Picomanaging – Fungi as possible parasites of photosynthetic picoeukaryotes

Photosynthetic picoeukaryotes (PPE) are major contributors to CO₂-fixation in marine ecosystems. These small organisms (here defined as < 5µm in diameter) not only fix carbon but some mixotrophic species also help regulate bacterioplankton abundance. While the molecular diversity of PPEs is relatively well known, the factors controlling their abundance are not. However, recent studies suggest a top-down regulation instead of other limiting factors such as nutrient abundance. One possible regulating mechanism might be eukaryotic parasites. In addition to members of the Alveolata clade, marine fungi are well-known eukaryotic parasites. In their paper, Lepère et al. (2015) examined the effect of eukaryotic parasitism on PPEs in the marine environment.  

Water samples were taken from 0-88 m depth along the AMT19 transect in the Southern Atlantic Ocean, including the southern subtropical gyre (SG) and the southern temperate region (ST) off the coast of South America. The abundance and distribution of free-living Syndiniales (an order of dinoflagellates), Perkinsozoa (protists) and fungi were investigated using filtered samples. Additionally, the PPEs were divided into small (Plast-S) and large fractions (Plast-L) and the interactions between PPEs and putative parasites were analysed using flow-cytometric cell sorting, tyramide signal amplification - FISH (TSA-FISH) and wheat germ agglutinin (WGA) chitin staining.

The analysis of PPE community structure along AMT19, yielded similar findings to previous studies. Prymnesiophycae (48%) dominated the Plast-L populations, while Pelagophyceae (30%) and Chrysophyceae (20%) made up the majority of the Plast-S populations.
Syndiniales were only detected at low abundance, albeit with high variation between sites. As expected, Perkinsozoa were generally absent from the sea-water, as they are preferentially found in sediments. In order to avoid 18S rRNA copy number bias, the authors examined free-living fungi with three specific FISH probes, specifically targeting different fungal sequences. The average abundance of free-living fungi was 9.3% of the total Eukarya, most of which were zoospores. The highest abundance of fungi was detected in the ST region (14%). Generally, Chytridiales accounted for 3.5% of the total eukaryote community. In contrast, no positive signals were detected for Cryptomyocota and the clade doesn’t appear to be abundant in surface sea-water. 

Dual labelling TSA-FISH showed no association between PPEs and Syndiniales dinospores and they are generally found to parasitize larger organisms e.g. other dinoflagellates. The authors were able to demonstrate associations between fungi and PPEs, in which sporangia seemed to be attached to the surface of the algae. Targeting of rRNA showed the algal cells to be active and not under saprotrophic feeding by the fungi. However, these interactions were only detected in Plast-L populations, where. 3% of the Prymnesiophyceae and 6.4% of the Chrysophyceae had fungi attached to their surfaces. These interactions were observed in both the ST and SG. Moreover, the authors identified possible different stages of fungal infection. 

Lepère et al. are the first to demonstrate presumable parasitic fungi and their impact on PPEs. In addition to grazing, picophytoplankton abundance might therefore also be regulated by parasitism. However, this study merely suggests a possibility and the authors acknowledge some limitations in their study e.g. the availability of suitable oligonucleotide sequences. Nevertheless, the authors don’t mention why they deviated from the general size classification of picoplankton (0.2 - 2 µm instead of < 5 µm), or why six years passed between the sampling in 2009 and publishing in 2015. 

Reviewed Paper:

Lepère, C., Ostrowski, M., Hartmann, M., Zubkov, M. V., & Scanlan, D. J. (2015). In situ associations between marine photosynthetic picoeukaryotes and potential parasites–a role for fungi?. Environmental microbiology reports. Link: http://onlinelibrary.wiley.com/doi/10.1111/1758-2229.12339/abstract