The phytoplankton community affects the energy metabolism and immunomodulation strategy of oyster during breeding seasons.

The mass mortality of Pacific oyster Crassostrea gigas has become a severe ecological and economic concern to Chinese aquaculture, which is proposed to be linked to the phytoplankton community in the farming waters. In the present study, both field and laboratory experiments were conducted to identify the phytoplankton taxa associated with oyster mortality and explore the molecular mechanism by which they affect the physiological health of oysters. The field experiment showed that more serious mortality of oysters was observed in the North Yellow Sea from July to September in 2018 (average survival rate of 75.11 %) than in 2019 (average survival rate of 85.78 %), with the proportion of Bacillariophyta (diatoms) in the phytoplankton community in 2018 lower than that in 2019. In comparison to 2019, reduced dry weight, lower glycogen and triglyceride contents in hepatopancreas, lower 17ß-estradiol and testosterone concentrations in gonad, as well as a generally weaker immune response against Vibrio splendidus stimulation were detected in the oysters sampled in 2018. The treatment of oysters with either starvation (starvation group) or Nitzschia closterium f. minutissima feeding (N. closterium group) was conducted to verify the field findings, with individuals reared in natural seawater as control. After 40 days of N. closterium feeding, dry weight, glycogen and triglyceride contents in hepatopancreas significantly increased, as well as the biosynthesis of sex hormones and gonadal maturation were promoted compared to the control and starvation groups. Moreover, a much stronger immune response against V. splendidus stimulation was observed in the oysters of N. closterium group, with the fold-changes of norepinephrine content in serum, SOD activity in hepatopancreas, and the mRNA expression level of IL17-5 and HSP70 in haemocytes higher than those in the control and starvation groups. Collectively, these results suggested that lack of diatoms in the farming waters suppressed the energy storage and gonadal maturation of adult oysters, and also resulted in a compromised immune response against bacterial infection, which may be a leading cause of the mass mortality of oysters living in diatom-deficient waters during breeding seasons.

Acquisition and maintenance of resistance to viruses in eukaryotic phytoplankton populations.

Viruses are known to play a key role in the regulation of eukaryotic phytoplankton population densities; however, little is known about the mechanisms of how they interact with their hosts and how phytoplankton populations mediate their regulations. Viruses are obligate parasites that depend on host cell machinery for their dissemination in the environment (most of the time through host cell lysis that liberates many new particles). But viruses also depend on a reliable host population to carry on their replication before losing their viability. How do hosts cells survive when they coexist with their viruses? We show that clonal lines of three picoeukaryotic green algae (i.e. Bathycoccus sp., Micromonas sp., Ostreococcus tauri) reproducibly acquire resistance to their specific viruses following a round of infection. Our observations show that two mechanisms of resistance may operate in O. tauri. In the first resistant type, viruses can attach to their host cells but no new particles develop. In the second one, O. tauri acquires tolerance to its virus and releases these viruses consistently. These lines maintained their resistance over a 3-year period, irrespective of whether or not they were re-challenged with new viral inoculations. Co-culturing resistant and susceptible lines revealed resistance to be associated with reduced host fitness in terms of growth rate.

An immunological approach to detect phosphate stress in populations and single cells of photosynthetic picoplankton.

In the marine cyanobacterium Synechococcus sp. strain WH7803, PstS is a 32-kDa cell wall-associated phosphate-binding protein specifically synthesized under conditions of restricted inorganic phosphate (P1) availability (D. J. Scanlan, N. H. Mann, and N. G. Carr, Mol. Microbiol. 10:181-191, 1993). We have assessed its use as a potential diagnostic marker for the P status of photosynthetic picoplankton. Expression of PstS in Synechococcus sp. strain WH7803 was observed when the P1 concentration fell below 50 nM, demonstrating that the protein is induced at concentrations of P1 typical of oligotrophic conditions. PstS expression could be specifically detected by use of standard Western blotting (immunoblotting) techniques in natural mesocosm samples under conditions in which the N/P ratio was artificially manipulated to force P depletion. In addition, we have developed an immunofluorescence assay that can detect PstS expression in single Synechococcus cells both in laboratory cultures and natural samples. We show that antibodies raised against PstS cross-react with P-depleted Prochlorococcus cells, extending the use of these antibodies to both major groups of prokaryotic photosynthetic picoplankton. Furthermore, DNA sequencing of a Prochlorococcus pstS homolog demonstrated high amino acid sequence identity (77%) with the marine Synechococcus sp. strain WH7803 protein, including those residues in Escherichia coli PstS known to be directly involved in phosphate binding.