Spatial and seasonal metal variation, bioaccumulation and biomonitoring potential of halophytes from littoral zones of the Karachi Coast.

Coastal wetlands primarily serve as natural sinks of trace metals and their importance for phytoremediation is well known at global level. There are some reports on trace metal availability in soil sediments of the Karachi coast but studies on accumulation and translocation to harvestable (Shoot) parts among halophytes of the littoral zones have not been conducted. Hence, phytoremediation potential of six naturally existing halophytes (Aeluropus lagopoides, Arthrocnemum macrostachyum, Atriplex stocksii, Avicennia marina, Cressa cretica and Suaeda fruticosa) was assessed for cleaning metal (Mn, Zn, Pb and Cr) polluted soils of the Karachi coast. Seasonal (winters, pre and post-monsoon summers) and spatial (three littoral zones: viz., site - I: Sandspit, site - II: Do-Dariya/Clifton and site - III: Korangi creek) variations in soil and plant metals of the Karachi coast were studied. Soil Zn, Pb and Cr were generally higher in winters, Mn and organic matter in summers (7-11%) while pH values ranged between 7.15 and 7.5 in all seasons at site - III. All tested species had potential for cleaning Pb through their harvestable part (shoots) with A. stocksii as prominent candidate (16 mg kg-1) at site - I. Cressa cretica emerged as exclusive candidate for Zn phytoremediation (96 mg kg-1) at site - I, while S. fruticosa, A. macrostachyum and A. lagopoides showed bioaccumulation in pre-monsoon summers at site II. Aeluropus lagopoides with higher Mn in post monsoon summers (62 at site - III and 53 mg kg-1 at site - II) and Cr (7.1 mg kg-1 at site - II and 14 mg kg-1 at site - III) appeared exclusive bioindicator with potential of for cleaning all metals (Mn, Zn, Pb and Cr) at different sites. Metal bioaccumulation at study sites appeared species specific and varied seasonally among tested halophytes.

Comparative assessment of heavy metal accumulation and bio-indication in coastal dune halophytes.

This study aimed at assessing heavy metals (Fe, Mn, Zn, Cu, Cr and Pb) in four perennial halophytes (viz. Heliotropium bacciferum, Halopyrum mucronatum, Ipomoea pes-caprae and Salsola imbricata) growing at two sites on the Karachi coast. Site - II, closer to the Industrial area had higher bioavailability as well as translocation factor (TF) for most of the heavy metals and Na+ where soil sediments had lower pH (approximately 7.5), higher salinity (EC) and organic matter (OM). Site - I which was far from Industrial area had comparatively higher bio-concentration factor (BCF) and lower TF for metal ions and soil pH of 8.1-9. Metal accumulation in plants was both site and species specific. Extractable concentration of shoot Pb in all tested halophytes was above normal of the threshold values (i.e., >0.3 mg kg-1) while Mn (<50 mg kg-1) and Cu (<40 mg kg-1) were within permissible limits. Salsola imbricata had highest Na+ at both sites (site - I = 73; site - II = 98 mg kg-1) with and 10 mg kg-1 extractable shoot Pb at site - I. Ipomea pes-caprae also accumulated shoot Pb higher than normal (site - I = 3.3; site - II = 0.8 mg kg-1) with lowest Na+ content. Heliotropium bacciferum had higher extractable Pb (site - I = 10.5; II = 2.75) with >20 mg kg-1 Na+ in shoot while maintaining > 1 TF for Pb, Cu, Mn and Zn at site - I and all tested metals at site - II. Halopyrum mucronatum had highest shoot Fe (644 mg kg-1), Zn (63 mg kg-1) and Cr (9.2 mg kg-1) at site - II and above threshold values of Pb at both sites (site - I = 8.2; site - II = 2.5 mg kg-1) which makes this species an ideal bio-indicator candidate while other species could be potentially used for Pb phytoremediation.