Adaptation of Marine and Estuarine Organisms

Abstract

The intertidal zone can be considered as the zone of intersection between land and the sea, which remains inundated during high tide and becomes naked (exposed to air only) during low tide. The region may be rocky (Fig. 8.1), sandy (Fig. 8.2) or muddy (Fig. 8.3).

Annexure 8A: Signals of Salinity Fluctuation in Nypa fruticans Seedling Growth, a Case Study from the Lower Gangetic Plain

8.1.1 1. Introduction

The mangrove community is greatly affected by climate change-induced salinity fluctuation, dilution by precipitation and run-off, temperature oscillation, atmospheric carbon dioxide concentration, etc. Of all the outcomes from changes in the atmosphere’s composition and alterations to land surfaces, relative sea level rise may be the greatest threat (Field 1995; Lovelock and Ellison 2007) as the event increases the soil and aquatic salinity often beyond tolerance of the sensitive mangrove species. Excessive saline conditions retard seed germination and impede growth and development of mangroves. Indian Sundarbans, the famous mangrove chunk of the tropics, is gradually losing Heritiera fomes (commonly known as Sundari) owing to increase of salinity in the central sector of the delta complex around the Matla River (Mitra et al. 2009a). Reports of alteration of growth in mangroves due to difference in salinity between western and central sectors of Indian Sundarbans are available (Mitra et al. 2004). However, no study has yet been carried on the effect of salinity fluctuation on the photosynthetic pigments and carotenoid level of Nypa fruticans under culture conditions from this part of the Indian subcontinent. The effects of salinity on mangroves have been studied in relation to antioxidative enzymes (Takemura et al. 2000; Parida et al. 2004b), leaf structure, rates of transpiration, stomatal conductance and rates of photosynthesis (Santiago et al. 2000; Parida et al. 2004a) and changes in chloroplast structure and function (Parida et al. 2003). Tanaka et al. (2000) reported that Na⁺/H⁺ antiport catalysed exchange of Na⁺ for H⁺ across the vacuolar membrane of the cells of Bruguiera sexangula offered tolerance to ionic stress imposed by NaCl and this mechanism was important for cellular salinity adjustments. Also, the mechanism of acclimation to salt in mangroves was suggested to be linked to the changes in the vacuolar size in B. sexangula (Hotta et al. 2000). Further, one of the biochemical mechanisms by which mangroves counter the high osmolarity of salt was accumulation of compatible solutes (Takemura et al. 2000).

In this paper, we present the effect of salinity on pigments in Nypa fruticans under hydroponic culture with an aim to obtain insights into the changes in chlorophyll and carotenoid level with salt acclimation. Such study is important from the point of sea level rise and subsequent saline water intrusion into the islands of Indian Sundarbans as the lower Gangetic delta complex is extremely vulnerable to climate change-related effects owing to its location below the mean sea level and experiencing a sea level rise of 3.14 mm/year. Moreover unlike other mangrove species, Nypa fruticans prefer extremely low saline condition and hence can act as signature of climate change-induced sea level rise and subsequent intrusion of saline water into the islands.

8.1.2 2. Materials and Methods

8.1.2.1 2.1 Plant Materials and Culture Conditions

Seeds of Nypa fruticans were collected from Sundarbans mangrove system of India. Seedlings were raised in the laboratory condition by diluting the water collected from the lower stretch of Sundarbans adjacent to Bay of Bengal in the southern part (salinity = 30 psu) under photosynthetically active radiation (PAR) of 1220–1236 μmol m⁻² s⁻¹. Two-month-old healthy seedlings were selected for hydroponic culture in Hoagland’s nutrient medium (pH = 5.8–6.0). The preliminary experiments were carried out in the selected species at five different salinities (2 psu, 5 psu, 10 psu, 15 psu and 20 psu) in order to determine the optimum range of salinities in context to photosynthetic pigments and carotenoid. The cultures were aerated continuously with an air bubbler. The hydroponic cultures were maintained in a culture room under a 14 h photoperiod at PAR of 300 μmol m⁻² s⁻¹, 26 ± 3 °C and 80 % RH. The culture medium was changed every 7 days. Leaves were harvested at 7-, 14-, 21- and 30-day intervals to measure the pigment concentrations.

8.1.2.2 2.2 Extraction and Estimation of Pigments

We followed the method of Porra et al. (1989) for chlorophyll estimation and Arnon (1949) for carotenoid estimation in the leaves of Nypa fruticans. The procedures for pigment estimation are shown here in flow charts.

8.1.2.3 2.3 Statistical Analysis

Statistical analysis of the results was carried out according to Duncan’s multiple range tests. Data were also subjected to analysis of correlation coefficient (r) in order to evaluate the interrelationship between salinity and selected pigments following the method of Sokal and Rohlf (1995).

8.1.3 3. Results and Discussion

All the collected seedlings of Nypa fruticans tolerate maximum salinity up to 15 psu and could be maintained for more than 30 days. On exposure to salinity of 20 psu, the leaves began to fall off after a week or so, and thus all the experiments were done up to 30 days in the salinity level 2, 5, 10 and 15 psu treated plants. The unhealthy conditions of the experimental seedlings of Nypa fruticans at 20 psu may be attributed to their ambient salinity in the western sector of deltaic Sundarbans region from where they were collected which usually ranges between 2 psu and 10 psu (Mitra 2000). Such a low saline belt in the western part of Indian Sundarbans may be attributed to freshwater supply from the Ganga–Bhagirathi channel, which originates from the Gangotri glacier of the Himalayan range.

The concentrations of chlorophyll and carotenoid pigments decreased significantly with the increase in salinity. The total chlorophyll expressed, on unit fresh wt. basis, decreased by 63.44 %, 73.33 %, 63.39 % and 63.89 % at 7-, 14-, 21- and 30-day intervals, respectively. The Chl a:b ratio in the plant, however, remained almost constant for the species and varied only marginally during the period under observation. In our experiments with differential salinity exposure, the Chl a:b ratio yielded a value of 3.00–3.41. It, thus, appears that high salinity did not affect Chl a:b ratio even though the total chlorophyll content decreased at high salt concentration. A similar trend in carotenoid content, expressed in fresh wt. basis, was observed. The pigment decreased by 36.84 % at the end of 7 days, 33.33 % at the end of 14 days, 33.33 % at the end of 21 days and 27.78 % at the end of 30 days. The decrease of the selected pigments with aquatic salinity is statistically significant. The decrease in chlorophyll content at higher salinity might possibly due to changes in the lipid protein ratio of pigment–protein complexes or increased chlorophyllase activity (Iyengar and Reddy 1996). Our results agree with several reports of decrease content of chlorophyll and carotenoids by salinity as reported in a number of glycophytes (Gadallah 1999; Agastian and Kingsley 2000). As the Chl a:b ratio remained unaffected at high saline condition in the selected species, it appears that the light-harvesting complexes (LHCs) of thylakoid membranes are little altered by salt exposure.

The adverse impact of salinity on leaf chlorophyll of mangrove species significantly affects the rate of photosynthesis. Various studies have shown that a number of mangrove species grow best at salinities between 4 psu and 15 psu (Connor 1969; Clough 1985; Downton 1982; Burchett et al. 1984; Clough 1984). Till date there have been few studies on the effect of salinity on photosynthetic gas exchange in mangroves. Clough (1985) stated in his communication that the rate of light-saturated photosynthesis decreases with increasing salinity of ambient media, attributing this to co-limitation of assimilation rate by stomatal conductance and photosynthetic capacity in response to differences in water status induced by the various salinity treatments. Thus, on the evidences available so far, it is most likely that salinity exerts its effect on photosynthesis mainly through changes in leaf water status and this study confirms that the photosynthetic process may be affected at high saline condition due to decrease in chl a and b concentrations in mangroves. The present study is different from several works as the salinity of water has been altered naturally (through rainwater dilution) keeping all the constituent salts of brackish water constant unlike several previous studies where the plants were exposed to different NaCl concentrations (Mishra and Das 2003; Netondo et al. 2004) that are not the real image of ambient seawater.

Our results show that Nypa fruticans of Indian Sundarbans region can easily be propagated under low salinity conditions. At 15 psu, the plants become acclimated to salt after 1–2 weeks of exposure, but at 20 psu the seedlings could hardly adapt.

Indian Sundarbans and its adjacent estuaries at the apex of Bay of Bengal are one of the less studied regions of the world ocean in context to impact of rising salinity fluctuation on mangrove floral community, although the region sustains the 5th largest mangrove chunk in the world (2120 km² in the Indian part and 4500 km² in the Bangladesh part). The present study is extremely important from the point of view of rising salinity in the central sector of Indian Sundarbans over a period of two decades (Mitra et al. 2009b) due to complete obstruction of the freshwater supply of Ganga-Bhagirathi-Hooghly River as a result of heavy siltation since the late fifteenth century (Chaudhuri and Choudhury 1994) and rising sea level (Hazra et al. 2002) at the rate of 3.14 mm/year, which is higher than the global average sea level rise of 2.12 mm/year. The pigments, being the key machinery in regulating the growth and survival of the mangroves, require an optimum salinity range between 4 and 15 psu (Downton 1982; Burchett et al. 1984) for proper functioning. Nypa fruticans, the freshwater-loving mangrove species, prefers an optimum salinity between 2 and 5 psu (Mitra et al. 2004). It appears that the growth of the species would be better if freshwater of the western sector of Indian Sundarbans is channelized to the central sector.

8.1.3.1 Acknowledgments

The financial assistance from the Clean Blue Planet Consultancy Services, Kolkata, is gratefully acknowledged.

Table 8B.1 Effects of different salinities on pigment concentrations in Nypa fruticans

Table 8B.2 Interrelationships between salinity and selected pigments in Nypa fruticans