Opening the black box on predator-induced phytoplankton defenses: Mechanisms, traits, and trade-offs

Much like terrestrial plants, phytoplankton in the oceans have developed a wide range of defense mechanisms to decrease their predation mortality. These are often divided into three major types: physiological (e.g. the production of various chemical substances), morphological (e.g. altering shape, increasing shell thickness), and behavioral (resting stages, motility). By deterring predation pressure to less defended prey, defense mechanisms play an important role in structuring the plankton food web. In general, defense mechanisms allow for the co-existence of defense- and competition specialists on limiting resources due to the increased predation on undefended compared to defended cells. As a result, more resources become available and may increase diversity and have major impact on phytoplankton community composition. In this Ph.D. work, I provide a mechanistic understanding of a few, common predator-induced phytoplankton defenses and their associated trade-offs: toxin production, colony formation, and structural changes in the cell wall. Studies dedicated to defense mechanisms in phytoplankton have often focused on the benefits of the defense, but have rarely established potential associated costs (trade-offs). However, there must be costs; otherwise, non-defended species or strains would be outcompeted and all species would be equally defended, which they are not. In addition, many defense mechanisms are inducible; i.e. they are only harnessed in the presence of predators, further suggesting that costs are substantial. Nevertheless, the trade-offs of many defense mechanisms remain elusive and have not been established experimentally. An example of this are the toxins produced by the dinoflagellate genus Alexandrium. The toxins have been demonstrated to reduce predation mortality as many strains are frequently de-selected by copepods, but quantifying the associated trade-offs have proven difficult. A reason for this may be due to the use of nutrient replete conditions, while costs may be more evident during nutrient limitation. To explore this possibility, we grew Alexandrium minutum at different levels of nitrogen limitation and induced an increased toxin production with predator cues. We document several nutrient-dependent responses by Alexandrium, in addition to induced cells being more toxic and more frequently rejected by copepods. We find no evidence of direct costs in terms of reduced growth rate, but document several potential ecological costs that might not necessarily manifest in laboratory conditions. Another toxic phytoplankton is the diatom Pseudo-nitzschia that produces the neurotoxin domoic acid. Here, there has been evidence of a trade-off in terms of reduced growth rate in cells exposed to copepods in order to increase toxicity. However, in contrast to Alexandrium, no actual benefits of the toxins have been reported. A common issue in these experiments have been the use of “black box” bottle-incubation set-ups where the mechanisms of the defense cannot be studied. To explore the use of domoic acid as a defense mechanism against copepods, we directly observe individual predator-cell interactions using video filming, while feeding cells of different toxin content to copepods. We demonstrate that cells with increased toxicity have a much higher chance of being rejected by the copepod after capture, suggesting that domoic acid is indeed an efficient defense. We also observe a ’public good’ effect of the toxins, as copepods exposed to toxic cells reduce their feeding activity. We find that more strains than previously known respond to chemical cues from copepods and that despite a large variation in growth rate between strains, the subsequent cost in terms of reduced growth rate, is similar. Colonial phytoplankton often respond to predator-presence by increasing or decreasing colony formation. However, the effect of colony shape and size on predator behavior is still relatively unknown. To explore the defensive role of colony formation,
we examine the feeding behavior of zooplankton of different sized on colonial phytoplankton of different shapes and sizes. We find the adult copepod capture clearance rate on colony-forming phytoplankton dramatically increase with the size of the colony. However, we find no benefit of the colony size increase in elongated diatom chains, but the ingestion of the spherical colony-forming Phaeocystis globosa was completely reduced in the largest colonies. Thus, we argue that it is the width and shape, rather than the length of the colony that determines defensive value in response to adult copepods. In contrast, we demonstrate that colony formation in both diatoms and P. globosa is an efficient defense mechanism against smaller predators, such as copepod nauplii and heterotrophic dinoflagellates. Reduced copepod predation on diatom cells with higher silica content has been reported, but the mechanism behind the reduction is still largely unknown. We take a direct-observational approach and study individual predator-cell interactions using cells manipulated to contain different amounts of biogenic silica. We demonstrate that increased silica content reduces the fraction of cells that are ingested by copepods and increases the time it takes the copepod to handle and ingest their prey.