In the present study, the chlorophyll content of the selected plant is 0.38 ± 0.01 mg g− 1d.w. in the leaves. In a study carried out by Sen et al. (2017), biochemical analysis revealed similar values of chlorophyll content for different plant types. Different parameters such as biotic and abiotic stress of the studying sites like temperature, drought or aridity, light intensity, salt-induced osmotic stress, contamination of sediment, pollution load, accumulation of pollutants in plant leaves, age of plants etc. can regulate total chlorophyll content of plant species (Achakzai et al., 2017; Katiyar & Dubey, 2001; Kaur & Nagpal, 2017; Nadgorska-Socha et al., 2017; Zhang et al., 2016). In its regular state, chlorophyll undergo numerous photochemical pathways like oxidation, reversible bleaching etc. (Puckett et al., 1973). Reduction in chlorophyll content due to its denaturation caused by gaseous pollutants (such as SOX and NOX) and suspended particulate matter (enters through stomata) leads to the major damage to the plant (Rai, 2016a).
APTI
Hence, the calculated APTI value of the studied species was 22.25 ± 0.12 showing similarity with the APTI values (10.61–31.38) estimated for different plants by Nadgorska-Socha et al. (2017). According to the results obtained, the studied species, Alternanthera ficoidea is a tolerant species, which is anticipated as a potential eco-friendly biomonitoring tool.
Lipid Content
The partitioning of organic compounds into organisms is regulated significantly by its lipid content (Geisler et al., 2012). The stomata or outer cuticular lamellae present in the leaf surface uptake the vapor-phase PAHs and the particle-bound PAHs accumulated on the leaf surface consequently integrated into the lipid-rich cuticle layer (Lehndorff & Schwark, 2004; Simonich & Hites, 1995). Therefore, the cuticle layer covers the aerial parts of the plant act as a significant interface between parts of the plant and its atmosphere. This layer also assists the controlling path in the transportation of soluble organic compounds (Holloway, 1994; Martins et al., 1999). Hence, the lipid content of plant plays critical role in plant PAHs uptake (Simonich & Hites, 1994). In the present study, the lipid content (mean ± SD) of the different parts of the plant showed the following trends: leaf (38.34 ± 1.05 µg/g) > root (23.63 ± 1.51 µg/g) > stem (5.23 ± 0.23 µg/g). In this context, the lipid content of the plant become a key reservoir, as high level of PAHs accumulation within different parts of a mature plant attributed to high lipid content (Wang et al., 2020; Wei et al., 2021).
SEM Analysis
Atmospheric pollutant accumulation is regulated by the characteristics of accumulating surface (Riederer, 1990). Hence, the surface morphology of the plant leaf was studied by Scanning Electron Microscope (SEM of Alternanthera ficoidea plant leaves which reveals rough surface area on both abaxial and adaxial sides which help in retaining of irregular shaped dust particles generated from pollution activities in the studied area (Fig. 2). This might confirm the accumulation of dust laden atmospheric PAHs in the cuticular lipid layer of plant leaf which further enter into other parts of the plant. The study also reveals that dust particles causes blockage of stomatal openings. Stomata are dynamic route of plant and atmospheric interaction and a considerable route to trap atmospheric pollution in plant body most effectively (Kardel et al., 2010). In particular, stomata assist the diffusion of gaseous phase PAHs in plant leaf (Collins et al., 2006). Both abaxial and adaxial surfaces appear as rough probably due to wax crystalloids in ultrastructure.
PAHs in different tissues of Alternanthera ficoidea (L.) and rhizobium
Plants are exposed to environmental contamination by PAHs and are more likely to accumulate PAHs from sediment and atmosphere, which causes the subsequent transfer of these contaminants to higher trophic levels. Hence, plants can play a pivotal role in regulating the fate of PAHs and their remediation in the environment (Aina et al., 2006; Zuo et al., 2006). The distributional pattern of PAHs within the investigated plant leaf, stem, root and associated rhizobium is displayed in Fig. 3.
The total PAHs concentration measured in plant leaf was 420.23 ± 0.60 µg/g (Fig. 3). All the individual congeners were detected in the leaf sample. For instance, the distribution pattern of individual congeners was followed as: PYR > ANT > FLT > B[a]A > ACY > D[a,h]A > ACE > CHR > B[a]P > NAPH > IP > B[g,h,i]P > B[b]F > B[k]F > FLU > PHEN. The HMW PAHs was predominant in comparison to LMW PAHs with the concentration of 284.08 ± 0.01 µg/g and 136.15 ± 0.62 µg/g, respectively (Fig. 4). The volatility and bioavailability of LMW PAHs make them available in the plant leaf (Wild & Jones, 1992). High vapour pressure of LMW PAHs makes them abundant in gaseous phase in troposphere (Atkinson & Arey, 1994). Due to this property, LMW PAHs are susceptible for long-range atmospheric transportation (Majumdar et al., 2017). This might be causing the less concentration of LMW PAHs in the leaf and sediment. The 2-, 3-, 4-, 5- and 6- rings PAHs account for 5.72%, 26.68%, 36.97%, 19.99% and 10.64% of the total concentration respectively. The abundance of all the PAHs with log Koa>6 (except-NAPH) suggests foliar uptake as the primary route of their entry in the plant (Cousins & MacKay, 2001). The deposited particles on plant leaves also act as significant contributor in the desorption of the particle-bound PAHs from particles into the leaf with log Koa >11 including B[b]F, B[k]F, B[a]P, D[a,h]A, B[g,h,i]P and IP (Collins et al., 2006). The physiological parameter like high amount of leaf lipid content (38.34 ± 1.05 µg/g) might enable leaf to absorb high concentration of PAHs as lipophilic PAHs are generally accumulated in the lipid-rich cell. The present findings showed a similar magnitude of PAHs concentration as obtained in our previous research with the plant species Murrya paniculata, carried out in the urban zones of Kolkata affected by heavy traffic load (Mukhopadhyay et al., 2023). The high level appearance of FLT and B[g,h,i]P in the leaf sample indicates high traffic flow in Kolkata, as vehicular exhaust is comparatively enriched with these PAHs (Gerdol et al., 2002). Moreover, the abundance of thermodynamically stable PAHs like FLT, PHEN and CHR in leaves and as well as sediment indicates petroleum as an important source in the study area along with pyrolytic activities such as petroleum, coal, wood and grass combustion. (Magi et al., 2002; Sicre et al., 1987). Hence, plant leaf can be widely used as an appropriate passive sampler in biomonitoring of atmospheric PAHs pollution.
The total PAHs concentration recorded in stem was 230.83 ± 4.87 µg/g (Fig. 3) which is almost half of that in plant leaves. However, the recorded concentration level of PAHs in stem is not showing consistency with its lipid content (5.23 ± 0.23 µg/g) as lipid fraction of a cell regulates the PAHs accumulation predominantly. This might be attributed by the PAHs uptake by translocation via root (from sediment) and/or leaf (from air) (Burken & Schnoor, 1997). But it is worthy to mention that the large portion of this concentration level might be contributed from the leaves as it is difficult for the hydrophobic organic compounds (such as PAHs) to translocate to the stems from the plant roots taken up in its lipid constituent (Liu et al., 2019). The recorded level of LMW and HMW PAHs was 91.30 ± 3.27µg/g and 139.53 ± 1.59 µg/g respectively (Fig. 4). The dominance of HMW PAHs over LMW PAHs just like its distribution available in leaves confirms the route of transfer of PAHs from leaves to stem again. But, the low rate of PAHs transfer from epidermis layer to phloem may cause the lower concentration of HMW PAHs in stem than that of the leaf (Simonich & Hites, 1995). The sequence of the studied congeners in the stem was ACE > B[a]P > D[a,h]A > B[b]F > B[k]F > ACY > PYR > NAPH > B[g,h,i]P > FLU > ANT > IP > B[a]A > PHEN. Specifically, the PAHs like FLT and CHR remain undetected. In particular, the percentile contribution of the 2-, 3-, 4-, 5- and 6-rings PAHs accounted for 2.48%, 37.07%, 2.72%, 54.89% and 2.83% of the total concentration respectively.
It is interesting to note that the PAHs concentration in the root was 68.45 ± 3.17µg/g (Fig. 3), much lower than that of the leaf and stem. The distribution pattern of the individual congeners in root was IP > B[a]P > ACY > B[k]F > D[a,h]A > FLU > PHEN > B[b]F > FLT > B[a]A > PYR ≈ B[g,h,i]P > ACE. In particular, root contains 6.75 ± 0.32µg/g and 61.70 ± 2.86µg/g of LMW and HMW PAHs respectively (Fig. 4). The distribution of 3-, 4-, 5- and 6-rings PAHs was 9.86%, 2.10%, 24.60% and 63.43% of the total concentration respectively with almost negligible concentration of ANT and CHR. In point of the fact that the translocation of 3 rings PAHs from the root to aboveground plant parts might attributed to their lowest availability in root (Tian et al., 2018). Moreover, the lower concentration of PYR (0.1 ± 0.09 µg/g) in root may refer their migration to aerial parts of the plant (Wang et al., 2020). The lesser solubility of HMW PAHs makes them strongly attached to plant roots and resists their translocation through xylem. HMW PAHs get fixed on the root epidermal surface and outside root tissues, which contributed to high level of HMW PAHs concentration in plant root in comparison to the LMW PAHs (Kang et al., 2010). Lipid, an important regulator of PAHs sorption in the root (Zhang & Zhu, 2009), plays a pivotal role in the adherence of PAHs to the root surface from associated rhizobium. Hence, the high content of root lipid (23.63 ± 1.51 µg/g) in our studied species played significant role in PAHs sorption through adsorption and absorption of the rhizobium and sediment PAHs at the root system level (Wang et al., 2010). A study conducted by Gao & Zhu (2004) showed a significantly positive correlation between root lipid content and PHEN uptake by the ryegrass root. In addition, HMW PAHs with high Kow values are more susceptible to bound to root surface strongly and have less tendency to translocate via xylem; hence get trapped at the epidermis surface of plant root (Kang et al., 2010). This could also act as a profound reason for high concentration of HMW PAHs in the plant root.
The total recorded concentration level of PAHs in the rhizobium was 68.66 ± 4.35 µg/g (Fig. 3) which is comparable with that in root. This trend also suggests that root absorb the PAHs from sediments. This may also be the effect of the degradation, transformation and mineralization of pollutants by the microorganisms present in the plant root zone (Joner et al., 2002; Ma et al., 2010; Xie et al., 2012). The detected order of the studied PAHs was ACY > PYR > B[g,h,i]P > B[a]P > IP > B[a]A > NAPH > ACE > B[k]F > FLT > PHEN > ANT > FLU. PAHs like CHR, B[b]F and D[a,h]A were absent in rhizobium. The calculated concentration of LMW and HMW PAHs in rhizobium was 25.25 ± 1.51 µg/g and 43.21 ± 2.84 µg/g respectively (Fig. 4). The percentile contribution of 2-, 3-,4-,5- and 6- rings PAHs was 6.25%, 30.53%, 27.53%, 13.11% and 22.59% of the total concentration respectively. The lower lipid solubility and higher water solubility of LMW PAHs make them available for microorganisms (Lee et al., 2008) which might cause the less concentration in rhizobium. High lipophilicity of HMW PAHs causes their less bioavailability and make it difficult to remove by microorganisms (Dachs & Eisenreich, 2000), might resulted into higher level of concentration in rhizobium. This may also contribute in the high level of HMW PAHs. The total concentration of LMW PAHs in above-ground plant parts (leaf and stem) found around five times higher than that of plant root, rhizobium and sediment. This is in the line of existing nature of LMW PAHs in the gaseous phase which introduced themselves through stomatal opening and makes them prevalent for the aerial parts of the plant. As well as LMW PAHs from the contaminated sediment surface can reach to the plant leaves through volatilization (Huang et al., 2018; Kang et al., 2010; Tian et al., 2018).
Therefore, the order of overall concentration level of PAHs follows leaves > stem > root ≈ rhizobium. Aboveground parts of the plant (leaf and stem) exposed to atmospheric air, showed high level of pollution than the root, might be suggesting large surface area for PAHs accumulation affected the concentration level (Tian et al., 2018). The overall concentration of PAHs pollution in the present study showed the following order leaf > stem > sediment > root ≈ rhizobium. Significant differences (p (2.7×10− 5<0.5)) and F (7.77) > Fcrit (2.49)) were found between the concentrations of PAHs in different matrices as obtained by one-way ANOVA [as in APPENDIX I]. The study conducted by wetland plants, more particularly mangrove plant Avicennia marina displayed different trend of PAHs accumulation along eastern coast of the Red sea in the order of: leaf > root > sediment (El-Amin Bashir et al., 2017). Further, a study performed using surface sediment and mixed of mangrove plants (including Kandelia obovata, Aegiceras corniculatum, Bruguiera gymnorrhiza and Avicennia marina) in Shenzhen, China, observed the following pattern of distribution: leaf > root > sediments (Li et al., 2014). Therefore, it could be inferred that uptake, accumulation and distribution of PAHs among wetland plants significantly governed by factors like types of species and initial PAHs contamination load in sediment (Ryan et al., 1988; Zhang et al., 2010). As the chemicals with log Kow <2.5 are more susceptible for the root uptake via transpiration (Cousins & Mackay, 2001), therefore the abundance of PAHs, having log Kow in the range of 3.36–6.65 (Table 1) might suggesting foliar uptake as the dominating route of PAHs exposure in the studied bioindicator. Additionally, the lipophilic PAHs have the propensity to get accumulated in the lipid storing cell, hence, plant lipid is acting as major factor for observed difference in the PAHs concentration level between sediment and different parts of plant (Chiou et al., 2001). Atmospheric factors prevailing in the study area, including low temperature and decrease in rate of photodegradation during study period (winter) also attributed to higher level of PAHs accumulation in the leaves Alternanthera ficoidea (L.) (Miura et al., 2019). Hence, this plant is a useful and ideal model to implement or cultivate to monitor and mitigate the regional PAHs pollution in the wetland. The studied species might be considered as keystone species, due to its assistance in the development and maintenance of wetland ecosystem through the abatement of wetland pollution. This can also integrate into the list of species with the bioremediation potential of pollutant which promote the management of the impact of air pollution in wetland in a socio-economic and unconventional route.
PAHs source apportionment in the wetland
PAHs introduced in wetlands through various routes like anthropogenic activities, atmospheric deposition, urban/surface runoff, municipality discharge, etc. Hence, it becomes extremely necessary to identify the sources of PAHs contamination to study its fate and transport in the study area to control and to assign the remediation activities (Chen et al., 2015; Moyo et al., 2013). LMW PAHs are mainly released from both petrogenic activities and low to moderate temperature combustion process (such as coal burning). On the other hand, HMW PAHs are generally released from pyrolytic activities taking place at high temperature (Mai et al., 2003; Marris et al., 2020). The compositional pattern of PAHs imparts useful data on accountable sources of PAHs pollution (Cao et al., 2005). Hence, widely accepted method was administered in the present study to analyse the sources of PAHs in plant leaves and sediment of the studied area to obtain a long-term valuable record of natural and anthropogenic activities in the study sites. In order to analyse the sources, the diagnostic ratios like LMW/HMW PAHs, PHEN/ANT, ANT/(ANT + PHEN), FLU/(FLU + PYR), FLT/(FLT + PYR), B[a]A/(B[a]A + CHR), B[a]P/B[g,h,i]P, IP/(IP + B[g,h,i]P), B[b]F/B[k]F, B[b]F/B[a]P and B[k]F/B[a]P have been applied in the present data. The molecular diagnostic ratios used in the present study and the obtained result are presented in Table 3. The ratio of LMW/HMW PAHs was considered as the primary index of source identification in the present study. In case of sediment and leaf, the LMW/HMW PAHs ratio is less than one which reflects the dominance nature of HMW PAHs generated from pyrogenic activities (Muel & Saguem, 1985). The ratios like PHEN/ANT, ANT/(ANT + PHEN), B[b]F/B[a]P and B[k]F/B[a]P also indicate pyrogenic sources as profound contributor of pollution (Table 3). The outcome of the ratio like FLU/(FLU + PYR) of both sediment and leaf as well as the ratio of IP/(IP + B[g,h,i]P) obtained from sediment indicates petroleum emission and combustion as other source of PAHs pollution in the studied wetland. The IP/(IP + B[g,h,i]P) ratio of leaf also indicates grass/wood/coal combustion activities as sources of PAHs pollution (Table 3). On the other hand, the B[b]F/B[k]F ratio suggests diesel emissions as sources of PAHs contamination. Further, the ratios such as FLT/(FLT + PYR), B[a]A/(B[a]A + CHR) and B[a]P/B[g,h,i]P implies vehicular emissions as prominent sources of PAHs pollution is study area (Table 3). Hence, the diagnostic ratios inferred pyrogenic activities including vehicular emissions, petroleum emissions/combustion and grass/wood/coal combustion as the dominated sources of PAHs pollution in the studied wetland. This may be explained by the fact that the study area is adjacent to a busy state highway (SH 3, Ultadanga - Dhapa via EM By-Pass) with high traffic density. Coal burning during the road construction activities also contributed to a significant level of pollution. The obtained data also revealed the presence of PAHs from biomass (grass/wood) burning in and around the areas of Captain Bheri.
Table 3
Source apportionment of PAHs using diagnostic ratio
(Sources: Baumard et al., 1998; El Deeb et al., 2007; Ohura et al., 2015; Tobiszewski & Namiesnik, 2012; Yunker et al., 2002).
Diagnostic ratio | Sediment & Leaf | Captain Bheri |
Value | Standard Range | Source |
LMW/HMW PAHs | Sediment | 0.18 ± 0.007 | < 1 | Pyrogenic |
Leaf | 0.48 ± 0.00 | < 1 | Pyrogenic |
PHEN/ANT | Sediment | | | |
Leaf | 0.001 ± 0.001 | < 10 | Pyrogenic |
ANT/ANT + PHEN | Sediment | | | |
Leaf | 0.99 ± 0.00 | > 0.1 | Pyrogenic |
FLU/FLU + PYR | Sediment | 0.16 ± 0.007 | < 0.5 | Petroleum emission |
Leaf | 0.36 ± 0.16 | < 0.5 | Petroleum emission |
FLT/FLT + PYR | Sediment | 0.48 ± 0.08 | ≤ 0.5 | vehicular emission |
Leaf | 0.44 ± 0.007 | ≤ 0.5 | Vehicular emission |
B[a]A/B[a]A + CHR | Sediment | 0.97 ± 0.001 | > 0.35 | Vehicular emission |
Leaf | 0.56 ± 0.00 | > 0.35 | Vehicular emission |
B[a]P/B[g,h,i]P | Sediment | 1.78 ± 0.40 | > 0.6 | Vehicle emission |
Leaf | 1.21 ± 0.02 | > 0.6 | Vehicle emission |
IP/IP + B[g,h,i]P | Sediment | 0.48 ± 0.04 | 0.2–0.5 | Petroleum combustion |
Leaf | 0.53 ± 0.01 | > 0.5 | Grass/wood/coal combustion |
B[b]F/B[k]F | Sediment | 2.98 ± 1.07 | > 0.5 | Diesel emissions |
Leaf | 1.04 ± 0.01 | > 0.5 | Diesel emissions |
B[b]F/B[a]P | Sediment | 0.17 ± 0.03 | < 0.5 | Pyrogenic |
Leaf | 0.48 ± 0.04 | < 0.5 | Pyrogenic |
B[k]F/B[a]P | Sediment | 0.05 ± 0.03 | < 0.5 | Pyrogenic |
Leaf | 0.46 ± 0.03 | < 0.5 | Pyrogenic |
*PHEN/ANT and PHEN/(PHEN + ANT) ratios could not be calculated due to non-detection of ANT in sediment sample.
The bioconcentration and translocation factors
Bioconcentration factor (BCF) can be defined as the accumulation of any pollutant in an organism with respect to its surrounding medium and expressed as the ratio of pollutant concentration in the organism to the pollutant concentration in the medium (Paraiba, 2007; Paraiba & Kataguiri, 2008). In the present study, BCF for plant leaf and root were calculated based on PAHs concentrations of leaf, root and sediment. The calculated values of BCFLeaf of Alternanthera ficoidea (L.) for LMW PAHs, HMW PAHs and ∑16PAHs was 10.51, 3.92 and 4.92 respectively, whereas, the such values determined for BCFRoot of Alternanthera ficoidea (L.) of LMW PAHs, HMW PAHs and ∑16PAHs was 0.52, 0.85 and 0.80 respectively. The high value of BCFLeaf may indicate the accumulation of PAHs in the plant leaf through both the mechanism including translocation from atmospheric deposition and root/sediment (Lohmann et al., 2011; Wang et al., 2008). Our finding finds consistency with other study done by Li et al. (2014). Alternanthera ficoidea inhabiting in the studied wetland with higher sedimentary PAHs pollution level showed low BCFRoot value suggesting less effective sediment-to-root PAHs transference pathway. This pattern implies air-to-leaf transfer as a dominant and major pathway of PAHs accumulation in the studied species over root-to-leaf transfer.
Process of transfer of PAHs from root to leaf was determined by translocation factor. The translocation factor (TF) for Alternanthera ficoidea (L.) was observed for LMW PAHs, HMW PAHs and ∑16PAHs was 20.17, 4.6 and 6.13 respectively. The observed values of TF (> 1) indicated transfer of PAHs from root to photosynthetic tissues, based on transpiration stream flux related to individual PHAs solubility than that of Kow. LMW PAHs due to their high water solubility is more available to the roots, whereas HMW PAHs with weak water solubility and high Henry’s Law constant and KOW get adsorbed on the root surface epidermis rather than being translocated into the xylem (Fismes et al., 2002; Kang et al., 2010; Meudec et al., 2007). High value of TF for LMW PAHs supports lowest concentration of LMW PAHs in root which might indicate increased level of LMW PAHs in stem. Additionally, higher percentage of LMW PAHs (Fig. 4) in stem (39%) than that of leaf (32%) exposed under the same atmospheric condition probably confirms its uptake from root through translocation. Based on the aforementioned outcome it could be inferred that both the pathways including: air-to-leaf and root-to-leaf are playing significant role in translocation of PAHs in the studied species, however air-to-leaf translocation pathway acting predominantly.