Morten Moldrup:
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Phototaxis provides phytoplankton with the means to orient themselves in a light gradient. This is accomplished using an eyespot and associated organelles. For the dinoflagellate Kryptoperidinium foliaceum, which has been described as having one of the most elaborate eyespot complexes known, positive phototaxis has hitherto not been reported. In this study we show that a newly isolated strain of K. foliaceum is indeed capable of positive phototaxis with a mean vector (±95% CI) of 352°±2.2° where 0/360° indicate the direction of the light source. A study of three strains (UTEX1688, CCMP and MBL07) of Kryptoperidinium foliaceum, showed that the eyespot in two of these strains have degenerated following decades in culture. Thus, previous studies have failed to report positive phototaxis due to loss of directionality caused by the degenerated eyespot. The results are discussed in a broader context; and we conclude that studies on algal morphology and physiology may result in erroneous conclusions if based on algal cultures maintained under laboratory conditions for extended periods.
Until recently it not believed to be phototactic and its spectral sensitivity therefore remains unstudied. We find the dynamic range of phototaxis to be ~2 log units. Additionally, we find indications that the spectral sensitivity behind the phototaxis is based on a single opsin with peak sensitivity around 500 nm. The spectral sensitivity agrees reasonably well with the absorption curve of a theoretical opsin. This is maintained although the expected peak in the near UV range is missing probably due to some sort of shading/filtering of harmful UV radiation. Interestingly, the phototaxis could be temporarily overruled by tactile stimuli. After physical contact with the light guide the cells escaped the area. They may do this as some sort of predator avoidance.
Field studies have indicated that dinoflagellates are key degraders of copepod fecal pellets in the sea, however, direct evidence of pellet feeding by dinoflagellates have not been published prior to this study. Feeding and growth of dinoflagellates on copepod fecal pellets was studied for 3 species of mixotrophic dinoflagellates and 4 species of heterotrophic dinoflagellates using a combination of video recordings of feeding behavior and classic incubation experiments. Fecal pellets offered were produced by adult Acartia tonsa on Rhodomonas salina as a food source. One out of 3 mixotrophic species (Karlodinium armiger) and 4 out of the 6 the heterotrophic dinoflagellates (Gyrodinium dominans, G. spirale, Diplopsalis lenticula, Protoperidinium depressum) studied fed on fecal pellets. Using natural concentrations of dinoflagellates and copepod fecal pellets, ingestion rates of 0.21 and 0.11 pellets cell-1 d-1 and clearance rates of between 0.21 and 0.32 ml cell-1 d-1 were obtained for Gyrodinium spirale and Protoperidinium depressum, respectively. Pellet feeding resulted in growth rates of 0.69 and 0.08 d-1 with a growth yield of 0.58 and 0.50, for the Gyrodinium spirale and Protoperidinium depressum, respectively. The main determining factors for the grazing impact of the dinoflagellates on fecal pellets were the dinoflagellate to pellet size ratio in combination with the feeding mechanism employed by the dinoflagellate species, pellet age, and pellet concentration. This study reveals a new trophic role for dinoflagellates as detritivores, which provides a less energy consuming pathway for recycling of pellet material than the several trophic levels which are transversed through the microbial loop; and proves that dinoflagellates can function as an effective “protozoan filter” for fecal pellets in the water column. The photosynthetic dinoflagellate Karlodinium is known to form massive blooms worldwide and often these are associated with fish kills. Here we show that Karlodinum armiger can reverse the traditional trophic pathway from primary producers to copepods, by attacking, immobilizing and engulfing the much larger metazoan grazers. Copepod immobilisation is fast but dependent on cell density of K. armiger, suggesting the presence of a potent paralytic toxin. Karlodinium armiger immobilises copepods by direct cell-contact, before ingesting parts of the copepod through a feeding tube. The common copepod Acartia tonsa was immobilised within a few hours and died 12 hours after exposure to ecologically relevant bloom cell densities in the laboratory. Karlodinium armiger increases its growth rate when exposed to copepods and most cells contain large visible food vacuoles following engulfment. Our results show a novel trophic pathway at the base of planktonic food web which reverses the typical flux of organic matter. Behavioural observations of K. armiger have also revealed a novel mechanism for faunal kills by this phototrophic microalga.
The red tide ciliate Mesodinium rubrum (=Myrionecta rubra) is known to contain a symbiont of cryptophyte origin. Molecular data have shown that the symbiont is very closely related or similar to free-living species belonging to the “Teleaulax clade”. This suggests that the symbiont of M. rubrum is either a temporary symbiont or a quite recently established symbiont. Here we present data from a number of experiments in which we tried to replace the symbionts in M. rubrum by supplying a number of different cryptophyte species belonging to different cryptophyte clades. Growth and ingestion rates of M. rubrum fed these cryptophytes were measured. In addition, cells of M. rubrum were analyzed for type of chloroplast using transmission electron microscopy and DNA sequences of the nucleomorph LSU. We found that M. rubrum ingested all the offered cryptophyte species, but it was unable to incorporate any of the offered prey species as symbionts. Also, M. rubrum can only sustain growth on cryptophyte species belonging to the “Teleaulax clade”.