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Indian Journal of Marine SciencesVol. 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 HidirDepartment 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 (0.4760.080 mg O2/hr/g fr wt or

0.5710.182 g O2/hr/cm2) than at 2.0 m depth (0.2920.030 mg O2/hr/g fr wt or 0.4260.135 g O2/hr/cm

2). 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 mol/m2/sec with a light compensation point at 20-40 mol/m2/sec. The

in situ light measurement recorded at 2.0 m depth was 108.339.18 mol/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

ecosystems that contribute significantly to the coastal

water carbon balance1. In many coastal areas,

seagrasses form extensive meadows, and are

recognised to be important in stabilising sea floor2.

The growth, distribution and abundance of seagrasses

are influenced by current regime3, nutrient

availability4, light intensity5, water temperature6 and

salinity ranges7 where they are growing. The seagrassbiomass generally decreases with increasing depth

due to light attenuation and the vertical distribution of

different seagrasses also depends on different light

intensity9,10.

The seagrass bed of Batu Tujuh (Port Dickson),

which consists seven of the 13 seagrass species

reported from Malaysia11,12 are distributed along a

depth gradient ranging from intertidal zone down to

about 6 m. Among the seven seagrasses, C. serrulata

(Magnoliophyta/Potamogetonales/ Cymodoceaceae)

grows in intertidal area and never found in deeper

area with Halophila decipiens and big leavesvariantHalophila ovalis in this seagrass bed12. Therefore, it is

assumed that light availability is a contributing factor

that controls the penetration ofC. serrulata in deeper

area in this seagrass bed. Hence, this study was

undertaken to detect the rate of photosynthesis ofC.

serrulata at different depths and the adaptational

responses of this seagrass to different light intensities.

This study will reveal the possible contribution of

light on this intertidal species in one of the seagrass

bed at Port Dickson, Malaysia.

The present study was conducted at Port Dickson,

Negeri Sembilan, Malaysia. It is an inshore tidal area

along the straits of Malacca (lat. 2 27 N ; long. 101

51 E). Presently, in situ photosynthesis study at

different depths was conducted under natural lightintensity from 1100 to 1400 hrs. Shoots of seagrass of

this species were collected and placed in the glass

cylinder (30 cm height, 2.6 cm diameter) filled with

seawater. The mouths of cylinder were closed with

rubber stopper ensuring that no air bubble was

present. Some of the cylinders were wrapped with

aluminium foil to generate the dark condition for dark

respiration. Ambient seawater was used for both light

and dark bottle experiments. Three replicates were

used at each depth for both photosynthesis and

respiration measurement. Other glass cylinders were

used as blanks including seawater without plants todetect the water photosynthesis and respiration by

phytoplankton and bacteria. All cylinders were

incubated for 3 h at 0.5 m and 2.0 m depth of

seawater. After incubation for 3 h, the oxygen

produced or consumed was detected by oxygen

electrode methods (Rank Brothers Limited, UK). The

light intensity was determined by using a light sensor

(LICOR, Model 189). For the light response of

photosynthesis study, experiment was carried out in

the laboratory immediately after collection of

*Corresponding authorE mail: hena71@yahoo.com

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INDIAN J MAR. SCI., VOL. 30, No. 4, DECEMBER 2001254

specimens. The rate of photosynthesis was determined

as O2 evolution13. About 1.5 to 2.0 cm long leaf

segment was placed in the cuvet chamber. Three

replicates were used for this detection and the mean

value was used. The photosynthesis measurement wascarried out at 28C with the light source provided by

250 watt halogen lamp. Light intensity (20 - 1600

mol/m2/sec) was varied by adjusting the distance of

light source from the chamber. Total chlorophyll

content was measured by the procedure described by

Arnon14.

Photosynthesis is a process of energy fixation that

is strongly affected by environmental factors,

temperature and light intensity. The rate of

photosynthesis of C. serrulata was higher at 0.5 m

than at 2 m (Fig. 1). This difference could be

attributed to higher light intensity at the depth of 0.5m than 2 m (Fig. 2). However, the reduction of

photosynthesis at 2 m depth was not consistent with

the light attenuation. The light attenuation was almost

linear (y = -132.33x+363.46, r2 = 0.974, P < 0.05)

with the depth to 2 m below the water surface. The

light intensity at 2 m reduced to around 72% of the

light at 1 m. However, the photosynthetic rate at 2 m

reduced only by about 39% (based on fr wt) or 26%

(based on leaf area). This could possibly be due to

difference in light quality at various depths as a result

of light absorption by water molecule and various

suspended matter in the water body. Since samples

used in the experiment were collected from the samelocality, the variation in sample could be ruled out.

In contrast, respiration rates for both fresh leaf

tissue and leaf surface area of this species were not

significantly different (t-test, P > 0.05) between the

two depths (Fig. 1). Respiration remains

approximately the same provided that temperature

and other factors are essentially unchanged15, which

support the present finding. The normal oxygen

requirement for respiration ofC. serrulata was almost

equal to theHalophila stipulacea (0.20 mgO2/hr/g dry

weight16. The respiration rates for other seagrasses

Halophila ovalis and Halodule uninervis were0.920.13 and 0.340.13 mgO2/hr/g dry weight,

respectively16, with a much oxygen requirement when

compared to C. serrulata.In laboratory study of C. serrulata, the maximum

photosynthetic rate recorded was 39.869.57 gO2/min/g fr wt, 0.0620.02 g O2/min/cm

2 leaf areaor 40.946.54 g O2/min/mg chlorophyll at the lightintensity of 800 mol/m2/sec (Fig. 3). No netphotosynthesis was observed at 20 and 40mol/m2/sec. Photosynthetic rates decreasedgradually when the light intensity increased above800 mol/m2/sec. The light compensation of

C. serrulata was at the light intensity of 20-40 mol/m2/sec. Clarke15 stated that at light intensities belowthis value, photosynthesis may still go on but the plantcannot survive because the energy lost due to theactivities of catabolic process represented byrespiration, which exceed the gain in energy, brought

Fig. 1Photosynthesis and respiration rate at 0.5 and 2.0 m

depths of seagrass Cymodocea serrulataA) based on fresh

weight, B) based on leaf surface area

Fig. 2Light intensity of different depths during the experimentaltime (July 10, 1999) of seagrass Cymodocea serrulata of Batu

Tujuh seagrass bed, Teluk Kemang, Port Dickson.

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SHORT COMMUNICATION 255

by the anabolic process of photosynthesis. The lightcompensation of the present study was comparablewith the values of other seagrass reported byDennison9 and Bulthuis17. Pollard & Greenway18found that high light compensation for Cymodoceaserrulata, Thalassia hemprichii and Zosteracapricorni were due to high respiration demand of theplants, which resulted from the high watertemperature. They also found that the lightcompensation points were 80 to 98 mol/m2/sec dueto high water temperature (29-33C) in Australianseagrass beds. In present experiment the temperaturewas 28C throughout the experimental period.

The light response of seagrass C. serrulata showed

increased photosynthesis correspondingly with the

light intensity from 40 to 200 mol/m2/sec and

photosynthesis peaked at light saturation at 200-800

mol/m2/sec. The photosynthesis irradiance (PI)

curve relation revealed that photoinhibition was set

when the light intensity increased beyond

800 mol/m2/sec forC. serrulata. The light intensity

at 1 cm below the surface during the study period was

around 370 mol/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 atvery 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.

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different light intensities,A) based on fresh weight, B) based onleaf surface area, C) based on chlorophyll content

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