South Afncan Journal of Botany 2001. 67 46()-464 Pflnred Ifl South AfrICa - All nghfS reserved

Copyrtght C NISC Ply Ltd ·SOUTH AFRICAN JOURNAL

OF BOTANY ISSN 0254-6299

Evaluation of Padina boergesenii (Phaeophyceae) as a bioindicator of heavy metals: some preliminary results from Mauritius

R Dulymamode*, N sukhoo, I Bhugun

Faculty of Science, University of Mauritius, Reduit, Mauritius

k Corresponding author, e-mail: rafic @uom.ac.mu

Received 16 May 2001 , accepted in revised form 3 June 2001

The suitability of the brown macroalga Padina boerge­senii was assessed as an indicator of eight heavy met­als (Fe, Mn, Zn, Cd, Cr, Pb, Ni and Cu) at 5 stations around Mauritius. The results were compared with metal concentrations in seawater. Iron and Manganese were found in greater concentrations, the highest ones being recorded at Baie du Cap and Bambous Virieux.

Introduction

Over the past two decades, Mauritius has witnessed a rapid increase in industrial and tourism development with the inevitable adverse consequences on the environment. Hotels and bungalows have mushroomed around Ihe coast and the lagoons are under serious Ihreat. In the light of this, there is an urgency to monitor the biotic and abiotic compo­nents of the lagoons. Macroalgae are particularly indicated for this monitoring process as they concentrate metals from seawater. The variations in the concentration of metals in the algal thallus often reflect the metal concentrations in the sur­rounding waters (Phillips 1977) . On Ihis basis, macroalgae (especially the Phaeophyceae Table1) have frequently been used as indicators of heavy metal contamination (Lobban and Harrison 1997. Forsberg et a/ 1988, Engdahl et a/. 1998). Among the tropical seaweeds, different species of Padina have been used. (Engdahl et a/. 1998, Zhao et a/. 1994).

Both active and passive mechanisms are involved in metal uplake. Metals such as Pb and Sr are passively adsorbed 10 the polysaccharides of the cell wall while others (e.g. Zn, Cd) are actively taken up against intracellular concentration gra­dients (Eide et a/. 1980). The factors affecting metal uptake by algae have also been well studied. These include the age of the algal frond, the position of the alga on the shore (Ferrei ra 1991), temperature (Munda 1986), the season of the year (Forsberg et a/. 1988) and the presence of other pollutants in the surrounding water (Bryan 1969).

This study aims at assessing Padina boergesenii as a bioindicator of heavy metals and also at determining some of the factors affecting metal uptake. Padina boergesenii was chosen as the study material owing to its wide distribu­tion in the lagoons and its availability throughout the year.

Chromium, Nickel and Copper occurred as traces in most regions. The concentration factor in the alga for Fe and Mn at each site was 102-103 fold. In-vitro studies of metal uptake showed some evidence of a synergistic mechanism underlying CdlZn absorption in P. boerge­senii. The results of these studies confirm the suitabili­ty of P. boergesenii as a bioindicator of heavy metals.

The alga is lirmly attached to the substratum by a strong hold fast and its large thallus oHers a high absorptive area to volume ratio. We have also compared some of our resu lts with existing data from the region particularly from Zanzibar and Oar-es-Salaam.

Materials and Methods

Collection of material

Fresh specimens of Padina boergesenii were collected in August 1999 at 5 different localities around Mauritius name­ly Grand Baie, Palmar, Bambous Virieux, Baie du Cap and Wolmar. Six samples were taken at each location over a dis­lance of 100m. The water depth at collection sites varied between 10cm to 1 m below the low water mark. Each sam­ple consisting of a bunch of thalli with holdfasts was thor­oughly rinsed in seawater to remove adhering particles (epi­phytes, sand particles and crustaceans) and brought to the laboratory in 500ml low-density polyethylene plastic bottles containing seawater. An additional sample of seawater (100ml) was collected a few centimeters below the water surface in polyethylene containers for metal analysis (water was filtered in situ) .

For the in-vitro studies on metal uptake, healthy thall i still attached to the substrate rocks or corals were collected at La Prairie and transported back to the laboratory. The alga was rinsed with seawater, then placed in four clean aquaria (60 x 30 x 30cm) filled wi th seawater (60 litres) from the col­lection site. The aquaria were kept at room temperature (24-2S°C) under ordinary light and continuous aeration by small water pumps. The algae were left under these condi-

South Arncan Journal 01 BOlany 200 1. 67. 460-464 46 1

Table 1: Some Phaeophyceae used as biOlndicators

Species Loca tion Reference

Fucus vesicu/osus Bristol Channel Morris and Bale 1975 Fucus veSIGufosus South coast of Finland Kangas et al. 1982 FUGUS vesicu/osus Humber Estuary Barnett and Ashcroh 1985 Fucus veslGulosus Archipelago 01 Stockholm Forsberg et al. 1988 Fucus veSlcu/osus Northern Baltic sea and Soederlung et at. 1988 Padma boergesenll Sepetiba bay. Brazi l Karez et al. 1 994 Padma boergesenJl Zanzibar Channel Engdahl ef al. 1998 Padina boryana Seychelles Islands Dolgushina et al 1995 Padina tetrastomatlca Coastal and Estuari ne Zlngde et at. 1976

tions for th ree days before starting the treatments. These were:

Aquarium A: 0.7 mgfl of Zn (as ZnSQ,.7H,Q) Aquarium B: 0.7 mgl l of Cd (as CdSQ,.8H,Q) Aquarium C: 0.7 mg/I of Zn and 0.7 mgl l of Cd Aquarium D: Control (sea water only) Algal samples were taken from the 4 aquaria for analysis

prior to Ihe trealment and on day 1, 2. 5. 7 and 9 (after dosage).

Processing of material in the laboratory

The material was rinsed in distilled deionised water and holdfasts were excised and discarded. All samples were dried in a cabinet at 35-40"C for 48hrs until a constant weight was obtained. The samples were ground and homogenised with a mortar and pestle, then stored in labelled polyethylene plastic bottles.

Digestion of material

1.00g of the fine algal powder was dissolved in 5ml concen­trated HNQ, (60-65%) and 2ml of 30% H,O,. Digestion was performed in a high performance microwave digester (Milestone mls 1200 mega) for 30min. After cool ing, sam­ples were filtered (through commercial fast filter paper) and diluted to 25m I with distilled deionised water to optimise metal detection .

Metal analyses

The digested samples were analysed using Flame Atomic Absorption Speclrometry (Unicam 929) while Graphite Furnace Atomic Absorption Spectrometry (GF 90) was used for water analysis.

Determination of concentration factor (eF)

In order to characterise the extent to which the alga con­centrates the metal , a concentration lactor (CF) can be cal ­culated for each metal. The CF can be defined as follows:

Results and Discussion

The results for the determination of heavy metals in the alga collected at the five sites are summarised in Figure 1.

The levels of heavy metals in Padina boergesenii vary considerably (Figure 1). Iron is the most abundanl element, while Ni and Pb are present in trace amounts. In general, metal abundance in the alga at different locations decreas­es in the following order:

Fe > Mn > Zn > Cd > (Cu , Cr) > Pb, Ni High levels of Fe and Mn were recorded at Bambous

Virieux and Baie du Cap stations situated in the south east and south respectively (Figure ld. f). These two stations are situated at the foot of mountains and there fore receive con­siderable amount minerals as run-off during heavy rains. Additionally, a metal work station is situated close 10 the sampling site of Bambous Virieux which could account for the highest level of Fe in that region. However. the Fe level recorded at most stations in Mauritius is lower than cited records lor other countries (Table 2) , but Mn levels exceed. or are in the range of levels for Zanzibar and in the range of the lower ones of Dar-es-Salaam. The algal concentration of Zn at all the Mauritian sites overlap with those cited for Zanzibar and Dar-es-Salaam in 1997. Zn is a constituent of boat paints and galvanised steel used for fences and roofing in hotels and could find its way to the lagoon through rain-water.

Cadmium concentration was high in most places except Bambous Virieux where it reached around 0.74ppm. The lower levels of Cd at Bambous Virieux was either because no Cd was available (none was delected in Ihe water allhis stalion) or because Cd was outcompeted by higher concen­trations of Zn. Mn, and Fe. Bryan (1983) reached similar conclusions with the brown alga Fucus vesiculosus.

Lead was detected in trace amounts in the alga at Baie du Cap and Bambous Virieux and in water only at Baie du Cap (0.02ppm) . This is a rather surprising result as one would expect high Pb levels at all sites owing to boat engine and other vehicle exhaust emissions. Its unavailability to the alga and in seawater in most stations could be due to its tenden­cy to precipitate easily. The bioavailability of Pb is also con­trolled by pH values of water, which, in turn, determines whether the metal is in free form or form complexes with constituents of water (Ahrland 1985). The sampling site at Baie du Cap is located near a bus station (5m from the beach) and this can represent a possible contamination

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Figure 1: Concentrations of Cd (a) , Cr (b), eu (c) , Fe (d) , Pb (e). Mn (f). Ni (9), and Zn (i) in the alga Padina boergesenii (lJg g ' dried alga, mean = SO, n = 6) al 5 stations around Maurit ius. (1 = Grand Baie, 2 = Palmar, 3 = Bambous Virieux, 4 = Baie du Cap, 5 = Wolmar)

source. Compared to other metals, Ni, Cr and eu were detected at trace levels.

About 80% of the wet weight of the alga comprise of water and thus, in places where heavy metals were detected in water, the CF could be calculated . Padina boergesenii con­centrates Fe and Mn by some orders of magnitude (Tabte 3). It is evident that the alga also concentrates other metals but the CF could not be calculated because the levels of these jn the water were below detection limit.

In the laboratory uptake investigation, it was found that Zn and Cd were maximally absorbed in the first two days thus

making P. boergesenii a suitable bioindicator (Figure 2). This is in contrast with Cain et al.'s (1980) report of a lag phase (2-4 days) corresponding to very stight intake of Cd in Scenedesmus obliquus. Karez et al. (1996) reported that P. boergesenii had a great capacity for a long-term uptake of Zn relating to a mechanism of deposition in the cell wall. This mechanism could account for the near steady state observed for Zn and Cd (up to 9'" day) in the present study. The maximum amount of Zn absorbed in the presence of Cd is nearly the same (200ppm compared 215ppm). However, the absorption of Cd was enhanced in the presence of Zn to

South African Journal of Botany 2001, 67: 460- 464 463

Table 2: Comparison of heavy metals content in P. boergesenil in Mauritius and two other East African Regions (Min-Max values in ppm, NO' below detection limits). (Partly adapted from Engdahl et al. 1998)

Metal Mauritius (1999) (this study) Dar-es-Salaam (1997) Zanzibar (1997) Cd 0.7-4.5 0.2 1.6 0.4-2.7 Cr 0.3-1 .5 0.8-8.8 0.7-8.8 Cu 0.2-1.8 2.5-16.2 2.0-8.1 Fe 67-518 440-4050 87-1550 Pb < 0.3 NO NO Mn 25-107 100-461 3.2-52.1 Ni < 1.0 4.7-9.4 3.2-7.7 Zn 11.1-108 9.0- 47.1 2.9-27.8

Table 3: Concentration Factor (CF) of Fe and Mn in P. boergesenll at different sites

Fe concentration Station Alga Water

GrandBaie 146 0.05 Palmar 67 0.02 Bambous Virieux 581 0. 10 Baie du Cap 414 0.08 Wolmar 207 0.05

reach a maximum of 21 8ppm (165ppm with Cd only). The preliminary results obtained here suggest that mechanisms involved could be synergistic rather than antagonistic. Both antagonistic (Stromgren 1980) and synergistic effects (Munda and Hudnik 1986) have been reported in metal uplake amongst the algae. Further work is needed to eluci­date the complex mechanisms involved in metal uptake in algae and also to confirm the prel iminary findings of th is study.

Acknowledgements - The authors thank the University of Mauritius for funding this work. Dr Mats Bjork is thanked tor his useful com­ments on an early draft of this paper.

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Edited by M Bjork and RN Pienaar

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