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Assessing the role of parasitism in the regulation of picophytoplankton communities in open ocean environments

Final Report Summary - PICOPAR (Assessing the role of parasitism in the regulation of picophytoplankton communities in open ocean environments)

1) Main goal and specific aims as described in the original application

Given the key ecological role that photosynthetic picoeukaryotes (PPEs) play in global CO2 fixation and carbon cycling in the ocean, it is of fundamental importance to identify the factors controlling their diversity and distribution. The most common consumer strategy, namely parasitism, is usually left out of aquatic trophic food web functioning; however, recent results highlight the potential importance of this process in aquatic systems. The aim of the project was therefore to determine and characterize the major parasitic groups potentially associated with PPEs, assess their ecological relevance and determine their impact on microbial eukaryote population dynamics and planktonic food web structure.

Project objective 1: To assess the diversity and specificity of parasites in both the pico- and nano-phytoplankton size classes.
Project objective 2: To compare the extent of parasite versus virus-mediated infection of photosynthetic eukaryote communities.
Project objective 3: To determine the role of environmental variables on parasite infection of pico- and nano-phytoplankton.

2) Accomplishments including an overview of results and conclusions

The first objective of this project was to answer general questions as: What is the diversity of photosynthetic picoeukaryotic classes (potentially susceptible to parasitism) in the open ocean? Are parasites capable of infecting pico-sized eukaryotic phytoplankton, or is their action limited to larger cells? The underlying hypothesis was that parasites could infect small photosynthetic eukaryotes and thus regulate their diversity and distribution in aquatic systems.
Initially, we determined the population structure of the eukaryotic pico- (<5µm) and nano- (<20µm) phytoplankton fractions so as to assign the photosynthetic classes potentially susceptible to parasitism. The diversity of these groups has been assessed by PCR cloning-sequencing of plastid 16S rRNA genes and Fluorescent in situ Hybridization (FISH). The small fraction was dominated by Pelagophyceae and Chrysophyceae whereas Prymnesiophyceae were the principal component of the nano-fraction.
Besides the regular occurrence of sequences affiliated to Syndiniales (Marine alveolate, MALV, including virulent pathogens for a wide range of algae) in 18S rRNA gene libraries of marine ecosystems, their quantitative distribution has barely been studied in open waters. We used FISH with an oligonucleotide probe specific for Amoebophryidae (MALV II) to investigate the contribution to the eukaryotic community of the small free-living stage of Amoebophryidae (zoospore) and their potential impact on small phytoplankton along a transect in the Atlantic Ocean (SOLAS cruise). The FISH method showed that free-living zoospores were not abundant (average along transect =3% of total eukaryotes) at the stations studied. Moreover, for the first time we developed a dual labelling FISH protocol in this work (existing probes for the major phytoplankton classes + MALV II) to allow the visualisation of potential parasites both in the host (endoparasitism) and on the host (ectoparasitism). However, no parasitic association was detected in the pico or nano-size fraction of the eukaryotic phytoplankton. Within the alveolate super-group, Perkinsozoa were also investigated. However, we were not able to amplify (with specific primers) or detect by microscopy these parasites although they have previously been found in marine and lacustrine environments.