Marine Snow Consumption by Zooplankton Facilitates a Novel Entry Pathway for Domoic Acid into the Marine Food Web
Marine Snow Consumption by Zooplankton Facilitates a Novel Entry Pathway for Domoic Acid into the Marine Food Web
Abstract:
Domoic acid (DA) is a neurotoxin, produced by diatoms from the genera Pseudo-nitzschia and Nitzschia, responsible for amnesic shellfish poisoning. Its transfer through marine food webs is well documented and can pose a threat to human health. Various pelagic and benthic organisms act as vectors for DA and the toxin accumulates in the food web despite loss variables favoring quick degradation in the water column and depuration from animal tissue. The observation that DA can be detected in higher trophic level organisms when toxin-producer abundance and toxicity are low, suggests there may be an alternative entry pathway for DA into the food web. Using laboratory experiments, we establish that organic polymers (e.g. marine snow) can aggregate and scavenge dissolved DA (dDA) (2.57 ± 0.56 nmol DA: µmol C). We then demonstrate assimilation of this marine snow-bound DA by the copepod Acartia tonsa in both laboratory and field conditions —a pathway which does not require direct ingestion of the toxin-producing diatom. In the laboratory, copepods suspended in filtered seawater (no diatoms) spiked with DA accumulated 24.8 ± 4.7 pg DA copepod-1 in triplicate measurements. These laboratory results were validated in the field. Copepods were suspended in particle-free seawater with ambient dDA and accumulated 14.4 ± 3.8 pg DA copepod-1 via this indirect pathway, which represented ~34% of the total assimilated DA. Among the five field experiments, the successful detection of marine-snow DA assimilation corresponded with the highest Pseudo-nitzschia spp. abundance (~225,000 cells L-1) and cellular toxicity (maximum, 0.88 pg DA cell-1). Our results demonstrate the potential for DA to enter the marine food web through a novel pathway which decouples the presence of DA from Pseudo-nitzschia. This has considerable implications for understanding the transfer of DA through marine food webs, how we monitor DA, and whether other phycotoxins can enter the food web in this manner.