Accounting for mixotrophy within microbial food webs

Abstract

Protist plankton contribute to both primary and secondary production in the oceans fuelling life within pelagic food webs. Despite commonly perceived as ‘phytoplankton’ or ‘zooplankton’, most protist plankton are mixotrophs through the combination of photoautotrophy and phagotrophy within a single cell. Nevertheless, we lack a clear understanding of their biogeography and impact on ecosystem functioning. The aims of this thesis were: i) to investigate the biogeography of mixotrophs according to their functional diversity across oceanic biomes and to evaluate how it relates to environmental variability; ii) to explore the competitive outcomes between mixotrophs and their auto- or hetero- trophic counterparts and the effect of mixotrophy on ecosystem functioning; and iii) to investigate the seasonal succession of protist trophic strategies and the vertical distribution of mixotrophs within a coastal temperate sea. These aims were explored utilizing global databases, long-term monitoring datasets, and numerical models of plankton food webs across different spatio-temporal scales. Mixotrophs were found to be ubiquitous in the global oceans; however, different types displayed different distributions. Among non-constitutive mixotrophs, those that host prey populations dominate within oligotrophic seas while those that steal prey plastids dominate high-biomass systems. In turn, global databases were strongly biased by size, taxonomy, and oceanic biome, failing to represent the importance of smaller constitutive mixotrophs. The modelling studies showed that mixotrophs control nutrient regulation, trophic transfer, and the microbial loop. Size was an important trait determining the success of mixotrophs with an innate capacity for photosynthesis while the specificity of prey from which acquired phototrophs can photosynthesize affected their success. Model and data showed that mixotrophy is a persistent trait over the seasonal cycle and throughout the water column within coastal temperate seas. These findings significantly change our understanding of the functioning of marine food webs and biogeochemical cycling in the oceans, underscoring the need to integrate mixotrophy within marine ecology research