Temporal dynamics and growth of <i>Actinophrys sol</i> (Sarcodina: Heliozoa), the top predator in an extremely acidic lake

<jats:title>Summary</jats:title><jats:p>1. The <jats:italic>in situ</jats:italic> abundance, biomass and mean cell volume of <jats:italic>Actinophrys sol</jats:italic> (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001–03).</jats:p><jats:p>2. <jats:italic>Actinophrys sol</jats:italic> exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 × 10<jats:sup>3</jats:sup> Heliozoa L<jats:sup>−1</jats:sup>. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. <jats:italic>Actinophrys sol</jats:italic> cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems.</jats:p><jats:p>3. We determined the growth rate of <jats:italic>A. sol</jats:italic> using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single‐celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates <jats:italic>Chlamydomonas acidophila</jats:italic> and <jats:italic>Ochromonas</jats:italic> sp., the ciliate <jats:italic>Oxytricha</jats:italic> sp. and the rotifers <jats:italic>Elosa worallii</jats:italic> and <jats:italic>Cephalodella hoodi</jats:italic>. <jats:italic>Actinophrys sol</jats:italic> fed over a wide‐size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. <jats:italic>Actinophrys sol</jats:italic> neither increased in cell number (<jats:italic>k</jats:italic>) nor biomass (<jats:italic>k</jats:italic><jats:sub><jats:italic>b</jats:italic></jats:sub>) when starved, with low concentrations of single‐celled bacteria or with the alga <jats:italic>Ochromonas</jats:italic> sp. Positive growth was achieved with single‐celled bacteria (<jats:italic>k</jats:italic> = 0.22 ± 0.02 d<jats:sup>−1</jats:sup>; <jats:italic>k</jats:italic><jats:sub>b</jats:sub> = −0.06 ± 0.02 d<jats:sup>−1</jats:sup>) and filamentous bacteria (<jats:italic>k</jats:italic> = 0.52 ± <0.01 d<jats:sup>−1</jats:sup>; <jats:italic>k</jats:italic><jats:sub>b</jats:sub> = 0.66 d<jats:sup>−1</jats:sup>) at concentrations greater than observed <jats:italic>in situ</jats:italic>, and the alga <jats:italic>C. acidophila</jats:italic> (up to <jats:italic>k</jats:italic> = 0.43 ± 0.03 d<jats:sup>−1</jats:sup>; <jats:italic>k</jats:italic><jats:sub>b</jats:sub> = 0.44 ± 0.04 d<jats:sup>−1</jats:sup>), the ciliate <jats:italic>Oxytricha</jats:italic> sp. (<jats:italic>k</jats:italic> = 0.34 ± 0.01 d<jats:sup>−1</jats:sup>) and in mixed cultures containing rotifers and <jats:italic>C. acidophila</jats:italic> (<jats:italic>k</jats:italic> = 0.23 ± 0.02–0.32 ± 0.02 d<jats:sup>−1</jats:sup>; maximum <jats:italic>k</jats:italic><jats:sub>b</jats:sub> = 0.42 ± 0.05 d<jats:sup>−1</jats:sup>). The individual‐ and biomass‐based growth of <jats:italic>A. sol</jats:italic> was highest when filamentous bacteria were provided.</jats:p><jats:p>4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. <jats:italic>Actinophrys sol</jats:italic> are omnivorous predators supported by a mixed diet of filamentous bacteria and <jats:italic>C. acidophila</jats:italic> in the epilimnion. Heliozoa are important components in the planktonic food webs of ‘extreme’ environments.</jats:p>