The nuisance alga Gonyostomum semen (Raphidophyceae) is known to form dense blooms in many freshwater bodies, and has increased its distribution and abundance. However, information on the mechanisms behind bloom formation and maintenance is scarce. Field observations indicate that G. semen may be favored in humic lakes. In the present study, we performed controlled laboratory experiments on cultures to test whether G. semen growth is enhanced by humic acids. In addition, we tested the mixotrophic capability of G. semen by providing it with Rhodomonas lacustris as prey. G. semen increased its growth rate in the presence of humic substances due to either acquisition of carbon or some unknown growth factor. Moreover, growth rate was enhanced in the presence of R. lacustris, which in turn was negatively influenced when cultured together with G. semen. We found no evidence of phagotrophy, but observed lysis of R. lacustris cells exposed to G. semen pre-grown with R. lacustris. Thus, our experiments suggest that cell lysis (probably via trichocyst action) and osmotrophy could be 2 mechanisms involved in establishing and maintaining dominance of G. semen in humic lakes.

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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, ...

1. Five oligotrophic clear-water lakes on the Faroe Islands were studied during August 2000. Algal and bacterial production rates, community respiration, and CO 2 saturation were determined. In addition, we examined the plankton community composition (phytoplankton and heterotrophic nanoflagellates) and measured the grazing pressure exerted by common mixotrophic species on bacteria. 2. High respiration to primary production (6.6-33.2) and supersaturation of CO 2 (830-2140 latm) implied that the lakes were net heterotrophic and that the pelagic heterotrophic plankton were subsidised by allochthonous organic carbon. However, in spite of the apparent high level of net heterotrophy, primary production exceeded bacterial production and the food base for higher trophic levels appeared to be mainly autotrophic. 3. We suggest that the observed net heterotrophy in these lakes was a result of the oligotrophic conditions and hence low primary production in combination with an input of allochthonous C with a relatively high availability. 4. Mixotrophic phytoplankton (Cryptomonas spp., Dinobryon spp. and flagellates cf. Ochromonas spp.) constituted a large percentage of the plankton community (17-83%), possibly as a result of their capacity to exploit bacteria as a means of acquiring nutrients in these nutrient poor systems.