Phytoplankton adaptation in ecosystem models

Neutral Theory and Plankton Biodiversity

The biodiversity of the plankton has been interpreted largely through the monocle of competition. The spatial distancing of phytoplankton in nature is so large that cell boundary layers rarely overlap, undermining opportunities for resource-based competitive exclusion. Neutral theory accounts for biodiversity patterns based purely on random birth, death, immigration, and speciation events and has commonly served as a null hypothesis in terrestrial ecology but has received comparatively little attention in aquatic ecology. This review summarizes basic elements of neutral theory and explores its stand-alone utility for understanding phytoplankton diversity. A theoretical framework is described entailing a very nonneutral trophic exclusion principle melded with the concept of ecologically defined neutral niches. This perspective permits all phytoplankton size classes to coexist at any limiting resource level, predicts greater diversity than anticipated from readily identifiable environmental niches but less diversity than expected from pure neutral theory, and functions effectively in populations of distantly spaced individuals.

Rapid evolution allows coexistence of highly divergent lineages within the same niche

Marine microbial communities are extremely complex and diverse. The number of locally coexisting species often vastly exceeds the number of identifiable niches, and taxonomic composition often appears decoupled from local environmental conditions. This is contrary to the view that environmental conditions should select for a few locally well-adapted species. Here we use an individual-based eco-evolutionary model to show that virtually unlimited taxonomic diversity can be supported in highly evolving assemblages, even in the absence of niche separation. With a steady stream of heritable changes to phenotype, competitive exclusion may be weakened, allowing sustained coexistence of nearly neutral phenotypes with highly divergent lineages. This behaviour is robust even to abrupt environmental perturbations that might be expected to cause strong selection pressure and an associated loss of diversity. We, therefore, suggest that rapid evolution and individual-level variability are key drivers of species coexistence and maintenance of microbial biodiversity.

The need for unrealistic experiments in global change biology

Climate change is an existential threat, and our ability to conduct experiments on how organisms will respond to it is limited by logistics and resources, making it vital that experiments be maximally useful. The majority of experiments on phytoplankton responses to warming and CO₂ use only two levels of each driver. However, to project the characters of future populations, we need a mechanistic and generalisable explanation for how phytoplankton respond to concurrent changes in temperature and CO₂. This requires experiments with more driver levels, to produce response surfaces that can aid in the development of predictive models. We recommend prioritising experiments or programmes that produce such response surfaces on multiple scales for phytoplankton.

Eco-evolutionary responses of the microbial loop to surface ocean warming and consequences for primary production

Predicting the response of ocean primary production to climate warming is a major challenge. One key control of primary production is the microbial loop driven by heterotrophic bacteria, yet how warming alters the microbial loop and its function is poorly understood. Here we develop an eco-evolutionary model to predict the physiological response and adaptation through selection of bacterial populations in the microbial loop and how this will impact ecosystem function such as primary production. We find that the ecophysiological response of primary production to warming is driven by a decrease in regenerated production which depends on nutrient availability. In nutrient-poor environments, the loss of regenerated production to warming is due to decreasing microbial loop activity. However, this ecophysiological response can be opposed or even reversed by bacterial adaptation through selection, especially in cold environments: heterotrophic bacteria with lower bacterial growth efficiency are selected, which strengthens the "link" behavior of the microbial loop, increasing both new and regenerated production. In cold and rich environments such as the Arctic Ocean, the effect of bacterial adaptation on primary production exceeds the ecophysiological response. Accounting for bacterial adaptation through selection is thus critically needed to improve models and projections of the ocean primary production in a warming world.

The evolution of trait correlations constrains phenotypic adaptation to high CO2 in a eukaryotic alga

Microbes form the base of food webs and drive biogeochemical cycling. Predicting the effects of microbial evolution on global elemental cycles remains a significant challenge due to the sheer number of interacting environmental and trait combinations. Here, we present an approach for integrating multivariate trait data into a predictive model of trait evolution. We investigated the outcome of thousands of possible adaptive walks parameterized using empirical evolution data from the alga Chlamydomonas exposed to high CO₂. We found that the direction of historical bias (existing trait correlations) influenced both the rate of adaptation and the evolved phenotypes (trait combinations). Critically, we use fitness landscapes derived directly from empirical trait values to capture known evolutionary phenomena. This work demonstrates that ecological models need to represent both changes in traits and changes in the correlation between traits in order to accurately capture phytoplankton evolution and predict future shifts in elemental cycling.

Selective constraints on global plankton dispersal

Marine microbial communities are highly interconnected assemblages of organisms shaped by ecological drift, natural selection, and dispersal. The relative strength of these forces determines how ecosystems respond to environmental gradients, how much diversity is resident in a community or population at any given time, and how populations reorganize and evolve in response to environmental perturbations. In this study, we introduce a globally resolved population-genetic ocean model in order to examine the interplay of dispersal, selection, and adaptive evolution and their effects on community assembly and global biogeography. We find that environmental selection places strong constraints on global dispersal, even in the face of extremely high assumed rates of adaptation. Changing the relative strengths of dispersal, selection, and adaptation has pronounced effects on community assembly in the model and suggests that barriers to dispersal play a key role in the structuring of marine communities, enhancing global biodiversity and the importance of local historical contingencies.

Algal plankton turn to hunting to survive and recover from end-Cretaceous impact darkness

The end-Cretaceous bolide impact triggered the devastation of marine ecosystems. However, the specific kill mechanism(s) are still debated, and how primary production subsequently recovered remains elusive. We used marine plankton microfossils and eco-evolutionary modeling to determine strategies for survival and recovery, finding that widespread phagotrophy (prey ingestion) was fundamental to plankton surviving the impact and also for the subsequent reestablishment of primary production. Ecological selectivity points to extreme post-impact light inhibition as the principal kill mechanism, with the marine food chain temporarily reset to a bacteria-dominated state. Subsequently, in a sunlit ocean inhabited by only rare survivor grazers but abundant small prey, it was mixotrophic nutrition (autotrophy and heterotrophy) and increasing cell sizes that enabled the eventual reestablishment of marine food webs some 2 million years later.