IAP-25-065

Physics, biology and ecology of toxic plankton blooms

Microalgal populations in marine and fresh waters can grow explosively giving rise to plankton blooms. This has important ecological consequences: the biomass in blooms can stimulate life across the food web; however, blooms can be harmful (Smayda, 2019). Each year, toxin-producing Harmful Algal Blooms (HABs) kill vast numbers of animals (including fish and mammals), with substantial health and economic impacts on human society, including major losses in the aquaculture sector (Gianella et al. 2025). In addition, hanges in environmental conditions such as nutrient availability and water temperature have toxic HABs more frequent. Despite recent progress, toxic HABs remain poorly understood and hard to predict (Flynn & McGillicuddy, 2018). Conditions favourable to growth, e.g., nutrients, temperature and light, are necessary but not sufficient for bloom genesis and spatio-temporal evolution (Paerl, 1988). The vast majority of toxic HAB formers belong to a group known as dinoflagellates, which swim by means of flagella and have flexible feeding strategies switching between photosynthesis and predation. Swimming, particularly in calmer waters, can be biased by environmental factors (nutrient gradients, light levels) leading to competitive advantage and the potential to dominate blooms. However, current studies treat swimming simplistically, without accounting for recent progress in the physics of microswimmers, such as dinoflagellates. In the last four decades, enormous progress has been made in this area of biophysics (Bees & Croze 2014). However, only recently are its ecological implications beginning to be explored (Grunbaum, 2009). For example, it has been shown that ‘thin plankton layers’ in the ocean arise when biased swimming algae, including the HAB former Heterosigma asakiwo, get trapped in the strong shear which characterises these layers (Durham et al. 2009). In this interdisciplinary project you will combine state of the art biophysics, theoretical and experimental, with algal physiology and ecotoxicology to elucidate the mechanisms of toxic HAB formation using Karenia mikimotoi – a HAB dinoflagellate of increasing global concern (Li et al. 2019). Through a combination of lab-based studies and controlled marine environments (microcosms) you will investigate how light and nutrient availability shape blooms and their toxic content. You will also study the link between motility and mixotrophy, the ability of toxic algae to switch between photosynthetic growth and other types of nutrient acquisition (e.g. feeding on bacteria), which has not previously been investigated.

• Advanced microscopy (including differential dynamic microscopy) to characterise the motility of dinoflagellates and their ‘taxes’, biased responses towards light and other factors
• Microfluidics to quantify chemotaxis to nutrients and access to prey
• Quantitative macroscopy to chart the migration of dinoflagellates in lab-based millifluidic devices (cuvettes)
• Migration in microcosms will be quantified by macro imaging of the water surface, plus samples of the population density measured by calibrated optical density
• The growth and health of the microalgae in various nutrient regimes will be characterised by growth assays and Fv/Fm measurements
• Toxic chemicals will be characterised by LCMS
• Agent based and continuum models of microalgal migration in response to environmental taxes

Year 1

Year 1 work will be largely lab based involving:
• characterisation of Karenia mikimotoi growth, both phototrophic and mixotrophic simultaneously to photosynthetic health
• Measurement of toxin production in various stages of growth
• Determination by advanced microscopy of the biased swimming of Karenia to light (phototaxis), chemicals (chemotaxis) and bacteria (bacteriotaxis), and measurement of key taxis parameters.
• Development of an agent based model of bloom formation, and test in the lab

Year 2

Year 2 work will involve:
• Scaling up the artificial blooms to lab microscosms (square ponds)
• Quantification of blooms by top view imaging measuring algal accumulation and depth sampling to obtain the population depth profile
• Adaptation of the model to describe the microcosms (addition of fluid flow and temperature gradient).

Year 3

Year work 3 builds on years 1 and 2, allowing:
• Model testing for microcosms and adapting it to describe environmental blooms
• Use aerial and satellite data to check the model predictions and explore opportunities to incorporate your findings into existing K. mikimotoi bloom development and transport models.
• In this year you should also be looking for opportunities beyond the PhD

Year 3.5

In year 3.5, you will be writing your thesis and doing minor amounts of further analysis of to refine your analysis and presentation of results.

• School of Mathematics, Statistics and Physics and Newcastle University training (e.g. Health and Safety, How to write a literature review, Unix shell)
• Culture training visit to the Culture Collection of Algae and Protozoa in Oban, Scotland.
• Visit to Plymouth Marine Laboratory to explore whether project findings can be used to improve the predictive power of the S3 EUORHAB (https://pml.ac.uk/news/blooming-algae-off-the-south-west-coast/)
• Integration with the Sonic Intangibles programme (https://sonicintangibles.github.io/) to explore sonification of swimming data as an additional path for dissemination.