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What is fluorescence? Substances that are fluorescent absorb light at certain wavelengths and re-emit them at longer (lower energy) wavelengths. The process of fluorescence can be broken down into three stages which are excitation, excited state life and emission. When a fluorescent pigment is excited, it absorbs energy from light and its electrons are elevated to a higher energy state. During the nanoseconds long excited state, the molecular structure of the fluorescent pigment will undergo a specific conformational change which means that the protein will actually change shape. When the protein reaches a certain “target” shape, the high energy electron will release almost all of the absorbed light energy as it returns to the original, lower energy state and then the protein returns to its original shape. Remember that all this is nearly instantaneous. The released energy is manifested as the visible, brightly colored fluorescent emission.
During excitation, fluorescent pigments absorb relatively high energy light in the 350-500 nm range which corresponds roughly to violet, indigo and blue colored light. Fluorescent pigments do not absorb light at individuals wavelengths but they absorb it over small ranges with very sharp peaks of absorption. There are 5 major peaks of fluorescence excitation wavelengths. Most aquarists are unknowingly familiar with two of these because they correspond to the two different shades of actinic bulbs available in the aquarium market today. The traditional phillips actinic 03 style lamp has an emission peak at 420nm and these offer a purplish cast that is preferred by die hard vho users and veteran reefkeepers. Bulbs that resemble the actinic 03 lamp have predominantly been available only in standard T12 NO and VHO bulbs although newer PC and T5 bulbs are becoming increasingly available in this color. The lighter brighter blue actinic bulbs which were originally manufactured as power compacts have an emission peak around 470nm. Most T5 bulbs are found in this color (sometimes marketed as actinic blue, super blue or blue plus) and some newer PC bulbs are actually half actinic 03 and half “super blue”. The two different types of lamps and their major excitation peaks differ in the fluorescent colors that they excite. Actinic 03 bulbs are better at exciting green fluorescence and super blue bulbs are better at exciting red and orange fluorescence. Most of the blue metal halide bulbs will also have emission peaks that excite fluorescence and I believe that most of these will usually have peaks at 470nm or at least, they usually have a higher peak at this wavelength.
So why do organisms fluoresce anyway? Well, there a variety of reasons and these include communication, photochemical mechanisms, some are the unsolicited bi-product of biochemical reactions and there are cases where we simply don't know why fluorescences is present. For the sake of brevity I will concentrate on the importance of photochemical mechanisms.
Photosynthesis is possible because of photosynthetic pigments like chlorophyll. Chlorophyll behaves much like the fluorescent pigments I described earlier except that when the electrons return to their lower energy state, only a small amount of energy is released as fluorescence. The bulk of the energy is transferred as physical (kinetic) energy to be later transformed into stored (chemical) energy. Many fluorescent pigments are known as accessory pigments because they are accessories to photosynthetic pigments. In a way, accessory pigments can be thought of as light converters. Since photosynthetic pigments have a specific range of wavelengths over which they can capture light energy, some of the available light is not immediately harnessed for photosynthesis. In these cases, accessory pigments can capture light outside the range of photosynthetic pigments and they can convert it into wavelengths that are usable by photosynthetic pigments. This is especially true in red algaes which are commonly found in very deep water. In this darker environment, photosynthetic organisms need all the light they can get and the extra energy provided by the fluorescence of accessory pigments critical. In this context, the accessory pigments are closely associated with the photosynthetic pigments and in reference to corals, the accessory pigments are also called symbiont pigments. The fluorescence of accessory pigments is only detectable with specific equipment because even the small amount of fluorescence from chlorophyll is far brighter. When photosynthetic organisms are found in very bright environments, they are at risk of being stressed by an overabundance of light. The excess light can actually inhibit photosynthesis and in the case of UV, the light can be physically harmful in high doses. Once again, fluorescent pigments can be used to alter how the organism is affected by the light but in this case, the fluorescence is emitted at wavelengths that are not harmful or inhibitive to photosynthesis. Although most symbionts will have some suite of photoprotective pigments, the most visible pigments are those that belong to the host and these are the ones that most coral aquarists try to encourage by the use of intense lights.
During excitation, fluorescent pigments absorb relatively high energy light in the 350-500 nm range which corresponds roughly to violet, indigo and blue colored light. Fluorescent pigments do not absorb light at individuals wavelengths but they absorb it over small ranges with very sharp peaks of absorption. There are 5 major peaks of fluorescence excitation wavelengths. Most aquarists are unknowingly familiar with two of these because they correspond to the two different shades of actinic bulbs available in the aquarium market today. The traditional phillips actinic 03 style lamp has an emission peak at 420nm and these offer a purplish cast that is preferred by die hard vho users and veteran reefkeepers. Bulbs that resemble the actinic 03 lamp have predominantly been available only in standard T12 NO and VHO bulbs although newer PC and T5 bulbs are becoming increasingly available in this color. The lighter brighter blue actinic bulbs which were originally manufactured as power compacts have an emission peak around 470nm. Most T5 bulbs are found in this color (sometimes marketed as actinic blue, super blue or blue plus) and some newer PC bulbs are actually half actinic 03 and half “super blue”. The two different types of lamps and their major excitation peaks differ in the fluorescent colors that they excite. Actinic 03 bulbs are better at exciting green fluorescence and super blue bulbs are better at exciting red and orange fluorescence. Most of the blue metal halide bulbs will also have emission peaks that excite fluorescence and I believe that most of these will usually have peaks at 470nm or at least, they usually have a higher peak at this wavelength.
So why do organisms fluoresce anyway? Well, there a variety of reasons and these include communication, photochemical mechanisms, some are the unsolicited bi-product of biochemical reactions and there are cases where we simply don't know why fluorescences is present. For the sake of brevity I will concentrate on the importance of photochemical mechanisms.
Photosynthesis is possible because of photosynthetic pigments like chlorophyll. Chlorophyll behaves much like the fluorescent pigments I described earlier except that when the electrons return to their lower energy state, only a small amount of energy is released as fluorescence. The bulk of the energy is transferred as physical (kinetic) energy to be later transformed into stored (chemical) energy. Many fluorescent pigments are known as accessory pigments because they are accessories to photosynthetic pigments. In a way, accessory pigments can be thought of as light converters. Since photosynthetic pigments have a specific range of wavelengths over which they can capture light energy, some of the available light is not immediately harnessed for photosynthesis. In these cases, accessory pigments can capture light outside the range of photosynthetic pigments and they can convert it into wavelengths that are usable by photosynthetic pigments. This is especially true in red algaes which are commonly found in very deep water. In this darker environment, photosynthetic organisms need all the light they can get and the extra energy provided by the fluorescence of accessory pigments critical. In this context, the accessory pigments are closely associated with the photosynthetic pigments and in reference to corals, the accessory pigments are also called symbiont pigments. The fluorescence of accessory pigments is only detectable with specific equipment because even the small amount of fluorescence from chlorophyll is far brighter. When photosynthetic organisms are found in very bright environments, they are at risk of being stressed by an overabundance of light. The excess light can actually inhibit photosynthesis and in the case of UV, the light can be physically harmful in high doses. Once again, fluorescent pigments can be used to alter how the organism is affected by the light but in this case, the fluorescence is emitted at wavelengths that are not harmful or inhibitive to photosynthesis. Although most symbionts will have some suite of photoprotective pigments, the most visible pigments are those that belong to the host and these are the ones that most coral aquarists try to encourage by the use of intense lights.
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