Water Research 36 (2002) 2941–2948 Removal of ammonium and phosphorus ions from synthetic wastewater by the microalgae Chlorella vulgaris coimmobilized in alginate beads with the microalgae growth-promoting bacterium Azospirillum brasilense Luz E. de-Bashan a,b , Manuel Moreno a , Juan-Pablo Hernandez a , Yoav Bashan a, * a Environmental Biology Center for Biological Research of the Northwest (CIB), P.O. Box 128, La Paz, B.C.S. 23000, Mexico b Department of Biology, Pontificia Universidad Javeriana, Santafe de Bogota, Colombia Received 1 May 2001; accepted 1 November 2001 Abstract Coimmobilization of the freshwater microalga Chlorella vulgaris in alginate beads with the microalgae growth- promoting bacterium Azospirillum brasilense under semi-continuous synthetic wastewater culture conditions significantly increased the removal of ammonium and soluble phosphorus ions compared to immobilization of the microalgae alone. In continuous or batch cultures removal of these ions followed a similar trend but was less efficient than in semi-continuous culture. It is proposed that coimmobilization of a microalgae with microalgae growth- promotingbacteriacanserveasatoolindevisingnovelwastewatertreatments. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ammonium and phosphorus removal; Microbial immobilization; Microalgae; Plant growth-promoting bacteria; Wastewater treatment 1. Introduction The fresh water unicellular microalga Chlorella vulgaris is used for tertiary wastewater treatment mainly for the removal of nitrogen and phosphorus compounds and heavy metals [1–4]. It is also used for several industrial processes unrelated to wastewater treatment [5,6]. The interactions between the microalgae and other microorganisms in its ecological niche or during waste- water treatment have not been well studied. Pseudomo- nas diminuta and P. vesicularis, two obligate aerobes isolated from laboratory algal cultures, stimulated the growth of the microalgae Scenedesmus bicellularis and Chlorella sp., without releasing any growth-promoting substance [7]. Recently, we have shown that coimmobi- lization of C. vulgaris with the plant growth-promoting bacterium (PGPB, [8]) Azospirillum brasilense, used as aninoculantinagriculture[9],significantlyincreasedthe growth and pigment production of the microalgae [10,11], analogous to the effect of the bacterium on the growth of numerous terrestrial plants [12]. However, coimmobilization of the microalgae with its associative bacterium from the wastewater treatment pond, Phyllo- bacterium myrsinacearum, changed the metabolism of the microalga, but did not enhance its growth; the microalgae senesced and died earlier when associated with P. myrsinacearum than when associated with A. brasilense [11,13]. The aim of this study was to determine whether the growth-promotingassociationofthePGPB A. brasilense with C. vulgaris will also improve ammonium and phosphorus ion removal by the microalgae from batch, continuous and semi-continuous synthetic wastewater cultures. This has been done as a first step toward *Corresponding author. Fax: +52-112-54710. E-mail address: bashan@cibnor.mx (Y. Bashan). 0043-1354/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0043-1354(01)00522-X