Acquisition of photoautotrophy in kleptoplastic dinoflagellates-Nusuttodinium aeruginosum/acidotum as a case study

Field: Agricultural and Biological Sciences (Life Sciences)

The integration of stable plastids by endosymbiosis led to an enormous diversity of photosynthetic eukaryotic organisms on earth. However, the steps during the establishment of a stable endosymbiosis are poorly understood. To address this early step of evolution, many studies have been performed on organisms with transient plastids. Originally heterotrophic, are these organisms able to acquire photoautotrophy through the uptake of photosynthetic prey. In contrast to being digested, the prey preserves its photosynthetic ability and supplies the host with photosynthetic products. The prey can either be reduced to an endosymbiont or even to a single plastid. In the latter case the plastids are stolen from photosynthetic prey and therefore called ‘kleptoplasts’. Kleptoplasts are widespread among eukaryotes and occur in multicellular as well as in unicellular organisms. One of the most prominent groups harboring kleptoplasts is the dinoflagellates. This study focuses on freshwater isolates of the kleptoplastic genus Nusuttodinium with a special focus on the species N. aeruginosum/acidotum that acquires its kleptoplasts from the blue-green cryptophyte genus Chroomonas. Stable axenic co-cultures from different locations were established for N. aeruginosum/acidotum and for another species N. amphidinoides and their live cycles were studied. In contrast to all other systems described until now, N. aeruginosum/acidotum is able to harbor kleptoplasts of different morphology and stability making it an ideal model system to study possible steps during the acquisition of stable permanent plastids. Depending on the phylogenetic position of the prey either ‘granular’ or ‘U-shaped’ kleptoplasts are formed. Granular kleptoplasts are characterized morphologically by their multiple number and small size. Each kleptoplast is associated with a small cryptophyte nucleus, the ‘kleptokaryon’ and resembles recently ingested prey. Kleptokarya are stable for up to 10 days and kleptoplasts for up to 14 days. In the granular stage cells took up prey permanently, especially just after cell division that takes place during the dark phase. In contrast, the U-shaped situation is characterized by just one large U-shaped kleptoplast filling out the cell and a single large kleptokaryon located antapically in the curve of the U. Compared to the free living prey and the granular situation, the kleptoplast increases up to 20 fold in size and the kleptokaryon up to 16 fold. Whereas the kleptoplast is distributed equally to both daughter cells during cell division, just one inherits the kleptokaryon. A stability of up to 50 days was recorded for the kleptokaryon and up to 60 days for the kleptoplast. DNA fluorescence measurements and a PCR based approach with primers specific to the cryptophyte nucleus indicate polyploidization of the cryptophyte nucleus after ingestion by the dinoflagellate. In addition enlarged kleptokarya increase kleptoplast stability and influence the behavior of N.acidotum/aeruginosum. Prey uptake is inhibited by intact kleptokarya and dinoflagellates mostly show straight swimming behavior that clearly differs from the behavior in the granular situation. If prey availability is very high N. aeruginosum/acidotum does not form U-shaped kleptoplasts but instead forms granular kleptoplasts that are digested if prey availability remains high or, if prey availability decreases, are distributed equally to the daughter cells in which they develop into stable U-shaped kleptoplasts. In contrast to N. aeruginosum/acidotum, N. amphidinoides represents a more primitive stage during plastid acquisition as kleptokarya are digested shortly after ingestion and kleptoplasts are less modified and stable than the U-shaped plastids. Comparing the ‘primitive’ granular with the ‘advanced’ U-shaped situation with respect to literature allows the formulation of a new hypothesis after which prey scarcity is a trigger and polyploidization one of the most important steps towards the acquisition of stable plastids.