Vol. 76: 219–232, 2016 AQUATIC MICROBIAL ECOLOGY Published online February 22 doi: 10.3354/ame01780 Aquat Microb Ecol OPENPEN ACCESSCCESS Presence, abundance and bacterivory of the mixotrophic algae Pseudopedinella (Dictyochophyceae) in freshwater environments Marina Gerea1,*, Juan F. Saad2, Irina Izaguirre2, Claudia Queimaliños1, Josep M. Gasol3, Fernando Unrein4 1Laboratorio de Fotobiología, INIBIOMA, CONICET-Universidad Nacional del Comahue, Quintral 1250, (R8400FRD) Bariloche, Argentina 2Departamento de Ecología, Genética y Evolución, IEGEBA, UBA-CONICET, Ciudad Universitaria, (C1428EHA) Buenos Aires, Argentina 3Institut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta, 37−49. 08003 Barcelona, Catalonia, Spain 4Laboratorio de Ecología y Fotobiología Acuática, IIB-INTECH, UNSAM-CONICET, Av. Intendente Marino km 8,200, (B7130IWA) Chascomús, Argentina ABSTRACT: The genus Pseudopedinella has been described as mixotrophic; however, ecological information about this algal stramenopile (Heterokonta) is unclear. We investigate the environ- mental conditions that determine the presence, abundance and bacterivory rates of this genus in freshwater systems. To this end we analyzed 54 water bodies with different limnological features distributed along a latitudinal and trophic gradient in northern Patagonia (Argentina) and the Antarctic Peninsula. In addition, 14 grazing experiments were carried out in order to estimate in- gestion rates and impact on the bacterioplankton. Our results indicate that this genus is exclu- sively found in oligotrophic environments, and that it develops well in a wide range of tempera- tures. Average cell-specific grazing rate was 2.83 bacteria cell–1 h–1, with a maximum value of 6.74 bacteria ml–1 h–1. Interestingly, grazing increased with prey abundance and decreased with in- creasing nutrient availability. These patterns are common in highly bacterivorous protists that use phagotrophy as a main source of nutrient acquisition. Despite their usually low abundance (avg. 182 cells ml–1), this single genus was responsible for up to 24% (avg. 10%) of the total grazing im- pact exerted by all phagotrophs in these lakes. Overall, our results support the idea that Pseudo- pedinella is a highly bacterivorous group of freshwater protists, with the ability to develop well in oligotrophic conditions and with a potentially significant impact on bacterioplankton. KEY WORDS: Pseudopedinella · Bacterivory · Grazing rate · Patagonian lakes · Antarctic lakes INTRODUCTION al. 2008, Flynn et al. 2013). Mixotrophic flagellates (MF) are major grazers of bacteria in oligotrophic Among planktonic algae the phenomenon of mixo- systems, often being responsible for >50% of the trophic nutrition, that is the combination of phago - total bacterivory (Bird & Kalff 1986, Unrein et al. trophy and phototrophy, has been known for about a 2007, Hartmann et al. 2012). century (Biecheler 1936). Nevertheless, the rele- Mixotrophy is widespread among phytoplankton vance of these algae as bacterivores in pelagic (Tranvik et al. 1989, Nygaard & Tobiesen 1993, trophic webs was only recognized a few decades ago Stoecker et al. 1997) and it has been demonstrated in (Bird & Kalff 1986, Nygaard & Tobiesen 1993). Nowa- many algal groups, such as chrysophytes, hapto- days, mixotrophy is considered a key life strategy phytes, dinoflagellates, cryptophytes, prasinophytes among marine and freshwater organisms (Gasol et and raphidophytes (Bird & Kalff 1986, Stoecker 1999, © The authors 2016. Open Access under Creative Commons by *Corresponding author: [email protected] Attribution Licence. Use, distribution and reproduction are un - restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com 220 Aquat Microb Ecol 76: 219–232, 2016 Callieri et al. 2006, Unrein et al. 2007, 2014, Carvalho by nanoplanktonic flagellated algae (Izaguirre et al. & Granéli 2010, Jeong et al. 2010, Maruyama & Kim 1998, 2003, Queimaliños & Diaz 2014). In particular, 2013, McKie-Krisberg et al. 2015). Several studies Pseudopedinella sp. were detected in some oligo - have attempted to classify mixotrophs into categories trophic Antarctic lakes using molecular fingerprinting based on the relative importance of their phago- and microscopic techniques (Unrein et al. 2005). In trophic and phototrophic activities (Jones 1994, 2000, the present investigation, we studied 54 water bodies Stoecker 1998). While some discrepancies exist, most located along a latitudinal gradient from North Patag- studies agree that each taxon differs regarding the onia to the Antarctic Peninsula covering a wide relative magnitude of both modes of nutrition, and variety of lake types. The aim of this study was to ana- consequently in their potential impact on Bacteria and lyze in detail the cellular morphology of this genus, Archaea (from now on bacterioplankton for simplic- and identify the main environmental variables that ity). However, there are relatively few investigations constrain the presence, abundance, grazing rates and carried out on natural communities that comparatively impact of Pseudopedinella on the bacterioplankton. analyze in situ bacterivory rates and grazing impact We hypothesized that Pseudo pedinella might exhibit among various MF. The available data suggest that high in situ bacterivory rates comparable with other the chrysophytes are one of the most ‘heterotrophic’ heterotrophic organisms, and might significantly im- algae, with high relative contribution of phagotrophy pact on bacterioplankton communities. As mentioned to their growth (Sanders et al. 1990, Olrik 1998, Rot- above, different MF might differ in the use of phago- tberger et al. 2013) and high impact on the bacterial trophy for growth and consequently in their impact on community in oligotrophic lakes (Bird & Kalff 1986, bacterioplankton. Increasing our knowledge about Schmidtke et al. 2006, Unrein et al. 2010). Neverthe- the ecological requirements and grazing rates of each less, some differences could be expected among gen- MF taxa will help to better understand their role on era or species belonging to the same class. In this microbial trophic webs. In the present manuscript, we sense, comparative analyses evaluating the physio- contribute to the characterization of Pseudopedinella logical range of two strains of mixotrophic chryso- sp. as a potentially significant bacterivore. phytes (Dinobryon divergens and Poterioochromonas malhamensis) have demonstrated marked differences in the contribution of phagotrophy to their nutritional MATERIALS AND METHODS requirements (Rottberger et al. 2013). Heterotrophic nutrition in the stramenopile (Het- Study site erokonta) Pseudopedinella has been long recognized (Porter 1988, Havskum & Riemann 1996, Sekiguchi et A total of 54 lakes were sampled along a north- al. 2003, Stauffer et al. 2008). However, to the best of south transect of 2800 km, from 41º 03’ S in North our knowledge, its impact on the bacterioplankton Patagonia (Argentina) to 63° 23’ S in the Antarctic has been quantified only twice (Nygaard & Tobiesen Peninsula (Fig. 1). Patagonian lakes are categorized 1993, Havskum & Riemann 1996). The taxonomic sta- as belonging to 2 different geographical regions tus of this genus has changed during the last few according to their geological origin: ‘Patagonian decades. Within the group Stramenopiles, Pseudope- Plateau Region’ and ‘Andean Patagonian Region’ dinella was first considered a member of the class (Iriondo 1989, Quirós & Drago 1999). Lakes located in Chrysophyceae, while since the 1980s it has been in- the first area are generally shallow and mesotrophic cluded within the class Dictyochophyceae (Hibberd to eutrophic, where as Andean Patagonian lakes are 1986, Kristiansen & Sandgren 1986, Kristiansen 1990). of glacial origin and consequently deeper than the However, several ecological studies still include this Plateau lakes. Due to the rocky basin formation of the nanoplanktonic genus in the Chrysophyceae (see ref- Andean lakes, they often present oligotrophic and erences in Table S1 in the Supplement at www.int- ultraoligotrophic status. Mean annual temperature in res.com/ articles/ suppl/a076 p219 _ supp. pdf). Its usually Patagonia ranges from 12°C in the northeast to 3°C low abundance in natural samples and the misplace- towards the south. From the Andes and eastward, ment within the class Chrysophyceae in many studies total annual pre cipitation decreases exponentially might explain the lack of information about this genus from ca. 2000 to <200 mm yr−1 in the central portion with respect to its environmental requirements, distri- of Patagonia (Paruelo et al. 1998). The studied bution and impact on bacterioplankton. Antarctic lakes are located in Hope Bay at the North- As a general trend, the phytoplankton community ern end of the Ant arctic Peninsula. Most of them are of Antarctic and most Patagonian lakes is dominated shallow and of glacial origin. Extensive information Gerea et al.: The mixotrophic algae Pseudopedinella 221 again in January 2010. Finally, (5) the 6 Antarctic water bodies were sampled twice between January and February 2004. Detailed in formation about each water body is presented in Table S2 in the Supplement at www.int-res.com/ articles/ suppl/a076 p219 _ supp. pdf. Sampling data In the different campaigns three general types of water bodies were sampled: deep lakes (DL), shallow lakes (SL) and tempo- rary ponds (P). For deep lakes, integrated samples were collected within