Halophytes for phytoremediation of hazardous metal(loid)s: A terse review on metal tolerance, bio-indication and hyperaccumulation

Highlights

• This review outlined opacity regarding the terms bioindication, phytostabilization and metal hyperaccumulation among halophytes.

• Obligate and facultative metal hyperaccumulating halophytes have been enlisted for the first time.

• Based on shoot metal concentrations, multipurpose use of halophytes has been proposed.

Abstract

Phytoremediation is a cost-effective and environment friendly method for cleaning metal(loid)s from contaminated soils. Species with exceptionally higher shoot metal concentrations (hyperaccumulators) seem ideal for phytoremediation, though some metal tolerant species with ‘above normal’ values with higher translocation factor (TF) may also serve the purpose. Halophytes not only remove salts and metalloids from soils but may also be cultivated as non-conventional crops. Nurturing halophytes requires precise understanding of their nature and efficient management for sustainable use. Species with low metal concentrations in their edible parts (especially leaves) may be grown as forage and fodder, but those with metal hyperaccumulation could prove fatal due to their serious health hazards. Like other metallophytes, redundant use of the term ‘metal hyperaccumulation’ among halophytes needs to be revisited for its ambiguity and potential pitfalls. Similarly, understanding of metal tolerance and shoot accumulation nature of halophytes is needed prior to their use. This review is an attempt to compare halophytes with potential of metal bioindication, phytostabilization and hyperaccumulation (as per definition) as well as their ‘obligate’ and ‘facultative’ nature for appropriate uses.

Introduction

Halophytes have emerged as valuable plants with several economic benefits (food, feed, and fiber resources) besides having immense potential for phytoremediation of metalloids from contaminated soils. Just like other metallophytes importance of halophytes in cleaning polluted soils is well established (Manousaki and Kalogerakis, 2011). Metal contamination is associated both with natural (for example, parent material and volcanic eruption etc.) as well as anthropogenic (agricultural and industrial) activities (Sharma and Pandey, 2014). Plants affected with metals could prove to be fatal for living beings via food chain (Keller et al., 2015). Therefore, cleaning of soils from hazardous materials is essential. Literature suggests that metallophytes (plants naturally occurring in metalloid soils) are inherently efficient in extracting soil metals although some non-metallophytes (halophytes included) may also serve the purpose (Manousaki and Kalogerakis, 2011). True metallophytes absorb metals from the soil and transport them towards shoot where they are stored in extraordinarily higher concentrations for which the term ‘hyperaccumulation’ is often used (Baker and Brooks, 1989, Baker et al., 2000). However, plants having marginally higher metal concentrations than their original requirement (as micronutrient) are also useful. Metal tolerant plants that show correlative relationship with soil metals are called ‘bioindicators’ and they are species specific (Van der Ent et al., 2013, Bonanno et al., 2018). Moreover, such group of plants represent habitat types for one or more metal speciation (Reeves et al., 1983). Plants growing in the presence of higher soil metal concentrations, may become potentially toxic for some conventional crops as they may interfere with metabolism and enzyme functions (Clemens, 2001). Therefore, researchers suggest halophytes (salt resistant plants) alongside other plants for explicit use of depollution process provided that suitable species are selected for cleaning contaminated soils (Wang et al., 2014, Mujeeb et al., 2020, Mujeeb et al., 2021).

Halophytes are known to grow in extreme habitats (both water-logged and dry saline conditions) in which most of the glycophytes cannot (Heida, 2014). They can be used on marginal lands as forage crops and to yield biomass for bioenergy with additional property of soil phytoremediation (Van Oosten and Maggio, 2015, Mujeeb et al., 2020). Research on metal tolerant halophytes (e.g., species belonging to genera Atriplex, Tamarix, Salicornia etc.) suggest that these plants could be used as agronomic tools on contaminated soils by accumulating ‘above normal’ values of trace metals in their tissues (Van Oosten and Maggio, 2015). This group of plants may be cultivated without fresh water by using saline resources in arid regions (Wahla and Kirkham, 2008, Manousaki and Kalogerakis, 2011, Van Oosten and Maggio, 2015). Research articles suggest that many species of halophytes help in de-salinization of salt affected lands by hyperaccumulating toxic Na⁺ and Cl^- in addition to metal tolerance in their armory (Manousaki and Kalogerakis, 2011, Flowers and Colmer, 2008). Due to this property halophytes are rendered suitable for phytoremediation of salts. However, one needs to be careful while using the term ‘hyperaccumulation’ for metals as this trait is usually confined to high shoot metal concentrations (Van der Ent et al., 2013). A precise definition states that plants bearing extremely high metal concentrations in their living tissues, may be hundred or thousand times greater than normal (Reeves, 2003, Van der Ent et al., 2013) are called ‘hyperaccumulators’. Therefore, the relentless use of the term for plants bearing marginally higher metals may be misleading and creates ambiguity (Van der Ent et al., 2013). The current list of hyperaccumulators suggests more than 700 species at the global level (Reeves et al., 2017), therefore not all ‘metal tolerant’ plants may be placed in this category and halophytes are no different. Recent advances using X-ray ionomics on herbarium specimens however suggest that there may be some value additions to already existing list of hyperaccumulators (Van der Ent et al., 2019). Some other studies using phylogenetic relationship of closely related taxa also point towards addition of metal hyperaccumulators (Jin and Qian, 2019, Xu et al., 2020). Among halophytes some of the species may be having concentrations high enough to be considered metal hyperaccumulators. For reference, standard values (essential requirement) of trace metals as plant micronutrients, adequate levels in shoot tissues and suggested permissible (threshold) limits are highlighted in Table 1 (mentioning different references) beyond which the plants may be adjudicated for phytoremediation wherein, proposed values are taken from the pioneers in this field of research. This review is an attempt to provide clarity of the concepts (as per definitions) to eliminate redundant and overlapping use of the terms on metal hyperaccumulation and metal tolerance with special emphasis on halophytes around the world.

Metal tolerance under saline conditions may be attributed to cross talk mechanisms between salinity and metal stress (Hamed et al., 2013). Microbiota associated with plant roots may also help in biosorption of metals (Vacheron et al., 2013). Microbes with plant growth-promoting traits, the so-called plant growth promoting rhizobacteria (PGPR) are known to stimulate plant nutrient uptake, mitigate metal toxicity, immobilize/mobilize heavy metals in the soil and may improve plant health (Ferrarini et al., 2021). Resistance of perennial halophytes to trace metals is closely related with their deep rooting system and biomass (Anjum et al., 2014, Wang et al., 2014). Halophytes help in dissolution of soil calcites in the rhizosphere along with bio-sorption of metals (Ghnaya et al., 2005). Metal resistance is also facilitated by some of the inbuilt assets among plants such as enzyme mediated production of phytochelatins (PC’s) for metals (such as Cd, Ni, Cu, Zn, Ag, Hg, Pb etc.) and metallothionein (MT) genes encoding peptides (Sheoran et al., 2011). Some of the halophyte species are also reported to have these mechanisms which helps metal remediation in plant proximity (Sruthi et al., 2016). Root exudates containing aminos and organic acids (phytosiderophores) are helpful in metal chelation which increases metal availability in soil solution (Sruthi et al., 2016). Overproduction of phytochelatins increases bioavailability of soil metals (Wahla and Kirkham, 2008, Manousaki and Kalogerakis, 2009, Bankaji et al., 2015) helping in root metal absorption under saline conditions (Acosta et al., 2011). Once absorbed through roots, metal ions are transported towards shoots (Briat and Lobre´aux, 1997) where metallothioneins (MT’s) may help in sequestering metal ions in the vacuole or cell wall (apoplast) so that they cannot interfere with or damage vital metabolic functions (Sheoran et al., 2011). In this way plants restrict metal ion accumulation in cells by avoidance or exclusion strategy. Studies suggest that heavy metal and salt resistance are partly based on common physiological mechanisms (Fitzgerald et al., 2003, Almeida et al., 2006, Reboreda and Caçador, 2007, Pereira et al., 2009, Marques et al., 2011). In some halophytes, metal ions as well as salts may either be excreted at root level or via specialized leaf structures for example, salt glands and trichomes (Lokhande and Suprasanna, 2012) though this is achieved by different transport channels. Xerohalophytes of the drier habitats may also remove salts and metal ions with the help of different transporters and in case of hyperaccumulators they are transported to the shoot system (Mujeeb et al., 2020). However, physiologically metal ion detoxification is usually achieved by maintaining water potential gradient (Flowers and Colmer, 2008), vacuolar ion sequestration (Ahmed et al., 2013), synthesis of organic solutes for osmotic adjustment (Flowers and Colmer, 2008, Aziz and Khan, 2014) and increased antioxidant activity (Manousaki and Kalogerakis, 2009, Lokhande and Suprasanna, 2012, Kandziora-Ciupa et al., 2013). Although metal resistance is also common in salt marsh inhabitants but most of the species act as ‘natural sinks’ for their ability of metal ‘phytostabilization’ (Reboreda and Caçador, 2007). Such plants usually have poor ability of metal translocation towards shoot. Compared to the plants of drier habitats most of the saltmarsh taxa are not hyperaccumulators as they sequester most of the metal load in roots (Alam et al., 2021). It is therefore concluded that responses of metal tolerant plants may change with their habitat as well as biogeography. However, the assumption whether halophytes have metal hyperaccumulation potential needs thorough investigations.

To date 52 plant families from all over the world have been identified for metalloid hyperaccumulation (Reeves et al., 2017) and still counting. Plant list with known hyperaccumulation is dominated by ca. 83 members of Brassicaceae and 59 Phyllanthaceae (Reeves et al., 2017). Another study on closely allied species indicates more than 60 plant families from all groups (Xu et al., 2020) that have hyperaccumulation ability (including halophytes). Different studies suggest that some of the angiospermic plants from families Fabaceae, Aizoaceae, Polygonaceae, Brassicaceae, Papaveraceae, Euphorbiaceae, Amaranthaceae (formerly Chenopodiaceae) and Poaceae etc., show potential of metal hyperaccumulation with unusually high concentrations (Table 3(A), Table 3(B)). Plants given in the list are all salt tolerant (for example, Tephrosia villosa, Panicum antidotale, Acacia nilotica, Tamarix aphylla, Suaeda fruticosa, Arthrocnemum macrostachyum, Salvadora oleoides etc.), of which some are also found in Pakistan (Khan and Qaiser, 2006). Examples cited in the list of metal hyperaccumulators (both Table 3A and B) suggest that out of ~400 reported halophytes (Khan and Qaiser, 2006) there may be a few unexplored species representing true hyperaccumulation potential. Research on mining sites with sediment metal concentrations, biosorption by plants and their subsequent translocation towards shoot could provide further details in this regard. A case study on 5 perennial halophytic plants at southeast of Murcia Region (Spain) indicated Pb hyperaccumulation in many species while Fe, and As in a euhalophyte Arthrocnemum macrostachyum (Martínez-Sánchez et al., 2012). Although some annuals (e.g., Salsola kali) may also hyperaccumulate trace metals such as Cd and Fe (de La Rosa et al., 2004, Dragovic et al., 2014) such plants may only be harvested once in a year. Therefore, perennial halophytes with higher shoot metals in above ground biomass would be of more interest as they may be harvested multiple times. Studies suggest that halophyte species are metal tolerant with phytoremediation potential, but not all may be ascertained metal hyperaccumulators merely based on >1 TF (translocation factor) (van der Ent et al., 2013). Hence, shoot metal concentrations are also important as suggested for metallophytes (Table 1). For further clarity, halophytes with phytoremediation potential, metal tolerance and those with ‘in situ’ (lab conditions) or ‘ex situ’ (field conditions) hyperaccumulation are discussed in the next section.

Halophytes having ‘above normal’ or marginally higher metal concentrations than permissible limits in their shoots (Table 1) in real sense are ‘metal tolerant’ rather than hyperaccumulators as mentioned erroneously in some of the research articles. As for bioindicator potential, halophytes must accumulate metal concentrations in above ground parts while maintaining strong linear correlations with soil metals and without showing any phytotoxicity symptoms (Bonanno et al., 2018, Van der Ent et al., 2013). Hence, bioindicator halophytes for their metal specificity may help in assessing contamination levels in polluted soils though, they may not necessarily bear unprecedented values as found in hyperaccumulators (Salt et al., 1998). Based on the abovementioned criterion Hassan et al. (2015) advocated Calotropis procera (a salt desert native) as bioindicator of Cr polluted soils while another halophyte, Paspalum paspalodes showed bioindication of Cd and Phragmites australis for multiple elements such as Cd, Mn and Cr (Bonanno et al., 2018). As for phytoremediation potential, sensitivity indices of BAF (Bioaccumulation factors) such as BCF (bioconcentration factor) and TF (translocation factor) are often used to determine the nature of plants as phytostabilizer and / or accumulator (Manousaki and kalogerakis, 2011). Plant species with BCF > 1 are referred to as “phytostabilizers” while those with TF > 1 are called “phytoextractors” or “shoot accumulators” (Zaier et al., 2010, Manousaki and Kalogerakis, 2011). Higher BCF values are used to estimate plant’s ability to pump heavy metals from soils (ratio of soil: root metals) while TF > 1 indicates higher shoot metal transport that is associated with the ability of shoot metal accumulation (Zaier et al., 2010, Sheoran et al., 2011). In this review, we are presenting different lists of halophytes ( Tables 2,Table 3A and3B) with distinctive abilities of bioindication, phytostabilization and hyperaccumulation. Plants can accumulate trace metals in different plant parts and effectively resist them without death (Wang et al., 2014, Zhao et al., 2018). Table 2 highlights halophyte species with tissue metal concentrations (µg g⁻¹ ≈ mg kg^-1) with marginally higher or ‘above normal’ ranges along with concentration in soil sediments that would give an insight about metal accumulating nature of halophytes. Many of them may be placed in ‘phytoextractor’ or ‘shoot accumulator’ category for their translocation factor >1 and high shoot metal concentrations irrespective of the soil concentrations. Such plants include Atriplex stocksii for Zn (>60 mg kg⁻¹), Arthrocnemum macrostachyum (>700) and Polygonum aviculare (329 mg kg⁻¹) for Pb, Rhizophora mucronata and Salsola imbricata for Mn (>50 mg kg⁻¹), and Sporobolus virginicus for Cu (>40 mg kg⁻¹; Table 2). Although many other halophytes in the presented list showed high shoot metal concentrations compared to roots, their soil metal concentrations appeared comparable to shoots hence showing correlative relationships and for this reason they may be classified as ‘bioindicators’ according to definition. Examples of such plants are Derris trifoliata, and Dicanthium caricosum which accumulated Mn, Acanthus ebracteatus, Atriplex stocksii, Ceriops decandra and Halopyrum mucronatum accumulated Pb, Cressa cretica and Sporobolus virginicus Zn, and Aeluropus lagopoides, Zn and Cr metals respectively (Table 2). On the contrary, several halophyte species (mentioned in Table 2) with higher root metal concentrations, and >1 BCF exhibited ‘phytostabilization’ property. Interestingly most of phytostabilizer halophytes are native to the wetlands and coastal marshes e.g., Avicennia alba which had higher root Mn, Phragmites australis, Phragmites karka, Aeluropus lagopoides and Sporobolus virginicus with higher Pb (Table 2). These findings are also in agreement to the fact that marshes are natural sinks for the heavy metals and usually possess poor translocation towards shoot (Reboreda and Caçador, 2007).

Although translocation factor (Tf) is commonly considered as an important index, it is not the only criterion for ascertaining ‘hyperaccumulator nature’ of the plants (Van der Ent et al., 2013) and the same may apply on halophytes. The argument on not merely using TF for hyperaccumulation emerged owing to difficulties in root sampling especially in trees besides structural and physiological complexities between roots and shoots. It means that a plant with lower TF but still maintaining high shoot metal concentration may not be relegated from hyperaccumulation category. The confusion usually arises when shoot metal concentrations are far above prescribed limits for hyperaccumulation though values in roots also exceed than the prescribed benchmark with BCF > 1. An example from our list (Table 3A) is Susuvium portulacastrum which had very high BCF for Pb (15.5) but it also accumulated up to 3400 mg Kg⁻¹ in shoot (Zaier et al., 2010), higher than the proposed benchmark (1000 mg Kg⁻¹). Similarly, Portulaca oleracea accumulated up to 1400 mg kg⁻¹ Cr (far greater than proposed limit of 300 mg Kg⁻¹; Table 1) though its BCF was 10–15 (Elshamy et al., 2019). For removing such ambiguities, Van der Ent et al. (2013) recommended to set the definition of hyperaccumulation based on shoot metal concentrations (refer to column 5 in Table 1) rather than BAC’s. They further suggested that some of the metals naturally exist in very low concentrations such as Cr (<50 µg g⁻¹) and Zn (50–500 µg g⁻¹) even on ultramafic (metal enriched) soils. Therefore, they proposed to set hyperaccumulation levels of 300 for Cr and 3000 mg kg⁻¹ for Zn. Besides other discrepancies, there is also a confusion in some of the research articles regarding the nature of hyperaccumulation in plants under lab and field conditions. To remove this confusion, Pollard et al. (2014) broadly classified plants into two groups based on biogeography and metal accumulation characteristics: (1) Obligate hyperaccumulators (plants naturally occurring on metalliferous soils) and (2) Facultative hyperaccumulators (plants growing on non-metalliferous and so called, normal soils). While explaining the nature of Cu and Co hyperaccumulators Lange et al. (2017) also explained both ‘in situ’ (plants of natural habitats) and ‘ex-situ’ (plants grown in experimental conditions) as ‘obligate’ and ‘facultative’ hyperaccumulators respectively. Likewise, it is proposed that halophytes grown in lab conditions with extraordinarily high metal concentrations may be considered as ‘facultative hyperaccumulators’ (Table 2A). Since many researchers have focused their research work on trace metals by giving different concentrations in lab grown plants, they may not be placed in the category of obligate hyperaccumulators. Examples of facultative hyperaccumulators include Sesuvium portulacastrum which accumulated up to 3400 mg Kg⁻¹ shoot Pb when grown in 1000 µM Pb (NO₃)₂ (Zaier et al., 2010) and Tamarix aphylla 285 Cd in 45 µM Cd (NO₃)₂ solution (Hagemeyer and Waisel, 1988) (Table 2A). On the contrary, examples with higher amounts of trace metals in natural conditions or those at the mining sites for example, Arthrocnemum macrostachyum with Cr (1379) and As (101) (Martínez-Sánchez et al., 2012, Lu et al., 2017); Brassica juncea 9400 Pb (Koptsik, 2014), Euphorbia cheiradenia 1138 Pb (Chehregani and Malayeri, 2007), Salsola kali 2075 mg Kg⁻¹ Cd (de la Rosa et al., 2004) etc., may be classified as ‘obligate’ hyperaccumulators’ (Table 2B). Some of the representative halophytes had >1 TF with unusually higher values in Polygonum aviculare for Cu (11.74) and Brassica nigra for Pb (5.37) while maintaining higher shoot concentrations of these metals (Table 2B). On the contrary, some of the halophytes (Suaeda fruticosa, Chenopodium morale, and members of Poaceae) accumulated higher metal concentrations in roots compared to shoots (with unusually higher BCF) although most of them maintained values well above the range suggested for shoot hyperaccumulation (Table 2B). Due to this discrepancy, it was suggested to set threshold criteria of shoot metals at least 2–3 times higher than soil (shoot:soil ratio) for hyperaccumulation rather than bioaccumulation factors alone (Van der Ent et al., 2013). Studies on some of the halophytes from Pakistan also indicated species having hyperaccumulation potential of Cr under field conditions (mining areas) with Suaeda fruticosa bearing highest Cr (1379 mg kg⁻¹) values though average amount of soil Cr was 19.5 mg kg⁻¹ (Bareen and Tahira, 2011). Other species included Trianthema portulcastrum, Chenopodium morale, Salvadora oleoides, Panicum antidotale etc. (Table 3B).

Based on the abovementioned facts, halophytes are widely advocated for revegetation and remediation of salt and metal affected lands (Flowers and Colmer, 2015, Manousaki and Kalogerakis, 2011). However, appropriate use of halophyte species for cleaning metal contaminated soils requires detailed agro-management based studies. For instance, harvestable shoot parts i.e., stem and leaves with higher metal concentrations are of more significance for phytoremediation due to higher metal translocation towards shoot. However, if shoots and other edible parts maintain low metal concentrations, and their values do not exceed then standard permissible limits (Table 1), they may be a good source of food and fodder for animals. More details of halophytes that may be consumed by humans and animals is discussed in our next section.

Plants listed in this review (Table 2, Table 3(A), Table 3(B)) indicate that some species can even accumulate more than two elements in high concentrations. Therefore, recommending plants as potential food and fodder or for the sole purpose of soil remediation requires thorough understanding. Plants accumulating very high amount of trace metals (hyperaccumulators) in their edible parts may not be rendered suitable for human or animal consumption as they may enter in the food chain, causing major health concerns, therefore, they can only be used for cleaning contaminated soils. A case in point is of Portulaca oleracea which is known to accumulate very high shoot Cr (1400) with >1 TF hence could be grown on Cr polluted soils for remediation purposes only. However, the same plant accumulates low shoot concentrations of Cu, Zn, Fe and Mn and a good phytostabilizer with BCF >1 and prevent metal load towards shoot (Alyazouri et al., 2013). Therefore, it may be grown on soils with higher Cu and Zn etc., to be used as vegetable and for medicines serving both purposes i.e., consumption as food and medicine for humans and remediation of some metals. Similarly, some other phytostabilizers with limited metal translocation may be recommended as fodder with added benefit of soil remediation. For instance, Avicennia marina, Suaeda maritima, Spartina maritima, S. alterniflora and S. densiflora which can grow on Mn affected soils and can accumulate this metal within acceptable limits (<500 mg kg⁻¹) or Sporobolus virginicus and Suaeda monoica accumulating Zn well within acceptable range (Table 4) may be used as fodder. However, if any plant has a tendency of accumulating 2 or more metals in edible parts it may not be deemed fit for consumption in case any of the available metals is marginally higher than the standard permissible limit which may be potentially threatening. In such conditions, species such as Suaeda monoica, Suaeda fruticosa, Tamarix aphylla etc., may be grown as a source of ‘bioethanol’ with high lignocellulosic biomass (Munir et al., 2021) while Suaeda fruticosa, Halopyrum mucronatum, Arthrocnemum macrostachyum, Cressa cretica etc., for obtaining seed oil for biodiesel purposes (Munir et al., 2021). A list of halophyte species with economic usages is presented in Table 4 which may be potentially grown on soils polluted with different metals. This information would help strategic planners of what to grow and on which types of soils for their sustainable use. Since hyperaccumulators usually have low biomass and searching them may be sometimes quite frustrating, selection of shoot metal accumulators would be a better alternative choice for soil remediation. Metal accumulators with higher shoot biomass may be harvested several times for remediation since they have supposedly higher metal concentrations than an average plant. Species such as Phragmites karka, Typha angustifolia, T. domngensis and Aeluropus lagopoides with relatively better growth rates under saline conditions seem better choice for trace metal extraction (Munir et al., 2021, Ahmed et al., 2013, Munir et al., 2021, Mujeeb et al., 2021). Species with salt secretion ability are also known to co-excrete metals (though by using separate pathways) thus proving efficient for phytoremediation of salts and metal ions. For instance, Avicennia marina excrete Zn and Pb (Ismail et al., 2014; Alam et al., 2021), Avicennia alba Zn and Cr (Kaewtubtim et al., 2016), Atriplex stocksii and Cressa cretica Zn and Pb, A. lagopoides Fe, Pb, Cr, Mn, Zn (Mujeeb, 2021, Mujeeb et al., 2021) and Halopyrum mucronatum for Fe, Zn, Pb and Cr (Mujeeb et al., 2020) which also remove salts efficiently. Above mentioned details suggest that there may be some more unexplored halophytes with the ability of salt and metal extraction other than metallophytes. Therefore, it is imperative to plan ‘in situ’ studies on halophytes to find out their relative potential of metal accumulation besides salt resistance in nature.