Phytoplankton defence mechanisms: traits and trade‐offs

Abstract

In aquatic ecosystems, unicellular algae form the basis of the food webs. Theoretical and experimental studies have demonstrated that one of the mechanisms that maintain high diversity of phytoplankton is through predation and the consequent evolution of defence mechanisms. Proposed defence mechanisms in phytoplankton are diverse and include physiological (e.g. toxicity, bioluminescence), morphological (e.g. silica shell, colony formation), and behavioural (e.g. escape response) traits. However, the function of many of the proposed defence mechanisms remains elusive, and the costs and benefits (trade-offs) are often unquantified or undocumented. Here, we provide an overview of suggested phytoplankton defensive traits and review their experimental support. Wherever possible we quantify the trade-offs from experimental evidence and theoretical considerations. In many instances, experimental evidence suggests that defences are costless. However, we argue that (i) some costs materialize only under natural conditions, for example, sinking losses, or dependency on the availability of specific nutrients, and (ii) other costs become evident only under resource-deficient conditions where a rivalry for limiting resources between growth and defence occurs. Based on these findings, we suggest two strategies for quantifying the costs of defence mechanisms in phytoplankton: (i) for the evaluation of defence costs that are realized under natural conditions, a mechanistic understanding of the hypothesized component processes is required; and (ii) the magnitude of the costs (i.e. growth reduction) must be assessed under conditions of resource limitation.

Competition–defense tradeoff increases the diversity of microbial plankton communities and dampens trophic cascades

“…Defense in unicellular plankton is expressed through a large variety of mechanisms, ranging from size and shape modifications to colony formation, toxin production or stealth behavior, and these are in many cases harnessed or upregulated in the presence of predators (Pančić and Kiørboe 2018). The tradeoffs are defense specific and are often not quantified (Viola et al 2010, Boots 2011, Pančić and Kiørboe 2018. It is therefore challenging to design a universal mathematical formulation of the tradeoff between defense and competitive ability.…”

The costs and benefits of multicellular group formation in algae*

“…7A). Notably, we found a relatively small influence of predator size in S. subspicatus, which is the only species out of the four known to produce spines, which may serve as an additional predator defense mechanism apart from group formation (Hessen and Van Donk 1993;Pančić and Kiørboe 2018).…”

On biological evolution and environmental solutions

“…Harmful algal blooms, which can persist for weeks or months, may foul drinking water, turn lakes anoxic and kill fish, and render lakes unacceptable for recreational use (Lewitus et al, 2012;Paerl et al, 2011). Ecological theory suggests that blooms develop when nutrient input releases phytoplankton from control by grazers (Abrams and Walters, 1996;Gragnani et al, 1999;Pančić and Kiørboe, 2018). Additionally, evolutionary processes are also relevant to the emergence, volume and toxicity of blooms.…”

“…Harmful algal blooms are characterised by an array of defense traits that are favored by natural selection. Some algae produce compounds that are toxic to grazers, such as the neurotoxins, saxitoxins and domoic acid produced by dinoflagellates, cyanobacteria, and diatoms, respectively (Pančić and Kiørboe, 2018;Xu and Kiørboe, 2018). Remarkably, toxin production varies widely both within and among populations.…”