The Controlling Parasites of Dinoflagellate Blooms

Photosynthetic dinoflagellates are important primary producers in marine ecosystems; however certain species can cause harmful agal blooms (HAB’s).   These species are usually found in warmer and nutrient rich waters. Since these HAB events are increasing in frequency, and can be a risk to human health, the search for some way to control or reduce  blooms has started. It has been proposed, by Taylor in 1968, that the use of specific dinoflagellate parasites (eg Syndiniales Amoebophrya) could be used as a biological control. However at the time his ideas were dismissed due to a lack of evidence. The aim of this study was to examine how the abundance and diversity of dinoflagellate parasites influenced their host populations in natural environments.

A  marine coastal estuary (the Penzé River, northern Brittany, France) was sampled for 3 consecutive years (2004 to 2006) in the months May to June. During those months, every year, the dominant dinoflagellate was observed to change weekly. These are the species: Heterocapsa rotundata, Scrippsiella trochoidea, Alexandrim minutum and H. triquetra.  There was infection by Syndiniales in all of the observed dinoflagellates, even when the population was small. The frequency of the parasitized dinoflagellates reached 46% with a mean value of 21% during the summer which corresponded to the previous literature. 

The specificity of the four mane parasitic clades was detected by using FISH, which, every year, identified four main clades of Syndiniales. Clade 1 is associated with the decline of H. rotundata, clades 2, 3, and 14 appeared in late June.  Each clade was associated the same single dinoflagellate species every year : clades 1, 2, and 14 infected the dinoflagellates H. rotundata, S. trochoidea, and H. triquetra, respectively. None of the major clades seemed to be specific to the toxic dinoflagellate A. minutum.

The parasites, in the case of A. minutum, spreads by deforming and expanding the dinoflagellate cells, until the host cell bursts the cell wall and elongates into a swimming structure (the vermiform stage). Each vermiform structure breaks into up to 400 dinospores within a few hours; each of which is able to re-infect host cells.  This enables the parasite to counter the rapid the growth rate of the host. These dinospores are only detectable within 10 days of their release. The dinoflagellate population decreased in correlation with the free-living form of the parasite. However the growth of the other species of dinoflagellate species did not decline despite the large amount of dinospores produced which suggests that the parasites are species specific. There were examples of when parasites attacked non-optimum host cells however no mature trophonts were found.

To explore the evolutionary history and the genetic microdiversity the authors employed Tajima’s test. Clades 1 and 3 had significantly negative results suggesting the recent evolution or rapid evolution divergence from strong selection pressure. Clades 2 and 14 were shown to be ancient, evolutionary wise. Clade 3 weren’t detected targeting any cells. 

A. minutum blooms in the Penzé estuary were first recorded in 1994. They re-occurred over the next 9 years until the parasites started to regulate the population. They are still present in the ecosystem however the blooms of A. minutum no longer occur. Although this wasn’t an anthropogenic introduction of the parasites, it could be an example of how parasitic control of HAB’s could be used to decrease the number or the extent of marine agal blooms.  

 

Chambouvet, Aurelie, et al. "Control of toxic marine dinoflagellate blooms by serial parasitic killers." Science 322.5905 (2008): 1254-1257.

http://www.sciencemag.org/content/322/5905/1254.short