Who's parasitizing the ocean's smallest primary producers? A closer look at the suspects

Amongst the several marine phytoplankton groups, there is a component of eukaryotes smaller than 5 µm, accordingly called photosynthetic picoeukaryotes (PPEs). These PPEs gained recognition over the past years for their role in the oceanic carbon cycle by O₂ fixation and their trophic role as bacterivores. This last feature proves the mixotrophic character of some PPE’s, since these organisms act as primary producers as well. In contrast to the relatively good understanding we have on PPE’s in general, there is less knowledge on environmental factors limiting these organism’s abundance. It’s already found that nutrient concentrations don’t show a controlling effect, which implies PPE abundance and carbon fixation regulation from higher levels in the trophic system. Lepère et al. (2016) investigated associations between marine PPEs and eukaryotic parasites, where the role for parasitic fungi was explored as well.

To evaluate abundance of PPE classes and parasites and associations between them, tyramide signal amplification-fluorescence in situ hybridization (TSA-FISH) was used. The probing of potentionally infected organisms could give new insights in the interactions between PPEs and parasites, therefore two probes are used at the same time: one labelling the potential parasite and one labelling the PPE host. The investigated potential parasites are: Syndiniales, Perkinsozoa and a wide range of fungi. To measure PPE abundance and composition, sea water samples were taken from the surface mixed layer in the Atlantic Ocean, along a transect passing ten stations in the southern subtropical gyre and the southern temperate region. The sampled organisms were separated in two size fractions of 2 µm and 3.1 µm, named ‘small, plastidic eukaryotes’(Plast-S) and ‘large, plastidic eukaryotes’(Plast-L), respectively.

Pelagophyceae and Chrysophyceae were the dominating fractions for Plast-S at all stations, while this was Prymnesiophyceae for the Plast-L PPE. Cryptophyceae was also detected in some Plast-L samples, albeit with much lower concentrations. At some stations, the dominating PPE classes were still measured in relatively low concentrations, which implies occurrence of other clades. Lepère et al. (2013) suggest some prasinophyceae clades could contribute at these stations as well, as they showed to play an important role within the PPE community in some oceanic regions. In our seminar two weeks ago, me and Eleni discussed the article of dos Santos et al. (2016), in which this contribution is researched for prasinophytes clade VII.

The investigated Syndiales parasites showed generally low concentrations. At six stations no Syndiales cells were found at all. However, these low numbers could be an underestimation, since the probe used to stain Syndiales was only specific for 75% of the described clades. Perkinsozoa is a phylum known to comprise aquatic parasites infecting a wide range of organisms. Yet, the FISH analysis barely gave any positive signals. This can be explained by the fact that Perkinsozoa mainly occur in sediments. The last group of potential parasites researched is separated by the use of three different FISH probes. Each one staining a specific kind of fungal parasites: Eumyocota, Chytridiales and some fungi within the Cryptomycota clade. The last two clades are known to contain some freshwater parasites. Eumyocota and Chytridiales represented respectively 9.3% and 3.5% of the total eukaryotic community, but no positive signals were found for Cryptomycota. This latter suggests that Cryptomycota don’t occur in the surface mixing layer of the Atlantic ocean.

Since it was known before that parasitism by Syndiales shows a preference for larger cells, it was not a surprise that there were no associations found between this parasite and PPEs. As mentioned before, Perkinsozoa was barely present in the surface mixing layer. The fungal organisms however, were the only group that showed parasitism of PPEs at some of the sampling stations, albeit only with the Plast-L fraction size. That is, Eumyocota and Chytridiales were associated with a small fraction of Chrysophyceae and Prymnesiophyceae, varying from 2.4% to 7.3% for the two PPE groups separately.

In the conclusion the authors state a quantitave importance of free-living fungi in the open ocean. However, to me it seems that the low percentages mentioned before don’t show a  particularly great impact of eukaryotic parasitism on PPEs. Also it occurred to me that the depths at which the PPE samples are taken, vary from five to 88 meters. I wonder if this difference shows in the results for parasitic associations found per station. In table 2 in the article, no associations – not even with fungal parasites – were found at a depth of 88 meters.  Nevertheless, this article gave innovative insights into the topic, where it was largely unexplored with regards to marine pelagic ecosystems before.

Article reviewed:

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.

Further reading (article discussed in seminar 3, by me and Eleni):

dos Santos, A. L., Gourvil, P., Tragin, M., Noël, M. H., Decelle, J., Romac, S., & Vaulot, D. (2016). Diversity and oceanic distribution of prasinophytes clade VII, the dominant group of green algae in oceanic waters. The ISME Journal.