The mixotrophic protists (= protists which combine heterotrophy and photosynthesis) are common in marine waters around the world. They vary widely in their photosynthetic and ingestion capabilities and they add a further complication to the marine planktonic food web. This thesis focused on 2 groups of mixotrophic protists: 1) symbiont-bearing dinoflagellates and 2) ciliates belonging to the genus Mesodinium spp.
The spatial distribution (horizontal and vertical) of symbiont-bearing dinoflagellates (order Dinophysiales, genera Ornithocercus, Histioneis Parahistioneis, Cithoristes, Amphisolenia, Triposolenia) was investigated along a transect from the deep ocean (Indian Ocean) to shallow coastal waters (North West off Australia), as well as on a transect outside Broome (Australia). The symbionts of these dinoflagellates are either prokaryotic (e.g. heterotrophic bacteria, cyanobacteria) or eukaryotic algae. Cell concentrations of these dinoflagellates were very low in these waters (< 4 cells L-1). The ectosymbionts-bearing dinoflagellates were most common and had the highest species diversity in waters characterized by high temperatures (> 28 ºC) and very low nitrogen concentrations (< 0.4µM). Using light and transmission electron microscopy, we could demonstrate that Ornithocercus spp. Ingested not only their ectosymbionts but also other prey items (i.e. ciliates). For future research on their physiology, the successful establishment of these organisms in laboratory culture is required.
The ciliate genus Mesodinium contains heterotrophic and mixotrophic species (so far only one symbiont containing species M. rubrum has been described). This study investigated the prey selection, photo and feeding physiology of a non-symbiont containing Mesodinium species, M. pulex. The results showed that Mesodinium pulex ingests a variety of prey cells, but that ingestion rates and especially growth rates varied depending upon the diet. The effects of light and prey concentration on photosynthesis, ingestion and growth rate of M. pulex was studied in detail when fed the dinoflagellate Heterocapsa rotundata. The photosynthetic performance of Mesodinium pulex was quite small, amounting for less then 4 % of its carbon uptake, indicating that M. pulex is primarily a heterotrophic species. Despite this, light affected ingestion rates. Ingestion rates increased by a factor of 2 in the light compared to in the dark. Consequently, growth rates also increased in the light.
The ciliate Mesodinium rubrum contains symbionts of cryptophyte origin. In the laboratory, our strain of Mesodinium rubrum is normally cultured on cryptophytes within the "Teleaulax clade". Prey selection of M. rubrum was investigated by offering different prey types (i.e. cryptophytes, dinoflagellate). Mesodinium rubrum ingested all the offered prey, but it could only maintain sustained growth when fed on Teleaulax amphioxeia. To test whether the symbionts of M. rubrum are permanent or temporary (replaceable), M. rubrum cultures were offered preys from 4 different cryptophyte clades. TEM pictures of M. rubrum revealed no evidence of sequestered chloroplasts from the prey. Also, the molecular data could not confirm that chloroplasts of M. rubrum can be replaced when offered other cryptophyte prey outer the "Teleaulax clade".