Elsa Breton

Microplanktonic Community Structure in a Coastal System Relative to a Phaeocystis Bloom Inferred from Morphological and Tag Pyrosequencing Methods

Background Massive phytoplankton blooms, like the recurrent Phaeocystis proliferation observed every year in the Eastern English Channel (EEC), have a significant influence on the overall planktonic community structure and their food web dynamics. As well as being an important area for local fisheries, the EEC is an ideal ecosystem for work on microbial diversity. This is because, although its environmental context is relatively complex, it is reasonably well understood due to several years of monitoring and morphological observations of its planktonic organisms. The objective of our study was to better understand the under-explored microbial eukaryotic diversity relative to the Phaeocystis bloom. Methodology and Principal Findings The community structure of microplankton (diatoms, haptophytes, ciliates and dinoflagellates) was studied through morphological observations and tag pyrosequencing. During the annual Phaeocystis spring bloom, the phytoplankton biomass increased by 34-fold, while the microzooplankton biomass showed a 4-fold increase, representing on average about 4.6% of the biomass of their phytoplankton prey. Tag pyrosequencing unveiled an extensive diversity of Gymnodiniaceae, with G. spirale and G. fusiformis representing the most abundant reads. An extended diversity of Phaeocystales, with partial 18S rDNA genes sequence identity as low as 85% was found, with taxa corresponding to P. globosa, but also to unknown Phaeocystaceae. Conclusions Morphological analyses and pyrosequencing were generally in accordance with capturing frequency shifts of abundant taxa. Tag pyrosequencing allowed highlighting the maintenance of microplankton diversity during the Phaeocystis bloom and the increase of the taxa presenting low number of reads (minor taxa) along with the dominant ones in response to biotic and/or abiotic changing conditions. Although molecular approaches have enhanced our perception on diversity, it has come to light that the challenge of modelling and predicting ecological change requires the use of different complementary approaches, to link taxonomic data with the functional roles of microbes in biogeochemical cycles.

No feeding on Phaeocystis sp. as solitary cells (post-bloom period) by the copepod Temora longicornis in the coastal waters of the English Channel

A Phaeocystis sp. (Prymnesiophyceae) bloom regularly occurs in April–May in the Eastern English Channel. In the literature, views are divided about the in situ appetence of copepods for this alga. In a study carried out in the coastal waters off the bay of Somme, at the end of the bloom, from 29 of April to 1 of May 1996, HPLC pigment analysis on both gut algal pigments and algal pigments from the water column shows that Temora longicornis adults did not feed on single cells of Phaeocystis sp. Alternatively, T. longicornis ingested diatoms and the gut content was correlated with the diatom biomass in the water. More, T. longicornis fed selectively on Dinophyceae and Cryptophyceae, which were scarcely present in the food environment. An inverse relationship was found between the concentration of Phaeocystis sp. in seawater and both gut content and abundance of young stages (CI–CIII copepodites) of T. longicornis. These results suggest an unfavourable impact of Phaeocystis sp. post-bloom on both feeding activity and distribution of T. longicornis.

Environmental Control on Fish and Macrocrustacean Spring Community-Structure, on an Intertidal Sandy Beach

The inter-annual variability of the fish and macrocrustacean spring community on an intertidal sandy beach near the Canche estuary (North of France) was studied from 2000 to 2013 based on weekly spring sampling over an 11-year period. Twenty-eight species representing 21 families were collected during the course of the study. The community was dominated by a few abundant species accounting for > 99% of the total species densities. Most individuals caught were young-of-the-year indicating the importance of this ecosystem for juvenile fishes and macrocrustaceans. Although standard qualitative community ecology metrics (species composition, richness, diversity, evenness and similarity) indicated notable stability over the study period, community structure showed a clear change since 2009. Densities of P. platessa, P. microps and A. tobianus decreased significantly since 2009, whereas over the period 2010-2013, the contribution of S. sprattus to total species density increased 4-fold. Co-inertia and generalised linear model analyses identified winter NAO index, water temperature, salinity and suspended particular matter as the major environmental factors explaining these changes. Although the recurrent and dense spring blooms of the Prymnesiophyte Phaeocystis globosa is one of the main potential threats in shallow waters of the eastern English Channel, no negative impact of its temporal change was detected on the fish and macrocrustacean spring community structure.

Parasitic Eukaryotes in a Meso-Eutrophic Coastal System with Marked Phaeocystis globosa Blooms

This study explored the potential interactions and relations of the putative Symbiotic/Decomposer/Parasitic community (SymbDec) with other taxa and predominant environmental parameters in the eastern English Channel (EEC) over a 2.5 years period (32 sampling dates). The EEC is a meso-eutrophic coastal system characterized by strong repeating patterns in plankton succession with recurrent massive blooms of the Haptophyte Phaeocystis globosa preceded and followed by communities of colonial diatoms and dinoflagellate grazers. The metagenomic sequencing results of the V2-V3 hypervariable region of the 18S rRNA gene allowed an overview of the planktonic community, which consisted of 32 high-level taxonomic groups. Forty-two % of the detected OTUs belonged to potentially parasitic taxa or decomposers such as Syndiniales (MALV – MArine ALveolates), Fungi, Cercozoa, Perkinsea and others, most of which were plankton parasites. We examined the Local Similarity Analysis (LSA) network of lag delayed correlations on the 142 most abundant OTUs (> 0.1 % of the total number of reads). LSA showed that 99 OTUs had highly significant connections, involving 26 OTUs characterized as potential parasites, and 2 decomposers. The parasitic network had small world characteristics, rendering the assemblage more resilient to environmental change, but more susceptible to the removal of highly connected taxa. The majority of the highly connected OTUs belonged to MALV, which were mainly connected with dinoflagellates, suggesting that it might play an important role on dinoflagellate top-down control. The bloom forming P. globosa was not observed in connection with parasitic OTUs in the network. Four environmental parameters (O2, N, T, and N/P) were strongly connected with only 11 OTUs, while correlations between microbes dominated the network.

Diversity and potential activity patterns of planktonic eukaryotic microbes in a mesoeutrophic coastal area (eastern English Channel)

The diversity of planktonic eukaryotic microbes was studied at a coastal station of the eastern English Channel (EEC) from March 2011 to July 2015 (77 samples) using high throughput sequencing (454-pyrosequencing and Illumina) of the V2-V3 hypervariable region of the 18S SSU rDNA gene. Similar estimations of OTU relative abundance and taxonomic distribution for the dominant higher taxonomic groups (contributing >1% of the total number of OTUs) were observed with the two methods (Kolmogorov-Smirnov p-value = 0.22). Eight super-groups were identified throughout all samples: Alveolata, Stramenopiles, Opisthokonta, Hacrobia, Archeaplastida, Apusozoa, Rhizaria, and Amoebozoa (ordered by decreasing OTU richness). To gain further insight into microbial activity in the EEC, ribosomal RNA was extracted for samples from 2013–2015 (30 samples). Analysis of 18S rDNA and rRNA sequences led to the detection of 696 and 700 OTUs, respectively. Cluster analysis based on OTUs’ abundance indicated three major seasonal groups that were associated to spring, winter/autumn, and summer conditions. The clusters inferred from rRNA data showed a clearer seasonal representation of the community succession than the one based on rDNA. The rRNA/rDNA ratio was used as a proxy for relative cell activity. When all OTUs were considered, the average rRNA:rDNA ratio showed a linear trend around the 1:1 line, suggesting a linear relation between OTU abundance (rDNA) and activity (rRNA). However, this ratio was highly variable over time when considering individual OTUs. Interestingly, the OTU affiliated with P. globosa displayed rRNA:rDNA ratio that allowed to delimit high vs low abundance and high vs low activity periods. It unveiled quite well the Phaeocystis bloom dynamic regarding cell proliferation and activity, and could even be used as early indicator of an upcoming bloom.

Realized niche analysis of phytoplankton communities involving HAB: Phaeocystis spp. as a case study

The link between harmful algal blooms, phytoplankton community dynamics and global environmental change is not well understood. To tackle this challenging question, a new method was used to reveal how phytoplankton communities responded to environmental change with the occurrence of an harmful algae, using the coastal waters of the eastern English Channel as a case study. The great interannual variability in the magnitude and intensity of Phaeocystis spp. blooms, along with diatoms, compared to the ongoing gradual decrease in anthropogenic nutrient concentration and rebalancing of nutrient ratios; suggests that other factors, such as competition for resources, may also play an important role. A realized niche approach was used with the Outlying Mean Index analysis and the dynamics of the species realized subniches were estimated using the Within Outlying Mean Indexes calculations under low (L) and high (H) contrasting Phaeocystis spp. abundance. The Within Outlying Mean Indexes allows the decomposition of the realized niche into realized subniches, found within the subset of habitat conditions and constrained by a subset of a biotic factor. The two contrasting scenarios were characterized by significantly different subsets of environmental conditions and diatom species (BV-step analysis), and different seasonality in salinity, turbidity, and nutrients. The subset L environmental conditions were potentially favorable for Phaeocystis spp. but it suffered from competitive exclusion by key diatom species such as Skeletonema spp., Thalassiosira gravida, Thalassionema nitzschioides and the Pseudo-nitzchia seriata complex. Accordingly, these diatoms species occupied 81% of Phaeocystis spp.s existing fundamental subniche. In contrast, the greater number of diatoms, correlated with the community trend, within subset H exerted a weaker biological constraint and favored Phaeocystis spp. realized subniche expansion. In conclusion, the results strongly suggest that both abiotic and biotic interactions should be considered to understand Phaeocystis spp. blooms with greater consideration of the preceeding diatoms. HABs needs must therefore be studied as part of the total phytoplankton community.