Ultraviolet Radiation On Photosynthetic Pigments Of Marine Flora Biology Essay

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There are three main classes of photosynthetic, pigments, chlorophylls (ch1) (including bacteriochlorophylls, Bch1), carotenoids and phycobilins (Phycobilipoteins, PBPS). All species capable of carrying out photosynthesis contain one or more type of chlorophyll or bacterio-chlorophyll and carotenoids. Phycobilins are found only in the red algae and blue green algae or cyanobacteria.

Chlorophyll

The unique reactions of photosynthesis are mediated by a class of molecules termed chlorophyll. Green algae and higher plants contain two types of chlorophylls, Chl a and Chl b. Both are soluble in organic solvents. Chl a is present in all photosynthetic organisms in which there is evolution of oxygen during photosynthesis.

The following types of Chl a are known. Chl a 660, Chl a 670, Chl a 680, Chl a 685, Chl a 690 and Chl a 700 - 720. The short wavelength forms of Chla are mainly presented in PSII, while the long wavelength forms are mainly present in PSI. Chl b is present in all green algae and higher plants. Most of the Chl b is present in PSII. There are two forms of chlorophyll-b, Chlb640 and Chlb650.

Carotenoids

The carotenoids are found in almost all photosynthetic organisms. They are yellow and orange pigments which are soluble in organic solvents. There are two types of carotenoids, namely carotenes and carotenols. Carotenes, eg. α-carotene and hydrocarbons. Most of the carotenes are present in PSI. Carotenols (Xanthophylls) are alcohol. Most of the xanthophylls are present in PSII. Carotenoids, chlorophyll, xanthophylls etc. are widely distributed in plants.

In the last two decades, there has been a great concern, about the depletion of stratospheric ozone layer (O3), caused primarily by the extensive release of chlorofluro carbon compounds and other ozone depleting substances. Stratospheric ozone, a thin gaseous layer, strongly attenuates ultraviolet (UV) radiation shorter than 310 nm. Reduction in stratospheric ozone allow the penetration of more solar UV radiation into the atmosphere to reach the earth's surface for every 1% of ozone loss, 1 - 2% more UV reaches the ground (Santhi, 2004).

Due to optical characteristics of ozone, it is the UV-B radiation, which is likely to increase at the earth's surface as a consequence of a decrease in stratospheric ozone concentration (Frederick et al., 1989). The effects of UVB on aquatic ecosystems are strongly dependent on the optical properties of the water body (Holm-Hansen et al., 1993 ; Hanelt et al., 2001). In coastal seawaters, UV radiation is strongly attenuated due to dissolved organic matter and this depends largely on the input of this material from the terrestrial ecosystem.

Most of the studies pertain to the effect of UV-B radiation only without taking into consideration, of UV-A, is a another component of solar UV radiation. UV-B exhibits both positive and negative effects on plant photosynthesis (Wellmann, 1983), whereas UV-B activates gene expression from PSII reaction centre proteins (Christopher and Mullet, 1994) it inflicts damage to photosynthetic apparatus (Joshi et al., 1997 ; Turcsanyi and Vass, 2000).

Seaweeds (also referred to as 'Marine Macroalgae') represent key components within coastal ecosystems (Luning, 1990). They are harvested by man for centuries, particularly in Japan and China where they form a part of the stable diet. The uses of seaweeds as food, fodder and manure are well known in many countries. These seaweeds are autotrophic, non-vascular, benthic macroscopic algae which forms an important component of the marine living resource. They are available largely in shallow coastal waters wherever there is a substratum on which they can grow and flourish. Seaweeds are marine plants because they use the light energy to produce carbohydrates from carbon dioxide and water.

Seaweeds are chlorophyll bearing plants with a plant body showing no differentiation into true tissues. It never form true, roots, stems, leaves and so it called a thallus on fond. The thallus has no elements for the transport of fluids. Seaweeds exhibit a great diversity in organization of their body. These are co-enocytic forms on siphonaceous form in which the cells are multinucleate without cross walls so that the entire plant consists of a variously ramified hallow tube.

Marine macroalgae contain more than 60 trace elements in a concentration much higher than terrestrial plants. They also contain protein, iodine, bromine, vitamins and substances of stimulating and antibiotic nature. Seaweeds are the only source for the production of agar, alginate and carrageenan. The phytochemicals are extensively used in various industries such as food, confectionery, textile, pharmaceutical, dairy and paper industries mostly as gelling, stabilizing and thickening agents. Apart from these, biochemicals other products such as mannitol, laminarin and fucadin are also obtained from marine algae. New attempts are being made for screening pharmaceutically active compounds from seaweeds. Seaweeds are applied as manure in agriculture plants, which can maintain a high level of available nitrogen in soils. Therefore to call these marine plants 'weeds' is incorrect, because they are essential in nature and directly valuable to humans (Santhi, 2004).

Based on their pigmentation, morphology and anatomy, seaweeds are classified into three groups viz. Chlorophyceae (Green algae), Rhodophyceae (Red algae) and Phaeophyceae (Brown algae). As primary producers these green, red and brown algae serve a multitude of ecosystem functions. Pigments are present in the crude extract of these marine macroalgae and are represented by chlorophylls and carotenoids (Smith, 1964).

Seaweeds are chlorophyll bearing plants with a plant body showing no differentiation into true tissues. chlorophylls in marine algae are of three types namely, chlorophyll-a, chlorophyll-b and chlorophyll-c. Basically chlorophyll is a large conjugated macro-cycle containing four convoluted nitrogen sub-cycles. The conjugated double bonds or electron system result in strong absorption of light, which in convoluted ring structure and the inner nitrogen atom produce a rich optical spectrum and even rich supply of redox levels. The chelated metal in chlorophyll is Mg2+ ion (Santhi, 2004).

Carotenoids are a class of natural fat soluble pigments generally found in plants, algae and photosynthetic bacteria, where they play a critical role in the photosynthetic process (Joseph et al., 2000). They are commonly thought of as plant pigments, but also occur widely in microorganisms and animals. Among the various classes of natural pigments such as the anthocyanins, flavonoids, pyrroles, blood and bile pigments ; the carotenoids are the most widely spread and structurally diverse pigment agents. To date, about 600 different natural carotenoids have been identified (Latscha, 1990 and 1991). Carotenoids are also known as lipochromes, produced in nature via isoprenoid pathway. Carotenoids are yellow to orange red pigments that are ubiquitous in nature (Guyomalch et al., 2000). Carotenoids also play an important potential role in health by acting as biological antioxidants, protecting cells and tissues from the damaging effects of free radicals and singlet oxygen (Di Mascio et al., 1989).

Carotenoids are synthesized by all photosynthetic organisms from bacteria to plants where they play atleast three essential functions: first, they act as accessory light harvesting pigments by absorbing light in the 450 - 570 nm regions. Second, they are important for the assembly and stability of some of light harvesting complexes. Finally they operate as photoprotectors by directly quenching both triplet excited chlorophylls and singlet oxygen.

Carotenoids are structurally related to retinol and b-carotene the main source of vitamin A for animals (Latscha, 1991). Carotenoids are defined by their chemical structure. The majority carotenoids are derived from a 40-carbon polyene chain, which could be considered the backbone of the molecule. This chain may be terminated by cyclic end groups (rings) and may be complemented with oxygen containing functional groups (Joseph et al., 2000). The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls.

The distinctive pattern of alternating single and double bonds in the polyene backbone of carotenoids is what allows them to absorb excess energy from other molecules, while the nature of the specific end groups on carotenoids may influence their polarity. The former may account for the antioxidant properties of biological carotenoids, while the latter may explain the differences in the ways that individual carotenoids interact with biological membranes (Britton, 1995).

The natural roles of carotenoids is colouration, photosynthesis and photoprotection are well established and of major biological importance. The structure of the carotenoid ultimately determines what potential biological functions that pigments may posses. The functions, applications and uses of carotenoids are due to the light absorbing properties of the polyene chromatophore (Joseph et al., 2000). Carotenoids are responsible to provide bright colouration, serve as antioxidants and can be a source for vitamin A activity (Ongand, 1992 and Britton et al., 1995). They also play an important role in medical as well as in industrial field.

The yellow orange carotenoid pigments of plants decrease during the senescence or the death phase of plants but do not always decrease to the point of near extinction as do the chlorophyllic pigments (Leffingwell, 2001). Carotenoids transfer most of the absorbed energy to12 chlorophyll-a for photosynthesis. In addition, the carotenoids protect the chlorophyll molecules from photooxidation under strong light. The presence of carotenoids in thylakoids is normally made by the co-existence of chlorophylls. All the chlorophylls and most of the carotenoids present in the thylakoid membrane are non covalently bound to protein forming pigment protein complexes (Thornber et al., 1979).

It is well known that light is one of the important physical factor which predominantly influences the architecture of the photosynthetic apparatus. In the process of photosynthesis, photoautotrophic organisms (like seaweeds) convert light energy into chemically bound energy, which is used for biomass production ; as a basic for all heterotrophic organisms. Changes in irradiance and light quality can either promote photosynthesis, but can also inhibit many biological processes, if radiation becomes excessive, or if short wavelength radiation with high energy content, such as UV radiation, is absorbed by biomolecules. Consequently, damage to important components in plant metabolism results in reduced photosynthetic and general metabolic activity and hence lead to a decrease in biomass production (Santhi, 2004).

In seaweeds, carotenoids, chlorophyll, xanthophylls are widely distributed. Carotenoids are responsible to provide bright colouration, serve as antioxidants and can be source for vitamin A activity (Ong and Tee, 1992 ; Britton et al., 1995). Carotenoids are responsible for many of the red, orange and yellow hues of plant leaves, fruit and flowers as well as the colours of some birds, fish and crustaceans. E.g. carotenoids colouration : orange colour of carrots and citrus fruits and the pink of flamingoes and salmon (Pfander, 1992).

Carotenoids are yellow to orange red pigments that are ubiquitous in nature (Guyomarch et al., 2000). Carotenoids also play an important potential role in human health by acting as biological antioxidants, protecting cells and tissues from the damaging effects of free radicals and singlet oxygen (Di Mascio et al., 1989).

Biological functions of carotenoids

Growth

Vitamin activity Cancer prevention

Protection Biological function s Water balance

Antioxidants Immune response

The cosmetic and food industries take advantages of the antioxidant properties of some carotenoids, which can load to their inclusion into nutraceutics or drug, since some carotenoids may prevent diseases like cancer or atherosclerosis, stimulate antibody response (Astrong, 1997 ; Clinton, 1998 ; Johnson and Schroeder, 1995).

Carotenoids have important functions in photosynthesis, nutrient and protection against photoxidative damage. Due to their enormous importance within coastal ecosystems a decrease in seaweed abundance related to environmental change e.g. under increased UV-irradiance. The physiological and ecological effects of present UV-B-irradiance is a precondition in order to be able to estimate future consequence of ozone depletion and increased UV-B levels.

Solar radiation is the most important prerequisite for life on earth. In the process of photosynthesis, photoautotrophic organisms (like seaweeds) convert light energy into chemically bound energy, which is used for biomass production ; as a side effect, molecular oxygen is generated as a basis for all heterotrophic organisms. Changes in irradiance and light quality can either promote photosynthesis, but can also inhibit many biological processes if radiation becomes excessive (Barber and Anderson, 1992), or if short wavelength radiation with high energy content, such as UV-B radiation, is absorbed by biomolecules (Vass, 1997). Consequently, damage to important components in plant metabolism result in reduced photosynthetic and general metabolic activity and hence lead to a decrease in biomass production.

Ever since the discovery of stratospheric ozone depletion in the Antartic in the 1970s (Farman et al., 1985), serious concerns have arisen about the impacts of increasing UV radiation on biosphere (Mandronich et al., 1998 ; Bjorn et al., 1999). Ozone is predominantly generated in the low latitudes, by photolysis of molecular oxygen. In the stratospere, ozone molecules are subject to UV-mediated photolysis and may also be degraded due to the reaction within catalytic cycles with No, Cl or Br serving as catalysts (Langer, 1999). The concentration of these compounds in the atmosphere increases mainly due to anthropogenic emissions, thus leading to ozone depletion.

Ultraviolet radiation includes the wavelengths below those visible to the human eye. Thus spectral range is according to the CIE definition divided into 3 wave bands : 315 - 400 nm UVA, 280 - 315 nm UVB and 190 - 280 nm UVC, which does not reach the earth's surface as it is completely absorbed on its way through the atmosphere. Due to the optical characteristic of ozone, it is the UVB range, which is likely to increase at the earth's surface, as a consequence of a decrease in stratospheric ozone concentration. Calculation based on the absorption characteristics of O3 indicate that a 10% decline in column ozone would result in an approximate 5% increase of surface irradiance at 320 nm while the same decline would be accompanied by a 100% increase at 300 nm (Frederic et al., 1989).

Biological effects of UV-B

The effect of UV-B exposure on the biological systems are manifold and reach from the molecular to the organism level, thereby affecting growth and production and consequently, ecosystem structure and function.

The physiological effects are also reflected on the ultrastructural level. UV-B radiation can lead to dramatic changes of the fine structure of chloroplast and mitochondria. Mild UV stress leads to a wrinkled appearance of the thylakoids, lumen dilatations and damage of the outer membranes. In the mitochondria, a swelling of the gistal is often observed (Poppe et al., 2003 ; Holzinger et al., 2004).

On the organism level, the effects mentioned above can result in reduced growth and production, as shown in higher plants, seaweeds, phytoplankton and ice algae (Caldwell, 1971 ; Worrest, 1983 ; Ekelond, 1995 ; Karentz et al., 1991a, b ; Makarou, 1999 ; Aguilera et al., 2000 ; Altamirano et al., 2000a, b). Other effects include the impairment of reproductive success on may even been lethal consequences. Consequently, all aspects mentioned may also affect ecosystem structures (Holm-Hansen et al., 1993 ; Johanson et al., 1995 ; Caldwell et al., 1998).

CONCLUTION

Now a day, the marine plants such as seaweeds, seagrasses, mangroves and microalgae etc., are found to be the source for food, medicines, oil and other economically important products. In this context, the world countries should conserve these marine flora from the deleterious effects of ultraviolet radiation as well the International nature conservation bodies should lend hands to hand in order to protect the most precious marine flora and fauna for the benefit of the future generations.

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