Diatom‐produced allelochemicals trigger trophic cascades in the planktonic food web

Abstract

Diatoms produce a series of cytotoxic secondary metabolites such as polyunsaturated aldehydes (PUA) in response to cell injury or stress. However, little information exists on the PUA effects on the pelagic food web. A set of experiments was conducted in the Chesapeake Bay and the coastal Atlantic waters using dissolved PUA (2E,4E-octadienal and 2E,4E-heptadienal), natural assemblages of phytoplankton and microzooplankton, and the copepod Acartia tonsa. The results demonstrate that PUA primarily acts as deterrent for microzooplankton herbivory on diatoms, while enhancing herbivory on picophytoplankton. This switch should favor PUA-producing diatoms by simultaneously reducing direct grazing losses and competition. Additionally, PUA stimulated copepod predation on microzooplankton, particularly on ciliates. Combined, these effects have the potential to disrupt growth-grazing equilibrium in the pelagic food web and create an opportunity for bloom development.

Seasonal Variability and Drivers of Microzooplankton Grazing and Phytoplankton Growth in a Subtropical Estuary

“…Though SiO 4 (>50 µM) was not limited, total chlorophyll in the fall was low (<3 µg L −1 ), which suggests diatom communities were likely stressed by other environmental factors (e.g., low NO 3 and/or sunlight). Diatoms have been shown to release polyunsaturated aldehydes in response to physiological (Ribalet et al, 2007) or mechanical stress (e.g., grazing or lysis; Ribalet et al, 2014), which can inhibit microzooplankton feeding and the growth of other phytoplankton species (Franzè et al, 2018). These inhibitory factors may explain low phytoplankton mortality in the fall compared to other seasons.…”

Types and Distribution of Bioactive Polyunsaturated Aldehydes in a Gradient from Mesotrophic to Oligotrophic Waters in the Alborán Sea (Western Mediterranean)

“…These PUAs (e.g., 2E,4E/Z-heptadienal, 2E,4E/Z-octadienal, or 2E,4E,7E/Z-octatrienal) are mainly derived from lipoxidation of essential fatty acids as eicosapentanoic acid (EPA) and docosahexanoic acid (DHA) [10]. Although PUAs are not the most abundant oxylipins produced, they were experimentally demonstrated to have a teratogenic effect on copepods [11,12], as well as stimulation predation on ciliates by copepods [13]. In nature, differential effects of PUAs on the bacterial community [14][15][16] and ciliates [17] were reported, although their role and ecological significance remain poorly understood.…”

“…However, it remains to be analyzed whether these compounds would persist long enough to have any potential effect on non-producer phytoplankton, bacterioplankton, or zooplankton as experimentally tested. Notwithstanding, during the last two decades, substantial research went into determining their biological functions in nature [13,14,17,[19][20][21].…”

Temperature-dependent impacts of allelopathy on growth, pigment, and lipid content between a subpolar strain of Synechocystis sp. CCBA MA-01 and coexisting microalgae

“…According to the literature, allelopathy is a potential driver of the development of blooms and plankton community dynamics (Kubanek et al, 2005;Prince et al, 2008;Hattenrath-Lehmann & Gobler, 2011;Franzè et al, 2018). The number of reports on the allelopathic activity of cyanobacteria has been steadily increasing (e.g., Brutemark et al, 2015;Costa et al, 2015;Dias et al, 2017).…”

“…However, there are still many unknown facts regarding the mechanism of action of allelochemicals, which might not need to reach high concentrations in order to be effective (Lewis, 1986). Recent papers showed significant effects of diatom allelochemicals, at low concentrations, in field planktonic food webs (Franzè et al, 2018).…”