Abstract
Particulate organic matter (POM) (fecal pellets) from zooplankton has been demonstrated to be an important nutrient source for the pelagic prokaryotic community. Significantly less is known about the chemical composition of the dissolved organic matter (DOM) produced by these eukaryotes and its influence on pelagic ecosystem structure. Zooplankton migrators, which daily transport surface-derived compounds to depth, may act as important vectors of limiting nutrients for mesopelagic microbial communities. In this role, zooplankton may increase the DOM remineralization rate by heterotrophic prokaryotes through the creation of nutrient rich “hot spots” that could potentially increase niche diversity. To explore these interactions, we collected the migratory copepodPleuromamma xiphiasfrom the northwestern Sargasso Sea and sampled its excreta after 12–16 h of incubation. We measured bulk dissolved organic carbon (DOC), dissolved free amino acids (DFAA) via high performance liquid chromatography and dissolved targeted metabolites via quantitative mass spectrometry (UPLC-ESI-MSMS) to quantify organic zooplankton excreta production and characterize its composition. We observed production of labile DOM, including amino acids, vitamins, and nucleosides. Additionally, we harvested a portion of the excreta and subsequently used it as the growth medium for mesopelagic (200 m) bacterioplankton dilution cultures. In zooplankton excreta treatments we observed a four-fold increase in bacterioplankton cell densities that reached stationary growth phase after five days of dark incubation. Analyses of 16S rRNA gene amplicons suggested a shift from oligotrophs typical of open ocean and mesopelagic prokaryotic communities to more copiotrophic bacterial lineages in the presence of zooplankton excreta. These results support the hypothesis that zooplankton and prokaryotes are engaged in complex and indirect ecological interactions, broadening our understanding of the microbial loop.
Highlights
The daily migration of zooplankton from their night-time habitat in the euphotic zone to the mesopelagic during the day is a well-documented phenomenon referred to as diel vertical migration (DVM; Lampert, 1989)
dissolved organic carbon (DOC)/dissolved free amino acids (DFAA) Net DOC production ( DOC) in the three zooplankton excretaenriched experiments was calculated as the change in DOC concentration in each zooplankton enriched treatment relative to that of the average of the control treatments at the final time point (Figure 2A and Supplementary Material 1)
DFAA-C production was significantly higher [Paired t-test, t(6) = 4.080, p = 0.007] in the zooplankton excreta treatment compared to the minimal change in control or carryover treatments and made up on average 18% of the DOC by copepods (Figure 2B)
Summary
The daily migration of zooplankton from their night-time habitat in the euphotic zone to the mesopelagic during the day is a well-documented phenomenon referred to as diel vertical migration (DVM; Lampert, 1989). Similar proportions have been estimated for zooplankton in the North Pacific by simultaneously measuring oxygen consumption and DOC excretion (Maas et al, in review), and pteropods in the Southern Ocean (Thibodeau et al, 2020) Applying this conversion factor to global estimates of zooplankton respiratory flux (Steinberg and Landry, 2017), migratory zooplankton DOM excretion is responsible for a flux of 0.26–10.08 mg C m−2 d−1, representing 0.5–18.5% of the particulate organic carbon (POC) flux from surface waters. This process appears to be important to total carbon flux during periods of high thermal stratification in oligotrophic systems, where POC flux and DOC delivery via mixing are typically low or negligible (Steinberg et al, 2000)
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