Phycological Research 2004; 52: 53–58 Research note Aggregation effects due to aluminum adsorption to cell walls of the unicellular green alga Scenedesmus obtusiusculus Maria Greger* and Monica Johansson Department of Botany, Stockholm University, S-106 91 Stockholm, Sweden SUMMARY Cells of the unicellular green alga Scenedesmus obtu- siusculus Chod. were cultivated for 2–24 h in nutrient media with low (60 µmol/L) or high (1000 µmol/L) phosphorus (P) concentration, and in the presence or absence of 222 µmol/L aluminum chloride (Al). Cell aggregation was studied by using light microscope, sedimentation and centrifugation. After 2 h, Al was adsorbed to the cell surface and cell aggregates were formed by the attraction of the cells to each other. Aluminum is bound by the negative charges of the cell walls, and studies at different pH showed that a high proportion of positively charged Al forms promote cell aggregation. This effect was most pronounced in low phosphorus cultures as phosphate reduces the effect of Al on cell aggregation by forming aluminum-phosphate. Algae cultivated in the absence of Al did not show any cell aggregation tendencies. Key words: aluminum, cell aggregation, phosphorus, Scenedesmus, unicellular green alga. Mobilization of aluminum (Al) is a well known problem in acidifying lakes (Dickson 1978; Persson and Broberg 1985). A decrease in pH increases Al solubility and its availability to aquatic organisms. Natural acidic waters may contain up to 48 µ mol/L Al (Bingman 1986). Aluminum is taken up by algae and decreases both cell division and photosynthesis (Pettersson 1989; Törn- quist 1989; Greger et al. 1992a). Aluminum affects nutrient metabolism by increasing net uptake of iron (Fe), magnesium (Mg), calcium (Ca) and phosphorus (P) and precipitates with P both inside and outside the cells (Pettersson 1989; Törnquist 1989; Greger et al. 1992b). Precipitation of Al with P on cell walls has been recorded for a wide variety of plant taxa including barley (McCormick and Borden 1974) and pea (Klima- shevskii et al. 1979) and also for cyanobacteria (Pettersson et al. 1985). In plant cells, aluminum is adsorbed by attachment to pectic substances in the cell wall and calcium binding sites on the cell surface (Wagatsuma 1983). Bradley and Parker (1968) measured a concentration of 1.2 mmol Al per gDW in the cell walls of the bacterium Staphylococcus aureus. Up to 80% of accu- mulated Al was present on the cell surface of the unicellular green alga Scenedesmus obtusiusculus, after cultivation in aluminum (Greger et al. 1992b). In the presence of Al, cells of Scenedesmus obtusi- usculus often form cell aggregates (Greger and Johans- son, unpubl. data, 1992). The formation of aggregates could be the reason underlying the decreased photo- synthesis and affected nutrient uptake in the presence of Al by shadowing and decreasing the absorption area for nutrients, which could be a great problem for algae living in lakes high in free Al (Greger et al. 1992a,b). Our hypothesis was that Al-cations bind to the negative charges of the cell walls and the more positive charges the Al-ion has the higher is the capacity of the Al-ion to bind to more than one cell and thus form cell aggre- gates. Furthermore, if phosphate is present this anion binds to Al in the solution and prevents the formation of cell aggregates. It has been shown that Al 3+ induces aggregation in phosphatidylserine-containing lipid ves- icles (Deleers et al. 1985). Flocculation of organic particles was performed with help of hydrolytically formed positively charged polynuclear Al or Fe hydroxy complexes (Bernhardt et al. 1985). Soil colloids, which have charged surfaces, form colloid aggregates in the presence of Al 3+ , Ca 2+ and Na + (Mengel and Kirkby 1982). Different cations have different capacities for aggregate formation, and Al 3+ has a high capacity relative to other cations (Mengel and Kirkby 1982). The aim of this study was to investigate if the aggregation caused by Al is a slow or fast process, if it is caused by extracellular-adsorbed Al and if the number of positive charges of the Al species influence the potential for aggregate formation. Because P pre- cipitates with Al, the influence of P on the aggregate formation, in the presence of Al, was also studied. The results are also discussed with respect to pH. *To whom correspondence should be addressed. Email: maria.greger@botan.su.se Communicating editor: I. Mine. Received 12 April 2002; accepted 20 September 2003.