost identical across the board. Ultimately, one of either the enzyme luciferase or photoprotein oxidizes the molecule luciferin, and the capture of the high-energy electron from the luciferin stabilizes the molecule, with the release of energy emitted as light. More specifically, the vacuole in bioluminescing cells is highly acidic, and its membrane– called the tonoplast– separates the acid from the luminescing part of the cytoplasm, or the scintillon. An external stimuli (mechanical, stress, etc) triggers an action potential across the tonoplast, which opens voltage-gated proton channels and allows protons to flow into the scintillon from the acid. This process, known as scintillon acidification, alters the structure of the luciferase that is present there and opens up its active site so that it can now bind to luciferin. The ensuing oxidation reaction produces a flash of light, or bioluminescence (2). Some organisms also possess luciferin binding protein, which allows reactions at neutral or alkaline pHs instead of acidic pHs to prevent auto-oxidation and reign more control over their bioluminescent mechanism.
Because luminescing organs heavily depend on enzymatic activity, they are heavily reliant on the right set of environmental conditions, including pH, temperature, and salinity (3). Studies have also shown that bioluminescence only occurs when the organism is in a low-light environment as to prevent energy-wastage from luminescing in already-bright conditions. This leaves these organisms vulnerable to light pollution, ocean acidification, thermal pollution, industrial waste, agricultural waste, wastewater sludge, and insecticides (among many other sources of pollution). Let’s take a look at exactly how these pollutants impact an organism’s ability to bioluminesce.
Ocean Acidification:
As the combustion of fossil fuels for energy releases more greenhouse gases into the atmosphere, the concentration of atmospheric carbon dioxide increases. Much of this carbon dioxide gets sequestered, or absorbed, by the ocean and reacts with water to form carbonic acid. Locations with higher carbonic acid content are growing more acidic (even though the ocean as a whole is expected to change from a pH of 8.1 to 7.8 by the end of the century– which is not a dramatic enough change to impact luciferase activity) (4). Since luciferase depends on a proton gradient and pH to be able to oxidize luciferin, changes in ocean acidity might inhibit the ability of organisms to bioluminesce. At the ideal pH of 8.2-8.4, bacteria luminesce yellow-green light, but in more acidic conditions, the wavelength shifts to red; below a pH of 6, there is practically no recorded activity (5). Luciferase also needs the tonoplast membrane voltage to recharge so that the proton gradient is re-established after the passing action potential, and this requires a steady pH of around 8 (6).
Thermal Pollution:
Power plants such as nuclear plants and combustion plants heat water to around 300 degrees Celsius for it to turn a turbine that generates electricity, and this extreme steaming process generates thermal pollution, or the harmful emission of heat. The discharge of heat from the plant (and often the water that is released into nearby environments) can increase the surrounding water temperatures to between 30-40 degrees Celsius, but luciferase activity is almost nonexistent past 34-35 degrees Celsius (with optimal activity at 26 degrees) (7). Higher water temperatures also cause the amount of dissolved oxygen to decrease, which can impact bacterial respiration. Thermal pollution has the potential to turn a water body into a dead zone, or an area with no life, by depleting dissolved oxygen content. Hot water cannot hold the same amount of oxygen as cooler water, so the heated water is less hospitable to aerobic organisms. When these organisms die, aerobic decomposers proliferate and deplete even more of the oxygen until the habitat can no longer foster life. This type of pollution greatly impacts the ability of organisms to survive and the activity of enzymes such as luciferase.
Industrial Waste:
Here, we refer to industrial waste as any type of pollution produced as a result of industrial and business-related procedures, including wastewater sludge, agricultural waste, livestock waste, and heavy metal discharge. Wastewater sludge contains various pollutants such as harmful strains of bacteria, heavy metals, viruses, and organic matter. If not treated properly, these contaminants make their way into the open environment and drastically reduce natural bacterial, fish, and algal populations. Heavy metals also change the light intensity and wavelength of the light emitted, which can inhibit communication and prey/predator interactions (8).
Agricultural waste includes runoff containing pesticides, insecticides, herbicides, and fertilizers. While the effect of pesticides on bioluminescent activity has not been extensively reviewed, some studies have shown that the main ingredient in insecticides– N,N-Diethyl-meta-toluamide or DEET– decreases bioluminescent abilities (9). Fertilizers contain high amounts of nitrogen and phosphorus– two essential elements that can cause an algal bloom by providing extensive nourishment to algae. If the bioluminescent species is algal, its population will thrive before returning to normal, but if not, the algal bloom can block sunlight from penetrating the water surface, kill off photosynthetic organisms, and thus decrease dissolved oxygen content (which can lead to a dead zone). Livestock waste needs to also be handled and treated carefully since the organic matter it contains can harbor dangerous strains of bacteria and viruses. In addition, concentrated animal feeding operations (CAFOs) pile several animals into tight spaces, which can bring on the onset of disease. As a solution, the livestock are heavily pumped with antibiotics, and if these antibiotics leach into groundwater or nearby streams, they can reach bioluminescent bacterial populations and impact their survival. Many bioluminescent organisms have light organs that utilize bioluminescent bacteria rather than themselves producing light, so antibiotic contamination can have a devastating effect on a plethora of luminescing species.
Finally, certain industries produce wastewater called reverse osmosis reject water. Reverse osmosis is a process used to filter water into two chambers- one that is dilute of contaminants, viruses, bacteria, minerals, etc and one is concentrated with them (thus it is reverse osmosis since it is accentuating solute concentration variance). The concentrated water is called RO reject water since the clean water is what gets utilized for production processes; reject water is often dumped from factories like dye industries, and some studies show that exposure to RO reject water significantly decreases bioluminescence (10).
Light Pollution:
Organisms only bioluminesce in low-light conditions so they do not waste energy producing light themselves. The purpose of bioluminescence for many organisms is to stun a predator or prey species or for communication, and self-produced light would not contrast with their surroundings in a well-lit environment. Normally, night-time would allow bioluminescence, but coastal industrialization has introduced constant light sources from harbors, boats, coastal buildings, etc to the shoreline. Many scintillons stay inhibited because of the extra light. Certain fishing methods use bright lights to blind their catch, which further contributes to light pollution in the waters.
Closing
While pollution from industries and agriculture have received more flack and inspired more strict business regulations recently, global industrialization is still transpiring at a high rate, introducing immense light and thermal pollution as well as atmospheric carbon dioxide. In order to protect the essential sensory tool of bioluminescence in marine organisms, reform in our energy use and production is nothing but an absolute requirement.
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