Monday, 7 April 2014

Copepods act as vectors for the for the accumulation of domoic acid in the Arctic food web

Out of the 37 described species of the diatom genus Pseudo-nitzschia, 12 are known to produce the neurotoxin domoic acid (DA), which accumulates in organisms which graze on the toxic Pseudo-nitzschia, such as copepods, krill, blue mussels and scallops, and are then passed through food webs when animals at higher tropic levels consume these grazers, and these even include whales and seabirds. Symptoms have even been found in humans that include several types of neurological disorders, and may even lead to mortality. One of these domoic-acid producing species, Pseudo-nitzschia seriata, was first discovered in Arctic waters in western Greenland. However, some of the earlier records of this species could have also included the morphologically-similar Pseudo-nitzschia delicatissima, which does not produce the toxin. P. seriata is a component of the Arctic phytoplankton community, but there is a lack of information about the frequency and intensity of blooms that form within Arctic waters.

Three species of copepod, Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus are known to be the most important metazoan grazers in Arctic Ocean and are key sources of prey to shrimps, including the commercially significant northern shrimp (Pandalus borealis) which is consumed by a large proportion of the local human population of Greenland and other countries which import them. It also has recently been shown that there is a risk of domoic acid accumulation in higher trophic levels via Calanus spp., as the North Atlantic right whale (Eubalaena glacialis) was found to be regularly exposed to domoic acid from its critical prey source, C. finmarchicus, and this has been suggested as one of the possible reasons for the reduction in reproduction success of this critically endangered whale population.

The authors of this current study looked at the risk of transferring DA through the Arctic food web via the three different species of zooplankton to determine whether they accumulate the toxin when fed with unialgal toxic cultures. They also examined whether the copepods are affected by DA and if this can be measured by looking for changes in grazing rates and/ or mortality, whether the three copepods vary in grazing rate and toxin accumulation, and finally if the copepods ingest toxic and non-toxic Pseudo-nitzschia at the same rate.

They found that the three species of Calanus graze on and ingest toxin-producing Pseudo-nitzschia seriata and that they can accumulate domoic acid after feeding, so they are indeed possible vectors. This paper is also the first to study the consequences of grazing on Pseudo-nitzschia spp. in Arctic waters, as previous studies have not attempted to find differences in clearance or ingestion rates between the toxic and non-toxic species. 

The weight-specific carbon ingestion rates of all three Calanus species were not significantly different when they fed on either toxic or non-toxic species, but the toxic P. seriata was egested/ cleared out of the gut and also ingested significantly less at 6-12 h than at 0-6 h by C. finmarchicus and C. hyperboreus. The amount of specific fecal pellet production (SPP) were very similar following each type of diet, which confirmed that both the toxic and non-toxic species were grazed by the copepods in roughly the same quantities. Although the copepods in this study were fed with unialgal cultures, the authors say one of the most critical ideas to investigate in future research is determining whether copepods graze on toxic Pseudo-nitzschia and accumulate DA when given a choice of food in the natural phytoplankton community. In addition, C. finmarchicus and C. hyperboreus seemed to stop grazing during the last 6 h of the experiment, which could be due to physiological incapacitation caused by toxin ingestion.

There may be an effect of size of the host on grazing rate, as C. finmarchicus was the smallest of the three species, which may be why it had the highest amount of DA per gram per copepod, but it also had the greatest weight-specific carbon ingestion of the toxic species. However, the different size ranges of the diatom species were not predicted to influence the grazing rate of any of the copepod species. Not all DA is egested by gut clearance but part of it is actually assimilated and accumulated into the vector’s tissues, so even if the guts of the copepods are clear of all toxic Pseudo-nitzschia species, it may still get transferred up into the higher trophic levels when these zooplankton are consumed by predators.

This study demonstrated a new and interesting insight into the effect of toxic microorganisms on the lower levels of the food web in Arctic waters. They mention that there is a threshold level where the Calanus species start to be affected by the toxin themselves, which seems to depend on a number of factors, including the size of copepod and concentration of domoic acid. In addition, the frequency and intensity of Pseudo-nitzschia species within Arctic waters is unknown. So I wonder whether certain external/ environmental conditions can enhance this impact, for instance in the face of global warming. An increase in sea temperature could cause a possible rise in phytoplankton blooms, and this could either cause a higher amount of DA to accumulate in copepods, or could lead to greater incidences of mortality. Either of these options could potentially impact the food web in some way, so the effect of temperature on the growth of blooms, and the subsequent influence of domoic acid on Calanus species and higher trophic levels, should be investigated.

Tammilehto, A., Nielson, T.G., Krock, B., Møller, E.F., and Lundholm, N. (2012) Calanus spp. – Vectors for the biotoxin, domoic acid, in the Arctic marine ecosystem? Harmful Algae, 20: 165-174

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