The question of whether the toxin-producing and bloom-forming dinoflagellate genus Dinophysis contains plastids that are permanent or contains temporary so-called kleptoplastids is still unresolved. We sequenced plastid 16S rRNA gene, the complete trnA gene and the intergenic transcribed spacer region located between the trnA gene and the 23S rRNA gene, and performed diagnostic PCR on cells of the genus Dinophysis. Dinophysis spp. were collected from five different geographical regions: the Baltic Sea, the North Sea, the Greenland Sea and the Norwegian fjord Masfjorden. In most cases the sequence analysis showed that the sequences were identical to each other and to sequences from the cryptophyte Teleaulax amphioxeia SCCAP K0434, regardless of the place of sampling or the species analyzed. The exception was some cells of Dinophysis spp. from the Greenland Sea. These contained a 16S rRNA gene sequence that was more closely related to the cryptophyte Geminigera cryophila. The cells of Dinophysis contained either one of the 16S rRNA gene sequences or both in the same cell. Our results challenge the hypothesis that the plastids in Dinophysis are permanent and suggest that they are more likely to be kleptoplastids.

We verified an active uptake of kleptoplastids in the toxic and bloom-forming dinoflagellates of the genus Dinophysis from its preferred prey, the ciliate Myrionecta rubra, using a quantitative real-time PCR technique. During a 65 d starvation/feeding experiment with Dinophysis caudata, changes in plastid 16S rRNA, plastid autofluorescence and plastid/nuclear DNA ratio through the cell cycle were followed with quantitative real-time PCR and flow cytometry. During starvation, the cultures initially showed a rapid growth and a 3.5-fold increase of number of cells ml -1 , while at the same time, plastid DNA cell -1 showed a 3.5-fold decrease, and a 3.6-fold decrease in phycoerythrin fluorescence cell -1 . The decrease in plastid DNA cell -1 d -1 closely followed culture growth rate (Pearson correlation, r = 0.91), indicating that existing plastids were diluted within the growing population and that no new plastids were synthesised by the cells. When starved cells were re-fed by the ciliate M. rubra on Days 43 to 51 of the experiment, plastid DNA cell -1 increased 7-fold up to 14 000 16S DNA copies per cell, thereby directly revealing the kleptoplastic behaviour. The implication is that not only availability of the prey M. rubra itself, but also the supply of suitable kleptoplastids might be an important controlling factor for Dinophysis spp. bloom formation and decline.

Kleptoplasty is the retention of plastids obtained from ingested algal prey, which can remain temporarily functional and be used for photosynthesis by the predator. With a new approach based on cell cycle analysis, we have addressed the question of whether the toxic, bloom-forming dinoflagellate Dinophysis norvegica practice kleptoplasty or if they replicate their own plastid DNA. Dividing (G2) and non-dividing (G1) D. norvegica cells from a natural population were physically separated with a flow cytometer based on their DNA content. Average numbers of nuclear and plastid rDNA copies were quantified with real-time PCR both in the G1 and G2 group. Cells from the G1 group contained 5800 +/- 340 copies of nuclear rDNA and 1300 +/- 200 copies of plastid rDNA; cells from the G2 group contained 9700 +/- 58 copies of nuclear rDNA and 1400 +/- 220 copies of plastid rDNA (mean +/- SD, n = 3). The ratio G2/G1 in average rDNA copies per cell was 1.67 for nuclear DNA and 1.07 for plastid DNA. These ratios show that plastid acquisition in D. norvegica is either uncoupled with the cell cycle, or plastids accumulate rapidly in the beginning of the cell cycle owing to feeding, as would be expected in a protist with kleptoplastic behaviour but not in a protist with own plastid replication. In addition, flow cytometry measurements on cells from the same population used for real-time PCR showed that when kept without plastidic prey, live Dinophysis cells lost on average 36% of their plastid phycoerythrin fluorescence in 24 h. Together these findings strongly suggest that D. norvegica does not possess the ability for plastid replication.

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Nostoc and Richelia belong to a group of heterocystous cyanobacteria and are unique within this group in forming intracellular symbioses with phototrophic hosts, the angiosperm Gunnera and the diatoms (algae) Rhizosolenia and Hemiaulus, respectively. The function of the cyanobiont is similar in the symbioses, namely providing fixed atmospheric nitrogen to their hosts; also the cyanobionts are contained in a host compartment, the symbiosome. The evolutionary timescale for the cyanobiont-endosymbiosis formation is in both instances about ≈90 Ma. However, the potentials for further co-evolution of host and microsymbiont, are different. Nostoc is regarded as preyed upon by its host, while in the RicheliaRhizosolenia symbiosis example the evolution towards a new type of permanent organelle is possible. It is proposed that symbiosis is ruled by divergent host strategies. In the case of Richelia-Rhizosolenia the evolution of a permanent symbiosis is linked to diatom hosts needing to carry the cyanobiont permanently, as it is not available free-living in the oceans. However, in the case of Nostoc/Gunnera, the host exploits an abundant cyanobacterial species. A model where the relative abundance of microsymbionts determines the nature of the symbiosis comes into view: If environmental ratios of host/microsymbiont are so that hosts are the dominating party, then the host has to carry the microsymbiont as luggage (vertical transmission). Likewise, if the ratio of microsymbiont is higher than host, than the host will prey on the microsymbiont (horizontal transmission). The article also discusses the retention of secondary plastids in dinoflagellates. We show that dinoflagellates are organisms that exemplify both types of strategies that is either preying or harbouring a permanent organelle. The difference from the cyanobacterial example is that only parts of the eukaryotic microsymbionts are kept, usually only the plastid. We emphasize that the dinoflagellates can obtain their plastids from various different organisms. The luggage theory offers an explanation to why some dinoflagellate species contain kleptoplastids, while others have permanent, secondary plastids and some have tertiary plastids.