Sublethal concentrations of ichthyotoxic alga Prymnesium parvum affect rainbow trout susceptibility to viral haemorrhagic septicaemia virus

Abstract

Ichthyotoxic algal blooms are normally considered a threat to maricultured fish only when blooms reach lethal cell concentrations. The degree to which sublethal algal concentrations challenge the health of the fish during blooms is practically unknown. In this study, we analysed whether sublethal concentrations of the ichthyotoxic alga Prymnesium parvum affect the susceptibility of rainbow trout Oncorhynchus mykiss to viral haemorrhagic septicaemia virus (VHSV). During exposure to sublethal algal concentrations, the fish increased production of mucus on their gills. When fish were exposed to the algae for 12 h prior to the addition of virus, a marginal decrease in the susceptibility to VHSV was observed compared to fish exposed to VHSV without algae. If virus and algae were added simultaneously, inclusion of the algae increased mortality by 50% compared to fish exposed to virus only, depending on the experimental setup. We concluded that depending on the local exposure conditions, sublethal concentrations of P. parvum could affect susceptibility of fish to infectious agents such as VHSV.

Gill Transcriptomic Responses to Toxin-producing Alga Prymnesium parvum in Rainbow Trout

“…The haptophyte P. parvum (Kalmar University Culture Collection, strain KAC 39) were cultured in F/2 medium (temperature 15°C, salinity 0.9%, photoperiod 14 h:10 h light:dark) as described previously ( 44 ). The exposure of fish to P. parvum was performed by adding exponentially growing cultures of P. parvum to the water to create environments with high (~4 x 10 4 cells per mL of water) and low (~1.5 x 10 4 cells per mL of water) densities of algae.…”

“…During the transfer, the viability of the algal cells was confirmed by microscopy, after which the cells were pipetted into the water column and gently mixed to ensure their homogenous distribution within the tank. The P. parvum densities were chosen to mimic natural blooms ( 46 ), with both doses expected to have sublethal effects on fish ( 44 ). The effects of high and low doses of algae were evaluated using 3 replicate tanks.…”

“…Sampling was alternated between the HH, HM and LL groups for experimental balance. The experiment was terminated at 5 h post-exposure to reduce the risk of fish suffering from the algal toxicity ( 44 ).…”

“…Both fish larvae (fathead minnow Pimephales promelas and zebrafish Danio rerio) as well as fish liver (Hepa-E1 and PLHC-1) and gill (G1B and RTgill-W1) cell lines were exposed to the sublethal doses of P. parvum to explore various aspects of the toxin-induced oxidative stress and antioxidant defence, including lipid peroxidation, oxidative DNA damage as well as gene expression and activity of antioxidant enzymes (41)(42)(43). It has also been demonstrated that the sublethal exposure of rainbow trout to P. parvum modulates susceptibility of fish to infectious agents such as viral haemorrhagic septicaemia virus (VHSV) (44). Surprisingly, little research has been done on the gill transcriptome, despite the gill being considered a key target tissue for P. parvum action.…”

The pathology associated with putative algal toxicosis in red snapper, Lutjanus species (Bloch 1790)

“…Reports of toxic algae‐related fish kills are on the rise. Toxic algae may predispose to secondary infectious disease or poor growth, and have a significant impact on aquaculture (Andersen et al., 2016; Noga, 1998; Place et al., 2012). This case was presented for investigation into persistent low‐grade mortality in red snapper, Lutjanus species Bloch, which did not respond to several therapeutic interventions.…”