Effects of prey abundance and light intensity on nutrition of a mixotrophic flagellate and its competitive relationship with an obligate heterotroph

Abstract

The mixotroph Poterioochromonas malhamensis was grown in batch and semicontinuous cultures in order to examine the dependence of phagotrophy versus phototrophy at bacterial densities similar to those found in oligo-and mesotrophic lakes in combination with differing light regimes. In addition, the growth rates and biomass accumulation of the mixotroph were compared to those of the heterotroph Spumella elongata grown under similar conditions. In P. malhamensis, primary production rates and cell-specific chlorophyll a concentrations were significantly higher when grown at low bacterial densities, and the carbon cell -1 gained by photosynthesis corresponded to 79% of the carbon cell -1 gained by bacterial ingestion. Furthermore, due to a longer exponential growth phase, cell densities and biomass of P. malhamensis were significantly higher when the mixotroph was cultured in high light than in darkness. As a result, the cumulative biomass of the mixotroph was significantly higher in the light treatments than the biomass of the heterotroph at similar bacterial densities. These results suggest that photosynthesis may contribute more to the nutrition of P. malhamensis in lakes with low bacterial abundances than previously suggested, and that the mixotroph's capacity for photosynthesis enables it to dominate in terms of biomass over the heterotroph. KEY WORDS: Poterioochromonas malhamensis · Spumella elongata · Phagotrophy · Photosynthesis · Competition Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 36: [247][248][249][250][251][252][253][254][255][256] 2004 mixotrophic nutrition (Raven 1995(Raven , 1997. Thus, in the competitive relationship between mixotrophs and obligate heterotrophs, the former are believed to have an advantage only when prey densities are low and light is available (Jones 2000).The aims of this study were (1) to examine and compare growth and biomass accumulation of the mixotrophic flagellate Poterioochromonas malhamensis and the heterotrophic flagellate Spumella elongata grown under similar conditions in batch and semi-continuous cultures and (2) to further investigate the effect of light and bacterial densities similar to those found in oligotrophic and mesotrophic lakes on the reliance of phagotrophy versus phototrophy in P. malhamensis. Small nanoflagellates belonging to the genus Spumella spp. are very abundant in many lakes (Auer & Arndt 2001), and have been reported to be important consumers of bacteria (Weisse 2002). The mixotrophs Poterioochromonas spp. and closely related Ochromonas spp. are abundant bacterivores that, as a result of their nutritional flexibility, have received considerable interest over the years (e.g. Myers & Graham 1956, Handa et al. 1981, Lewitus & Caron 1991, Sanders et al. 2001. P. malhamensis is an efficient bacterivore, and it has been suggested that the flagellate relies on phagotrophy to obtain most or all of the energy used for metabolism and growth ). Furthermore, when actively growing on bacteria, ...

Exploring strategies for nanoflagellates living in a ‘wet desert’

“…We used the conversion factor from Wetzel & Likens (1991) as this factor has been applied by other investigators working on P. malhamensis (e.g. Pålsson & Daniel 2004) and closely matches recently suggested carbon conversion factors (e.g. Menden-Deuer & Lessard 2000).…”

“…For instance, in the case of Poterioochromonas malhamensis this argument was used in regard to Type Strain 933/1a (e.g. Holen 1999, Pålsson & Daniel 2004), but careful re-evaluation of the data showed that the findings on the different strains used in the laboratory studies were not contradictory at all (see subsection on the contribution of phototrophy; see also Pringsheim 1952) and that only the interpretation of the results differed. In effect, even a 60 yr old laboratory strain still responds in the same way as recently isolated strains.…”

“…'ultramicrobacteria' between 0.01 and 0.05 µm 3 in size: Hahn et al 2003, Cottrel & Kirchman 2004, are much smaller than the bacteria commonly used as prey for flagellates in laboratory experiments (0.25 to 0.5 µm 3 ; e.g. Rothhaupt 1996b, Pålsson & Daniel 2004. Thus, even though bacterial abundances in laboratory experiments are close to those in nature, bacterial biovolume usually exceeds natural conditions many-fold because experiments are typically performed with cultured bacteria that are larger than those found in plankton.…”

Bacterial grazing by mixotrophic flagellates and Daphnia longispina: a comparison in a fishless alpine lake

“…This success is due to the versatility of their behaviour: phagotrophy allows predominantly phototrophic mixotrophs to gain limiting nutrients in both laboratory (Nyagaard & Tobiesen 1993) and field conditions (Stoecker et al 1997, Olrik 1998, whereas mixotrophs that are mainly heterotrophic can survive in low prey environments by increasing their chlorophyll content (Sanders et al 1989). In lakes with low bacterial abundances, the ability of MxFl to photosynthesize enables them to dominate over heterotrophs in terms of biomass (Palsson & Daniel 2004).…”

Microbial food webs in the dark: independence of lake plankton from recent algal production

“…Even though the light level provided in our L treatments was low, phototrophic growth of mixotrophic flagellates and ciliates probably contributed to higher protozoan biomass in the L treatments. At a similar light intensity, Pålsson & Daniel (2004) observed that photosynthesis by the widely occurring flagellated mixotroph Poterioochromonas malhamensis could support 80% of the gain in cell carbon when bacterial abundances were low. Nevertheless, higher flagellate biomass in the L treatments may also partly be a result of ingestion of bacteria growing in the presence of both 'old' and algal-derived DOM.…”