Diatom defence: Grazer induction and cost of shell‐thickening

Behavioral adaptations of cruise-feeding copepods to harmful algal blooms: Insights from the East China Sea

Harmful algal blooms (HABs) have become a global environmental concern, significantly impacting marine life and the fishing industry. However, the tolerance and adaptive mechanisms of zooplankton to HABs remain poorly understood. This study examines the behavioral and feeding responses of the cruise-feeding copepod Centropages dorsispinatus to summer HABs in the East China Sea (ECS), focusing on interactions with the blooming diatom (Skeletonema costatum) and dinoflagellates (Prorocentrum donghaiense, Karenia mikimotoi, and Alexandrium tamarense). Using short-term incubations and high-speed filming, we compared the ingestion rates and behaviors of C. dorsispinatus fed mono-algal diets and mixed diets containing neutral distraction particles (polystyrene beads). The results revealed that C. dorsispinatus obtained limited carbon from each algal diet (1.02-7.02 μg C cop.-1 day-1). The presence of distraction particles reduced carbon intake from S. costatum, P. donghaiense, and A. tamarense, but significantly increased intake from the healthy control, Platymonas helgolandica. Behavioral responses varied among algal diets: compared to P. helgolandica, C. dorsispinatus exhibited more frequent but shorter swims in S. costatum diets and less frequent swims in K. mikimotoi, and A. tamarense diets. These algal-specific responses were generally mitigated when copepods simultaneously exposed to the neutral distraction particles. Copepods achieved higher carbon intake with lower mechanical energy expenditure when grazing on large dinoflagellates compared to diatoms. We suggest that cruise-feeding copepods can actively adjust their behavior to adapt to varying food conditions, including the density, morphologic characteristics, and toxicity of algae. It allows copepods to better survive and forage in dinoflagellate HABs than in diatom HABs. However, the low ingestion rates observed limit the potential for cruise-feeding copepods to exert top-down control on HABs.

Eutrophication-Driven Changes in Plankton Trophic Interactions: Insights from Trade-Offs in Functional Traits

Understanding how plankton trophic interactions, particularly phytoplankton nutrient uptake and zooplankton grazing, respond to eutrophication is important for maintaining aquatic ecosystem functions and developing effective mitigation strategies. Phytoplankton exhibit trade-offs in functional traits between growth rate and antipredation defense, thereby regulating these trophic interactions. However, the combined effects of eutrophication and such trait-based regulation on plankton communities and interactions remain poorly understood. In the present study, we investigated these effects by integrating trait-based mechanistic modeling and field observations in China's eutrophic Pearl River Estuary. Our model predicted that the species with the weakest defensive capacities dominated under nutrient-poor conditions. As eutrophication increased, a concave growth-defense trade-off favored species with high growth rates and strong defense capacities, whereas a convex trade-off curve favored species that were either the least or the most well-defended. High grazing pressure accelerated these shifts. In the estuary, similar patterns emerged in the relative abundance of different phytoplankton species along a gradient of the nitrogen to phosphorus ratio (N:P), indicating changes from high nutrient uptake and low grazing under oligotrophic conditions to eutrophic conditions, in which some phytoplankton face considerable grazing pressure despite high nutrient uptake, whereas others grow slowly with less grazing pressure. These results enhance our understanding of trait-based plankton interactions in eutrophic bodies of water and provide support for more effective conservation and management strategies.

Multiple defense is an effective antipredator strategy in dinoflagellates

Phytoplankton have evolved myriad defenses against predators; yet, studies that simultaneously test for defense fitness costs and benefits are rare. We tested for relative fitness costs and benefits of defense in the marine dinoflagellate Alexandrium catenella using a framework that relates growth rates of prey genotypes (strains) that differed in constitutive toxin production (low, moderate, and high) to predator (copepod) concentration. Our approach is based on a novel molecular technique that allows one to disentangle the effect of predation mortality from the cell growth reduction due to toxin production. Results show that the strain with the highest constitutive toxin production was the only one that expressed inducible toxin production-a strategy that paid off as its fitness benefit outweighed its cost. Surprisingly, the moderate toxin strain that derived the highest relative fitness benefit increased cell division rate (akin to compensatory growth) and decreased cell size, while keeping its volume-specific toxin production constant in response to predation. These results suggest an effective antipredator defense portfolio.

Organismal trade-offs and the pace of planktonic life

No one is perfect, and organisms that perform well in some habitat or with respect to some tasks, do so at the cost of performance in others: there are inescapable trade-offs. Organismal trade-offs govern the structure and function of ecosystems and attempts to demonstrate and quantify trade-offs have therefore been an important goal for ecologists. In addition, trade-offs are a key component in trait-based ecosystem models. Here, I synthesise evidence of trade-offs in plankton organisms, from bacteria to zooplankton, and show how a slow-fast gradient in life histories emerges. I focus on trade-offs related to the main components of an organism's Darwinian fitness, that is resource acquisition, survival, and propagation. All consumers need to balance the need to eat without being eaten, and diurnal vertical migration, where zooplankton hide at depth during the day to avoid visual predators but at the cost of missed feeding opportunities in the productive surface layer, is probably the best documented result of this trade-off. However, there are many other more subtle but equally important behaviours that similarly are the result of an optimisation of these trade-offs. Most plankton groups have also developed more explicit defence mechanisms, such as toxin production or evasive behaviours that are harnessed in the presence of their predators; the costs of these have often proved difficult to quantify or even demonstrate, partly because they only materialise under natural conditions. Finally, all multicellular organisms must allocate time and resources among growth, reproduction, and maintenance (e.g. protein turnover and DNA repair), and mate finding may compromise both survival and feeding. The combined effects of all these trade-offs is the emergence of a slow-fast gradient in the pace-of-life, likely the most fundamental principle for the organisation of organismal life histories. This crystallisation of trade-offs may offer a path to further simplification of trait-based models of marine ecosystems.

Direct and indirect effects of copepod grazers on community structure

Ecological theory and empirical research show that both direct lethal effects and indirect non-lethal effects can structure the composition of communities. While the direct effects of grazers on marine phytoplankton communities are well studied, their indirect effects are still poorly understood. Direct and indirect effects are inherently difficult to disentangle in plankton food webs. In this study we evaluate the indirect effects of copepod grazers on community function and structure using isolated chemical alarm signals, copepodamides. We expose intact summer and spring communities to direct grazing from copepods, or to chemical alarm cues without the presence of grazers in controlled experiments. The effects of direct grazing on ecosystem function were moderate in both experiments as indicated by levels of chlorophyll and primary production. Indirect and direct effects resulted in changes in the composition of both the eukaryote and prokaryote communities as shown by metabarcoding of 18S and 16S rRNA. Size structure analysis suggests that direct grazing and copepodamide exposure both favoured smaller organisms (< 10-15 μm) corroborating the size-structuring effect of copepod grazers. We conclude that the well-established effect of copepods on phytoplankton communities results from a combination of direct and indirect effects. This is a first attempt to isolate indirect effects of copepods on community structure and the results suggest that a full mechanistic understanding of the structuring effect of copepods will require insights to both direct and indirect effects of consumers as demonstrated for other ecosystems components.

A siliceous arms race in pelagic plankton

Coevolution between predator and prey plays a central role in shaping the pelagic realm and may have significant implications for marine ecosystems and nutrient cycling dynamics. The siliceous diatom frustule is often assumed to have coevolved with the silica-lined teeth of copepods, but empirical evidence of how this relationship drives natural selection and evolution is still lacking. Here, we show that feeding on diatoms causes significant wear and tear on copepod teeth and that this leads to copepods becoming selective feeders. Teeth from copepods feeding on thick-shelled diatoms were more likely to be broken or cracked than those feeding on a dinoflagellate. When fed a large diatom, all analyzed teeth had visible wear. Our results underscore the importance of the predator-prey arms race as a driving force in planktonic evolution and diversity.

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

A review of the taxonomic diversity, host-parasite interactions, and experimental research on chytrids that parasitize diatoms

Diatoms (Bacillariophyta) are a major source of primary production on Earth, generating between 1/4 to 1/2 of all oxygen. They are found in almost all bodies of water, the ice of mountains, the arctic and the antarctic, and soils. Diatoms are also a major source of food in aquatic systems, a key component of the silica cycle, and are carbon capturers in oceans. Recently, diatoms have been examined as sources of biofuels, food, and other economic boons. Chytrids are members of the Kingdom fungi comprising, at a minimum, Chytridiomycota, Blastocladiomycota, and Neocallimastigales. Most chytrids are saprobes, plant pathogens, or parasites, and play an important role in aquatic ecosystems. Chytrid parasitism of diatoms has been reported to cause epidemics of over 90% fatality, though most of the information regarding these epidemics is limited to interactions between just a few hosts and parasites. Given the ubiquity of diatoms, their importance in natural and economic systems, and the massive impact epidemics can have on populations, the relative lack of knowledge regarding parasitism by chytrids is alarming. Here we present a list of the firsthand accounts of diatoms reported parasitized by chytrids. The list includes 162 named parasitic chytrid-diatom interactions, with 63 unique chytrid taxa from 11 genera, and 74 unique diatom taxa from 28 genera. Prior to this review, no list of all documented diatom-chytrid interactions existed. We also synthesize the currently known methods of infection, defense, and experiments examining diatoms and chytrids, and we document the great need for work examining both a greater breadth of taxonomic diversity of parasites and hosts, and a greater depth of experiments probing their interactions. This resource is intended to serve as a building block for future researchers studying diatom-parasite interactions and global planktonic communities in both fresh and marine systems.

Differential physiological response of marine and freshwater microalgae to polystyrene microplastics

Effects of microplastics on microalgae have not been compared from different habitat. To answer this question, three marine microalgae species (Chlorella marined, Nannochloropsis oculate, and Phaeodactylum tricornutum) and two freshwater species (Chlorella vulgaris and Tetradesmus obliquus) were selected and exposed to the environment relevant concentrations of polystyrene microplastics. The results indicated that microplastics have a significant concentration effect on the growth of microalgae. The attachment of microalgae to microplastics surface and the aggregation of microalgae with each other were observed. Under exposure of microplastics, the photosynthesis of microalgae was inhibited while the antioxidant system was activated, indicating that microplastics had a negative impact on microalgae. At the end of exposure, the oxidative stress status caused by microplastics in marine microalgae were alleviated, but the antioxidant system of freshwater microalgae was still at high levels, indicating a stress response. In addition, integrated biomarker response (IBR) indicated that the effects of microplastics on freshwater microalgae were severer than marine microalgae, which might relate to their differences in removing reactive oxygen species (ROS) effectively and membrane structure. Our study provides a reliable data for understanding the complex effects of microplastics on microalgae, and especially for comparing the differential effects of microplastics among different microalgae.

Elemental and biochemical nutrient limitation of zooplankton: A meta-analysis

Primary consumers in aquatic ecosystems are frequently limited by the quality of their food, often expressed as phytoplankton elemental and biochemical composition. However, the effects of these food quality indicators vary across studies, and we lack an integrated understanding of how elemental (e.g. nitrogen, phosphorus) and biochemical (e.g. fatty acid, sterol) limitations interactively influence aquatic food webs. Here, we present the results of a meta-analysis using >100 experimental studies, confirming that limitation by N, P, fatty acids, and sterols all have significant negative effects on zooplankton performance. However, effects varied by grazer response (growth vs. reproduction), specific manipulation, and across taxa. While P limitation had greater effects on zooplankton growth than fatty acids overall, P and fatty acid limitation had equal effects on reproduction. Furthermore, we show that: nutrient co-limitation in zooplankton is strong; effects of essential fatty acid limitation depend on P availability; indirect effects induced by P limitation exceed direct effects of mineral P limitation; and effects of nutrient amendments using laboratory phytoplankton isolates exceed those using natural field communities. Our meta-analysis reconciles contrasting views about the role of various food quality indicators, and their interactions, for zooplankton performance, and provides a mechanistic understanding of trophic transfer in aquatic environments.

Ecology of Fear: Spines, Armor and Noxious Chemicals Deter Predators in Cancer and in Nature

In nature, many multicellular and unicellular organisms use constitutive defenses such as armor, spines, and noxious chemicals to keep predators at bay. These defenses render the prey difficult and/or dangerous to subdue and handle, which confers a strong deterrent for predators. The distinct benefit of this mode of defense is that prey can defend in place and continue activities such as foraging even under imminent threat of predation. The same qualitative types of armor-like, spine-like, and noxious defenses have evolved independently and repeatedly in nature, and we present evidence that cancer is no exception. Cancer cells exist in environments inundated with predator-like immune cells, so the ability of cancer cells to defend in place while foraging and proliferating would clearly be advantageous. We argue that these defenses repeatedly evolve in cancers and may be among the most advanced and important adaptations of cancers. By drawing parallels between several taxa exhibiting armor-like, spine-like, and noxious defenses, we present an overview of different ways these defenses can appear and emphasize how phenotypes that appear vastly different can nevertheless have the same essential functions. This cross-taxa comparison reveals how cancer phenotypes can be interpreted as anti-predator defenses, which can facilitate therapy approaches which aim to give the predators (the immune system) the upper hand. This cross-taxa comparison is also informative for evolutionary ecology. Cancer provides an opportunity to observe how prey evolve in the context of a unique predatory threat (the immune system) and varied environments.

Predator-induced defence in a dinoflagellate generates benefits without direct costs

Inducible defences in phytoplankton are often assumed to come at a cost to the organism, but trade-offs have proven hard to establish experimentally. A reason for this may be that some trade-off costs only become evident under resource-limiting conditions. To explore the effect of nutrient limitation on trade-offs in toxin-producing dinoflagellates, we induced toxin production in Alexandrium minutum by chemical cues from copepods under different levels of nitrogen limitation. The effects were both nitrogen- and grazer-concentration dependent. Induced cells had higher cellular toxin content and a larger fraction of the cells was rejected by a copepod, demonstrating the clear benefits of toxin production. Induced cells also had a higher carbon and nitrogen content, despite up to 25% reduction in cell size. Unexpectedly, induced cells seemed to grow faster than controls, likely owing to a higher specific nutrient affinity due to reduced size. We thus found no clear trade-offs, rather the opposite. However, indirect ecological costs that do not manifest under laboratory conditions may be important. Inducing appropriate defence traits in response to threat-specific warning signals may also prevent larger cumulative costs from expressing several defensive traits simultaneously.