Biologia 63/6: 799—805, 2008 Section Botany DOI: 10.2478/s11756-008-0101-4 Siderocelis irregularis (Chlorophyta, Trebouxiophyceae) in Lake Tanganyika (Africa)* Maya P. Stoyneva1, Elisabeth Ingolič2,WernerKofler3 &WimVyverman4 1Sofia University ‘St Kliment Ohridski’, Faculty of Biology, Department of Botany, 8 bld. Dragan Zankov, BG-1164 Sofia, Bulgaria; e-mail: [email protected], [email protected]fia.bg 2Graz University of Technology, Research Institute for Electron Microscopy, Steyrergasse 17,A-8010 Graz, Austria; e-mail: [email protected] 3University of Innsbruck, Institute of Botany, Sternwartestrasse 15,A-6020 Innsbruck, Austria; e-mail: werner.kofl[email protected] 4Ghent University, Department Biology, Laboratory of Protistology and Aquatic Ecology, Krijgslaan 281-S8,B-9000 Gent, Belgium; e-mail: [email protected] Abstract: Siderocelis irregularis Hindák, representing a genus Siderocelis (Naumann) Fott that is known from European temperate waters, was identified as a common phytoplankter in Lake Tanganyika. It was found aposymbiotic as well as ingested (possibly endosymbiotic) in lake heterotrophs, mainly Strombidium sp. and Vorticella spp. The morphology and ultrastructure of the species, studied with LM, SEM and TEM, are described with emphasis on the structure of the cell wall and the pyrenoid. Key words: Chlorophyta; cell wall; pyrenoid; symbiosis; ciliates; Strombidium; Vorticella Introduction ics of symbiotic species in general came into alignment with that of free-living algae and the term ‘zoochlorel- Tight partnerships between algae and aquatic inver- lae’ was abandoned as being taxonomically ambiguous tebrates, including symbiotic relationships, have long (e.g. Bal 1968; Reisser & Wiessner 1984; Taylor 1984; been of interest and a number of excellent reviews are Reisser 1992a). It was proposed to replace this obsolete available. In the context of modern biology, Reisser notion by the prefix ‘symbiotic’, and the exact species or (1992b, p. xii) defined ‘symbiosis’ as the ‘living together genus designation or, in cases of uncertainty, by ‘sym- in physical contact of organisms of different species’, biotic green alga’ (Reisser 1984; Reisser & Widowski while the interaction between heterotrophic cells and 1992). The terms ‘endosymbiotic’ and ‘endosymbiont’ ‘symbiotic’ chloroplasts derived from ingested algae was were further clarified by Reisser (1992a, p. 2) to be used mentioned by him as “twilight-zones” of symbiosis in- in a general sense as ‘living within genetically different cluded there by tradition. organism... which will be called “host” without any Many symbiotic genera and species appear closely implications as to mutual harm or benefit. related to, or fall within, recognized free-living assem- Heterotrophic hosts seem to be very fastidious blages. However, by strong contrast with the free-living in choosing a potential autotrophic partner. Although (aposymbiotic) algae the descriptions of most of the the freshwater habitat offers a plethora of diverse symbiotic forms are still quite rudimentary and often cyanoprokaryotes and algae, they pick up exclusively contradictory (Reisser 1984). Following the studies of asexual green coccoid species (Reisser 1986). Due to Brandt (1882) the most commonly applied term to their simple morphology, classification by light mi- the green autotrophic partners was ‘zoochlorellae’. This croscopy alone is not easy. It has commonly been stated term, without any taxonomic value and arbitrarily ex- that the predominant symbiotic algal partners of fresh- tended to every green algal cell that lives in associa- water associations belong to the genus Chlorella (with tion with a heterotrophic organism, was characteristic some preference to the Chlorella vulgaris-sorokiniana of the confused state of knowledge on symbiotic forms cluster) and its representatives do not differ principally (Reisser 1984). After many discussions on whether exis- by morphology or cytology from free-living chlorellae, tence in a symbiotic association was a sufficient reason but are distinguished from them mainly by physiolog- to place these organisms in separate taxa, the systemat- ical characteristics (Reisser & Widowski 1992). More * Presented at the International Symposium Biology and Taxonomy of Green Algae V, Smolenice, June 26–29, 2007, Slovakia. c 2008 Institute of Botany, Slovak Academy of Sciences 800 M.P. Stoyneva et al. recently, the first molecular phylogeny based on 18S for a tropical coloured lake with abundance of ciliates rRNA sequences was reported and revealed paramae- up to 250 000 L−1. This suggests that mixotrophic cili- cian symbionts which were genetically close to so-called ates may play a substantial role in plankton photosyn- Chlorella vulgaris group (Hoshina et al. 2004, 2005). Oc- thesis. casionally, other green algae (species of Scenedesmus, The considerable role of ciliates is especially valid Ankistrodesmus, Pleurococcus, Oocystis, Choricystis, for the ancient, large tropical oligotrophic Lake Tan- Chlamydomonas, Symbiococcum and a few prasino- ganyika (Hecky & Kling 1981, 1987; Pirlot et al. 2005). phyceans) have been reported to occur in freshwater There ‘the biomass of Strombidium cf. viride nearly symbiotic systems (for references consult Reisser 1984; equaled or exceeded the phytoplankton biomass dur- Rahat 1992; Reisser & Widowski 1992; Nakahara et al. ing much of the stably stratified period; this protozoan 2004). However, these data, based mainly on field ob- probably has a symbiotic relationship with zoochlorel- servations, need careful corroboration by modern tax- lae, which were always present in it’ (Hecky & Kling onomic methods applied to material grown under lab- 1981, p. 548). More recently, ciliates are still found to oratory conditions in order to exclude the possibility be an abundant group in the lake (Descy et al. 2005) that the observed algae have only been taken as a food and in 2002, total autotrophic carbon was partly sus- (Reisser 1986; 1992a; Reisser & Widowski 1992). tained by the endosymbionts in Strombidium (Pirlot The hosts of freshwater endosymbiotic associations et al. 2005). When fixed phytoplankton samples were can be assigned to different groups but among them processed during the same study of Lake Tanganyika, the overwhelming are chromalveolate protists, predom- it was observed that this abundant ciliate contained inantly ciliates. This fact has long been linked with their numerous coccoid green algal cells. Similar green cells ability to feed via phagocytosis (Reisser 1992a). Ciliates were detected in the samples in different quantities as are commonly considered as herbivores, but they are ac- free-living or inside other zooplankters (e.g. Vorticella tually composed of more or less distinct trophic guilds spp.). Using light microscopy the alga was identified (Dolan 1991). The key role that ciliates may play in as Siderocelis irregularis Hindák, a first record for the food webs in lakes was stressed by many authors dur- lake (Descy et al. 2005). For the present study, selected ing the last few decades. These studies have revealed samples were prepared for scanning (SEM) and trans- that ciliate communities are composed of a high num- mission electron microscopy (TEM). Here we describe ber of species with different sizes that reflect a diverse the cell morphology, cytological peculiarities and repro- range of feeding strategies, ranging from phagotrophic duction mode of Siderocelis irregularis and we discuss ingestion of particles (including nanoplanktonic algae) its taxonomic position. to photosynthetic fixation of organic carbon. The pho- tosynthetic capability can be acquired by sequestration Material and methods of chloroplasts from ingested prey or by possession of photosynthetic cells as endosymbionts (Modenutti et al. The material was collected from the large deep ancient tec- 2000; Endo & Taniguchi 2006; Summerer et al. 2007). tonic tropical Lake Tanganyika. In 2002–2004, standard bi- This mixed nutrition, termed mixotrophy, may improve weekly phytoplankton sampling at a depth of 20 m was access to scarce nutrients since by excretion of carbohy- conducted in the regions of Kigoma and Mpulungu. Ad- drates host metabolism could be sustained (e.g. Stabell ditional samples through a longitudinal transect of the lake were collected during a cruise in July 2003. A Zeiss Ax- et al. 2002). Therefore mixotrophy can be viewed as a iovert 135 inverted microscope was used for phytoplankton kind of symbiosis and also as an adaptive strategy that counting. Detailed investigations on non-permanent slides provides greater flexibility in the planktonic environ- were carried-out on a Leitz Diaplan microscope with Differ- ment and particularly strong advantage in oligotrophic ential Interference Contrast. Staining involved Indian ink, waters (Jones 1994; Schoonhoven 2000). These easily Methylene Blue, Gentiane Violet and Iodine. TEM method- explains why ciliates appear in quite different types of ology followed G¨artner & Ingoli´c (2003). Micrographs were waters (Stoecker 1998) but are especially common in taken with a FEI Tecnai 12 TEM microscope with a Gatan oligotrophic waters where a close nutrient cycle within CCD camera Bioscan. For SEM studies algal cells were the symbiotic association would confer advantages to washed with distilled water and dehydrated in a series of ethanol with gradually increasing concentrations (from 10% both symbiotic partners (Fenchel 1987; Modenutti et to 96%; 30 min. each). The material was then transferred to al. 2000).