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Photosynthesis of Seagrass Cymodocea Serrulata (Magnoliophyta/ Potamogetonales/Cymodoceaceae) in Field and Laboratory

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Photosynthesis of Seagrass Cymodocea Serrulata (Magnoliophyta/ Potamogetonales/Cymodoceaceae) in Field and Laboratory Indian Journal of Marine Sciences Vol. 30 (4), December 2001, pp. 253-256 Short Communication Photosynthesis of seagrass Cymodocea serrulata (Magnoliophyta/ Potamogetonales/Cymodoceaceae) in field and laboratory M K Abu Hena *, K Misri, B Japar Sidik, O Hishamuddin & H Hidir Department of Biology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor D E, Malaysia Received 27 February 2001, revised 20 August 2001 In situ photosynthetic study for seagrass Cymodocea serrulata at two depths (0.5 m, 2.0 m) at Port Dickson, Negeri Sembilan was conducted. The photosynthetic rate at 0.5 m was comparatively higher (0.476±0.080 mg O2/hr/g fr wt or 2 2 0.571±0.182 μg O2/hr/cm ) than at 2.0 m depth (0.292±0.030 mg O2/hr/g fr wt or 0.426±0.135 μg O2/hr/cm ). Respiration rate was not significantly difference at the two depths. Laboratory study showed that the rate of photosynthesis varied with light intensity, exhibiting saturation at 200-800 μmol/m2/sec with a light compensation point at 20-40 μmol/m2/sec. The in situ light measurement recorded at 2.0 m depth was 108.33±9.18 μmol/m2/sec, which is comparatively higher than those at compensation light point, which suggests that this seagrass may inhabit the depth more than 2.0 m. However, based on field observation, this seagrass was only found at depth of 1.5-2.0 m HWL. Seagrass is a productive component in shallow marine This study will reveal the possible contribution of ecosystems that contribute significantly to the coastal light on this intertidal species in one of the seagrass water carbon balance1. In many coastal areas, bed at Port Dickson, Malaysia. seagrasses form extensive meadows, and are The present study was conducted at Port Dickson, recognised to be important in stabilising sea floor2. Negeri Sembilan, Malaysia. It is an inshore tidal area The growth, distribution and abundance of seagrasses along the straits of Malacca (lat. 2° 27′ N ; long. 101° are influenced by current regime3, nutrient 4 5 6 51′ E). Presently, in situ photosynthesis study at availability , light intensity , water temperature and 7 different depths was conducted under natural light salinity ranges where they are growing. The seagrass biomass generally decreases with increasing depth intensity from 1100 to 1400 hrs. Shoots of seagrass of due to light attenuation and the vertical distribution of this species were collected and placed in the glass different seagrasses also depends on different light cylinder (30 cm height, 2.6 cm diameter) filled with intensity9,10. seawater. The mouths of cylinder were closed with The seagrass bed of Batu Tujuh (Port Dickson), rubber stopper ensuring that no air bubble was which consists seven of the 13 seagrass species present. Some of the cylinders were wrapped with reported from Malaysia11,12 are distributed along a aluminium foil to generate the dark condition for dark depth gradient ranging from intertidal zone down to respiration. Ambient seawater was used for both light about ≥6 m. Among the seven seagrasses, C. serrulata and dark bottle experiments. Three replicates were (Magnoliophyta/Potamogetonales/ Cymodoceaceae) used at each depth for both photosynthesis and grows in intertidal area and never found in deeper respiration measurement. Other glass cylinders were area with Halophila decipiens and big leaves variant used as blanks including seawater without plants to Halophila ovalis in this seagrass bed12. Therefore, it is detect the water photosynthesis and respiration by assumed that light availability is a contributing factor phytoplankton and bacteria. All cylinders were that controls the penetration of C. serrulata in deeper incubated for 3 h at 0.5 m and 2.0 m depth of area in this seagrass bed. Hence, this study was seawater. After incubation for 3 h, the oxygen undertaken to detect the rate of photosynthesis of C. produced or consumed was detected by oxygen serrulata at different depths and the adaptational electrode methods (Rank Brothers Limited, UK). The responses of this seagrass to different light intensities. light intensity was determined by using a light sensor (LICOR, Model 189). For the light response of ⎯⎯⎯⎯ *Corresponding author photosynthesis study, experiment was carried out in E mail: [email protected] the laboratory immediately after collection of 254 INDIAN J MAR. SCI., VOL. 30, No. 4, DECEMBER 2001 specimens. The rate of photosynthesis was determined (based on leaf area). This could possibly be due to 13 as O2 evolution . About 1.5 to 2.0 cm long leaf difference in light quality at various depths as a result segment was placed in the cuvet chamber. Three of light absorption by water molecule and various replicates were used for this detection and the mean suspended matter in the water body. Since samples value was used. The photosynthesis measurement was used in the experiment were collected from the same carried out at 28°C with the light source provided by locality, the variation in sample could be ruled out. 250 watt halogen lamp. Light intensity (20 - 1600 In contrast, respiration rates for both fresh leaf μmol/m2/sec) was varied by adjusting the distance of tissue and leaf surface area of this species were not light source from the chamber. Total chlorophyll significantly different (t-test, P > 0.05) between the content was measured by the procedure described by two depths (Fig. 1). Respiration remains Arnon14. approximately the same provided that temperature 15 Photosynthesis is a process of energy fixation that and other factors are essentially unchanged , which is strongly affected by environmental factors, support the present finding. The normal oxygen temperature and light intensity. The rate of requirement for respiration of C. serrulata was almost photosynthesis of C. serrulata was higher at 0.5 m equal to the Halophila stipulacea (0.20 mgO2/hr/g dry 16 than at 2 m (Fig. 1). This difference could be weight . The respiration rates for other seagrasses attributed to higher light intensity at the depth of 0.5 Halophila ovalis and Halodule uninervis were m than 2 m (Fig. 2). However, the reduction of 0.92±0.13 and 0.34±0.13 mgO2/hr/g dry weight, 16 photosynthesis at 2 m depth was not consistent with respectively , with a much oxygen requirement when the light attenuation. The light attenuation was almost compared to C. serrulata. linear (y = -132.33x+363.46, r2 = 0.974, P < 0.05) In laboratory study of C. serrulata, the maximum photosynthetic rate recorded was 39.86±9.57 μg with the depth to 2 m below the water surface. The 2 light intensity at 2 m reduced to around 72% of the O2/min/g fr wt, 0.062±0.02 μg O2/min/cm leaf area light at 1 m. However, the photosynthetic rate at 2 m or 40.94±6.54 μg O2/min/mg chlorophyll at the light intensity of 800 μmol/m2/sec (Fig. 3). No net reduced only by about 39% (based on fr wt) or 26% photosynthesis was observed at 20 and 40 μmol/m2/sec. Photosynthetic rates decreased gradually when the light intensity increased above 800 μmol/m2/sec. The light compensation of C. serrulata was at the light intensity of 20-40 μmol /m2/sec. Clarke15 stated that at light intensities below this value, photosynthesis may still go on but the plant cannot survive because the energy lost due to the activities of catabolic process represented by respiration, which exceed the gain in energy, brought Fig. 1⎯Photosynthesis and respiration rate at 0.5 and 2.0 m Fig. 2⎯Light intensity of different depths during the experimental depths of seagrass Cymodocea serrulata⎯A) based on fresh time (July 10, 1999) of seagrass Cymodocea serrulata of Batu weight, B) based on leaf surface area Tujuh seagrass bed, Teluk Kemang, Port Dickson. SHORT COMMUNICATION 255 at 1 cm below the surface during the study period was around 370 μmol/m2/sec. This is far below the minimum intensity that may cause photoinhibition. It is predicted that based on laboratory experiments, C. serrulata is capable to carry out photosynthesis at very shallow water such as low tide, as well as below 2 m depth. On the other hand, this seagrass could also penetrate deeper area with Halophila decipiens and big leaves variant Halophila ovalis in this study area12. However, in the present study area the limited intertidal distribution of C. serrulata could possibly be affected by other environmental factors i.e. substratum, current movement or other factors. Authors are grateful to Malaysian Government for financial support (IRPA) project no. 08-02-04-019. References 1 McRoy C P & McMillan C, Production ecology and physiology of seagrass, in Seagrass ecosystems: A scientific perspective, edited by C P McRoy & C Helfferich, (Marcel Dekker, New York, Basel) 1977, pp. 53-87. 2 Fonseca M S & Fisher J S, A comparison of canopy friction and sediment movement between four species of seagrass with reference to their ecology and restoration, Mar Ecol Prog Ser, 29 (1986) 15-22. 3 Fonseca M S & Kenworthy W J, Effects of current on photosynthesis and distribution of seagrasses, Aquat Bot, 27 (1987) 59-78. 4 Short F T, Effects of sediment nutrients on seagrass: Literature review and mesocosm experiment, Aquat Bot, 27 Fig. 3⎯Photosynthesis rate of seagrass Cymodocea serrulata at (1987) 41-57. different light intensities,⎯A) based on fresh weight, B) based on 5 Dennison W C & Alberte R S, Photosynthetic responses of leaf surface area, C) based on chlorophyll content Zostera marina L. (eelgrass) to in situ manipulations of light intensity, Oecologia, 55 (1982) 137-140. by the anabolic process of photosynthesis.
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