Biological role of light for marine environment

Biological role of light for marine environment

Hanna Szpilewska, Marine Biology Center, Polish Academy of Sciences
ul.sw.Wojciecha 5, 81-347 Gdynia, Poland
e.mail: hanna@cbmpan.gdynia.pl

Seas and oceans cover approximatelly 70% of the Earth’s surface, corresponding to the content of water in human body. Marine organisms produce more than half of Earth’s oxygen, so necessary for aerobic respiration meaning a life of many organisms. During the day time oceans and seas are influenced by solar radiation. 98% of the sun light is used for heating of water, what consequently causes circulation of water masses, evaporation, circulation of air masses together with water vapour and finally a heating of colder Earth’s regions and a circulation of water in nature. From remaining 2% of sunlight not all light is able to penetrate deeply in a sea. Irradiance transmittance depends on the wind velocity, solar attitude, presence or absence of clouds.

Solar radiation energy is absorbed by different particles and yellow substances present in water. Thus comparing conditions in different seas daylight irradiance spectra may vary, for instance reaching maximum at 450 nm for clear oligotrophic waters of Sargasso sea and 530 nm for eutrophic waters of Baltic with similar irradiation transmission at sea depths of 200 and 20m correspondingly.

Seawater is more transparent to visible light to compare with other wavelengths of spectra.

Solar radiation absorbed by Earth is approximatelly equal to the energy emitted by Earth in the infra-red part of the radiation spectrum.

Environmental conditions (temperature, salinity, pressure, darkness) seem to be very similar at big depths of oceans, independently of geographical location.

Ocean regions may be divided in relation to their irradiation by Sun to sunlit water, „twilight" water and sunless water, which corresponds to the deepest regions of oceans.

Light in marine environment plays a role in the following processes: Photosynthesis, photodegradation, hotodamage, photoreactivation, phototaxis and in bioluminescence.

Sun light in visible range of 400-700 nm is used as a source of energy in photosynthesis. In this process carbon dioxide and water are substrates yielding simple sugars and oxygen as products. Process of photosynthesis differs according to different pigments involved. These pigments (chlorophyll a, b, c,d, carotenoids, xanthophyls, phicobilins...) are specifically used by different species, thus enabling them to absorb light of specific frequency. Photosynthesis supplies energy for cellular work.

4% of total Sun’s energy is in a range of UV light (200-400 nm). In surface waters there is a lot of dissolved organic matter with humic substances (being a product of degradation of plant and animal material) as the major fraction. UV light is absorbed very efficiently by them due to conjugated unsaturated bonds, presence of free electron pairs on heteroatoms. As a result of this absorption compounds of lower molecular size (aliphatic and aromatic mono- and dibasic acids, formic, acetic acids...) appear. They can be used directly by some marine organisms. On the other hand photoreactants like single oxygen, peroxyradicals, hydrogen peroxide, solvated electrons and hydroxyl radicals could be deletorious for living organisms, damaging biological molecules.

There are some processes of DNA damage repair which are induced by UV light irradiation, they are observed in whole spectra of living organisms from simple bacteria, sponges to higher organisms. Also another kind of process is observed in marine environment – photoreactivation, e.g. DNA repair in the presence of light.

Actually all organisms in environment react on light, being attracted or escaping from it. Even in organisms not having vision organs, some pigments were found, which probably play a role in phototaxis processes. Vision organs in marine organism differ in their complexicity, starting from very simple ones, which allow to recognize general change in light intensity (giving information for instance about a movement of other organisms in water) to very sophisticated sensitive eyes recognizing shape, distance of the object, light polarization etc.

Life in marine environment undergoes day cycle of life. Phytoplankton is located in the day time on the surface of water, where the process of photosynthesis takes place intensively, leading to primary production. Zooplankton moves vertically in water column, being generally (with some exeptions) at the bottom in the day time and at the surface layer at night. Induction of many proteins in marine organisms depends on external light and proceeds in circadian pace. There were found also examples of organisms, which have lucipherins with endonegous control of circadian rtytm, not depending on external light.

Another important source of light in marine environment results from luminescence processes taking place in numerous organisms. There are few distinct roles of bioluminescence in marine environment. Firstly it was postulated in the past that luminescence, which consumes a lot of oxygen, could serve in very remote past, as a defence against oxygen, which was toxic for early earthly organisms. Bioluminescence serves for illumination of the field of vision. Bioluminescence from light organs or eyes of fish, molluscs very often is a result of luminescent bacteria (e.g. Vibrio fisheri) located there. Bioluminescence of every organism posess very specific characteristic (frequency, amplitude). This may serve to recognize individuals of the same species, what may have an important meaning in reproduction. This may also attract prey. Very often bioluminescence is emitted from different parts of body, sometimes even moving along body in a direction of a mouth. Bioluminesce may also frighten enemies. Very often animals escape quickly upon sudden flashes of strong light. Such flashes of light are often induces by mechanical movement of water. There were also some investgations carried on symbhiosis of coral reef and algae, where bioluminescence light emitted by one organism might serve in photosynthesis of another one. It was found only recently, in case of marine bacterium Vibrio harveyi, that bioluminescence may play n important role by providing an internal light for DNA repair processes thus enhancing photoreactivation.

Measurements of bioluminescence may be done nowadays also in situ using special bioluminescent bathyphotometer.

Selected references:

Biology. The Unity and Diversity of Life. (1987) Elsevier.

Dera J. (1992) Marine Physics. PWN, Elsevier.

Dera J. (1995) Underwater Irradiance as a factor affecting primary production, IO PAS, Sopot.

Encyclopaedia of Contemporary Physics (Polish). (1983) PWN, Warszawa.

Engebrecht, J., Nealson K.H., Silverman M. (1983). Bacterial bioluminescence: isolation and genetic analysis of functions from Vibrio fischeri. Cell 32:773-781.

Lagler K.F., Bardach J.E., Miller R.R., May Passino D.R. (1977) Ichtiology, John Wiley and Sons, Inc.

Stryer L. (1981) Biochemistry. Freeman & Co. New York.

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