Research Article
Heavy Metal Levels in Saudi Arabian Spirulina
Department of Botany and Microbiology, College of Science, King Saud University, P.O. Box 2455, Riyadb, 11451, Saudi Arabia
The microalga (cyanobacterium) Spirulina (Arthrospira) is one of the most important microorganisms due to its suitable content of protein, vitamins, minerals, pigments and phytonutrients. Spirulina platensis was the first cyanobacterium to be commercially cultivated using modern biotechnology (Hu, 2004). It has been used as a food supplement for human and animal nutrition (Ciferri, 1983; Jassby, 1988a; Richmond, 1988; Belay, 1997; Al-Batshan et al., 2001; Becker, 2004). The final product of Spirulina must meets the quality criteria set by different producers and organizations in order to be used as a food supplement in local and international markets (Jassby, 1988b; Becker, 1988, 1994, 2004; Belay, 1997).
In Saudi Arabia, the interest in Spirulina started in 1999 when the Arabian Agricultural Service Company (ARASCO) established a small farm of this microalga (Al-Homaidan, 2002). The effects of the dry powder of this farm on chicken macrophage phagocytic function and nitrite production were examined (Al-Batshan et al., 2001). The impacts of temperature, pH and salinity on the growth and protein content of two locally isolated species of Spirulina were studied (Al-Homaidan et al., 2005). Sabbagh (2006) studied the effects of cadmium, copper, lead and uranium on chlorophyll, DNA, protein content and ultrastructure of S. platensis. He also measured the heavy metal content of five edible algae collected from Riyadh health food stores.
The purpose of this study was to determine the heavy metal concentrations in a locally grown strain of S. platensis and to decide whether or not this Spirulina is suitable as a food and feed supplements in Saudi Arabia and other markets.
Spirulina culture: Spirulina platensis strain was acquired from the university of Texas at Austin, U.S.A. (UTEX No. LB 2340). It was propagated in the laboratory using the modified Zarrouk medium (Vonshak, 1997a). Outdoor mass cultures were carried out according to the methods described by Gupta and Changwal (1992), Venkataraman and Mahadevaswamy (1992), Becker (1994), Fox (1996), Belay (1997), Vonshak (1997b) and Al-Homaidan (2002). They were conducted in 200-500 m2 oblong raceway ponds lined with polyvinylchloride plastic. The cultures were enriched with a commercial compound fertilizer containing equal amounts of N, P and K and less than 0.1% of micronutrients. The concentration of the Sodium bicarbonate which was added to the cultures was between 13 and 14 mg L1. The culture depth was maintained at 10-12 cm by daily addition of enriched freshwater. Paddle wheels were used to circulate the water at a speed of about 30 cm sec1. Harvesting was carried out by filtration through nylon filters (150-200 mesh). The biomass slurry was rinsed in acid water at pH 4.0 followed by freshwater. Drying was conducted under direct sunlight.
Sample preparation for atomic absorption spectroscopy: Algal samples were air dried at 90°C and 50 mg of the air dried samples were placed into digestion tubes. Twenty mL of concentrated nitric acid was added to each tube and the contents of the tubes were digested at 120°C for about 2 h. After cooling, 20 mL of double distilled water was added to each tube and the content was filtered through 0.45 μm millipore filters. The solutions were transferred to 25 mL volumetric flasks and the volumes were completed to 25 mL with double distilled water. Concentrations of As, Cd, Cu, Fe, Hg, Mn, Pb and Zn were determined in these aliquot samples (Lajunen, 1992; Sadiq and Zaidi, 1994). Two models of Shimadzu atomic absorption spectrometers were used for the measurement of metals. These were AA-6650F flame AAS with auto sampler and AA-6650G controlled furnace AAS with hydride vapor generator.
The average concentrations of As, Cd, Hg and Pb are shown in Table 1. Very low levels were found for these highly toxic non-essential metals. The concentration averages found for each contaminant, expressed in mg kg1 dry weight, were as follows; As, 0.002; Cd, 0.031; Hg, 0.008; Pb, 0.109. Higher concentrations were obtained for the essential metals. The average concentrations of Cu, Fe, Mn and Zn, expressed in mg kg1 dry weight, were 8.51, 394.0, 27.88 and 21.81, respectively (Table 2). Elevated amounts of heavy metals in algal biomass is one of the major problems that limit the large-scale utilization of microalgae (Jassby, 1988b; Becker, 1994). According to Becker (2004), no official standards exist for heavy metal levels in microalgal products. Some of the algal major producers have established their own guidelines for the heavy metal content of their microalgal products. For instance, guidelines for the concentrations of As, Cd, Hg and Pb in Spirulina were adopted by the major producers in different countries around the world (Boudene, 1975; Belay, 1997; Torres-Duran et al., 1998). A comparison between these levels and the heavy metal content of Saudi Arabian Spirulina is presented in Table 1. It can be seen that the heavy metal concentrations in the locally produced Spirulina is much lower than the guidelines established by major producers and it can be said that the local Spirulina will not cause any toxicological effects on consumers. Another comparison also can be made between the Cd and Pb contents of the local Spirulina and that obtained from Riyadh health food stores.
Table 1: | As, Cd, Hg and Pb content of Saudi Arabia Spirulina in relation to international standards (data given in mg kg1) |
Table 2: | Cu, Fe, Mn and Zn content of Saudi Arabia Spirulina in relation to Earthrise Farms (data given in mg kg1) |
Sabbagh (2006) found out that the concentrations of Cd in two brands of imported Spirulina ranged between 0.06 and 0.08 mg kg1 and at the same time the Pb levels in these two samples varied between 0.13 and 0.26 mg kg1. These levels are much higher than the concentrations found in this study.
The essential elements Cu, Fe, Mn and Zn are required in low concentrations by all kinds of life because they play important roles in metabolic processes taking place in living cells (Botkin and Keller, 2005). However, elevated levels of these elements are toxic to most organisms (Kaplan, 2004). To the author knowledge, no guidelines for the levels of these metals in microalgal products have been established by any major producer. By comparing the findings of this study with Earthrise Farms, which are the world largest Spirulina producers (Table 2) it is possible to say that the Saudi Arabian Spirulina content of Cu, Fe, Mn and Zn are within the expected limits for this alga. Similar conclusion will be achieved by comparing the results of this study with that of Jassby (1988a).
The author thanks King Abdulaziz City for Science and Technology (KACST) for providing funding for this project (Project No. PSG-1-1). The technical assistance provided by Dr. Gamal A. Salim of ARASCO is greatly appreciated.