Microbial Ecology Bacterial Uptake by the Marine Sponge Aplysina aerophoba Markus Wehrl 1 , Michael Steinert 2 and Ute Hentschel 1 (1) Zentrum fu ¨r Infektionsforschung, Ro ¨ntgenring 11, Universita ¨t Wu ¨rzburg, D–97070 Wu ¨rzburg, Germany (2) Institut fu ¨r Molekulare Infektionsbiologie, Ro ¨ntgenring 11, Universita ¨t Wu ¨rzburg, D–97070 Wu ¨rzburg, Germany Received: 7 March 2006 / Accepted: 21 April 2006 / Online publication: 31 January 2007 Abstract Sponges (Porifera) are filter feeders that take up micro- organisms from seawater and digest them by phagocytosis. At the same time, many sponges are known to harbor massive consortia of symbiotic microorganisms, which are phylogenetically distinct from those in seawater, within the mesohyl matrix. In the present study, feeding experi- ments were performed to investigate whether phylogenet- ically different bacterial isolates, hereafter termed Bfood bacteria,^ microbial seawater consortia, and sponge symbiont consortia are taken up and processed differently by the host sponge. Aplysina aerophoba retained high numbers of bacterial isolates and microbial seawater con- sortia with rates of up to 2.76  10 6 bacteria (g sponge wet weight) –1 h –1 , whereas the retention of sponge sym- bionts was lower by nearly two orders of magnitude [5.37  10 4 bacteria (g sponge wet weight) –1 h –1 ]. In or- der to visualize the processing of a food bacterium with- in sponge tissues, the green fluorescent protein-labeled Vibrio strain MMW1, which had originally been isolated from A. aerophoba, was constructed. Incubation of this strain with A. aerophoba and subsequent visualization in tissue cryosections showed its presence in the choano- cytes and/or endopinacocytes lining the canals but, un- like latex beads, not in deeper regions of the mesohyl, which suggests digestion of the bacteria upon contact with the host. Denaturing gradient gel electrophoresis (DGGE) was performed on the incubation seawater to monitor the changes in phylogenetic composition after incubation of the sponge with either seawater or sponge symbiont consortia. However, the DGGE experiment provided no evidence for selective processing of individ- ual lineages by the host sponge. In conclusion, this study extends early studies by Wilkinson et al. (Proc R Soc London B 220:519–528, 1984) that sponges, here A. ae- rophoba, are able to differentiate between food bacteria and their own bacterial symbionts. Introduction As filter feeders, sponges (Porifera) are capable of pro- cessing large volumes of seawater equivalent to many thousand of liters (kg sponge) –1 day –1 [27]. Heterotro- phic bacteria, cyanobacteria, and nano- and picoeukar- yotes are the most important components of the sponge diet [2, 18, 21–23, 28, 29, 40]. Particle uptake occurs at least at three functional sites: large particles (950 mm) are taken up at the sponge surface by epithelial pinacocytes, smaller particles (G50 mm) capable of entering the ostia are taken up by pinacocytes lining the progressively narrower canal walls, and even smaller particles (G5 mm) are trapped in the choanocyte chambers [26, 35]. After particle capture by pinacocytes and choanocytes, food particles are passed to the mesohyl cells by transcytosis. Once inside the mesohyl, food particles are phagocytosed by sponge cells, termed archaeocytes. Most studies on sponge feeding focused on in situ measurements of feeding/grazing rates under natural conditions. The general perception from these studies is that particle uptake by sponges is highly efficient but largely unselec- tive [i.e., 22, 26, 29]. Although particle size has been identified as one important factor for uptake in these and other studies [35], the potential effects of bacterial surface properties (i.e., membrane structure, membrane proteins, flagella) are still entirely unknown. Although sponges feed on microorganisms, large amounts of extracellular bacteria populate the mesohyl matrix of the so-called bacteriosponges [36, for recent reviews, see 11, 13, 16, 17]. For the purpose of this study, they are collectively referred to as Bsponge symbionts^ following the original definition of de Bary [4], although their benefits for the host sponge, if any, are not known. Sponge symbionts may constitute as much as 40–60% of Correspondence to: Ute Hentschel; E-mail: ute.hentschel@mail.uni- wuerzburg.de DOI: 10.1007/s00248-006-9090-4 & Volume 53, 355–365 (2007) & * Springer Science + Business Media, Inc. 2007 355