Accounting for mixotrophy within microbial food webs

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

A revised interpretation of marine primary productivity in the Indian Ocean: the role of mixoplankton

Traditional interpretations of marine plankton ecology, such as that in the Indian Ocean, mirror the plant-animal dichotomy of terrestrial ecology. Thus, single-celled phytoplankton produce food consumed by single-celled zooplankton, and these are in turn consumed by larger zooplankton through to higher trophic levels. Our routine monitoring surveys, research, models, and water management protocols all reflect this interpretation. The last decade has witnessed the development of an important revision of that traditional vision. We now know that the phytoplankton-zooplankton dichotomy represents, at best, a gross simplification. A significant proportion of the protist plankton at the base of the oceanic food-web can photosynthesise (make food ‘like plants’) and ingest food (eat ‘like animals’), thus contributing to both primary and secondary production simultaneously in the same cell. These protists are termed ‘mixoplankton’, and include many species traditionally labelled as ‘phytoplankton’ (a term now reserved for phototrophic microbes that are incapable of phagocytosis) or labelled as ‘protist zooplankton’ (now reserved for protist plankton incapable of phototrophy). Mixoplankton include various harmful algal species, most likely all the phototrophic dinoflagellates, and even iconic exemplar ‘phytoplankton’ such as coccolithophorids (which can consume bacteria). Like all significant revisions to ecology, the mixoplankton paradigm will take time to mature but to ignore it means that we fail to properly represent plankton ecology in teaching, science, management, and policy. This chapter introduces the mixoplankton functional groups and provides the first insight into the biogeography of these organisms in the Indian Ocean. A first attempt to consider the implications of the mixoplankton paradigm on marine primary productivity and ecology in the Indian Ocean is also given

The evolution of trait correlations constrains phenotypic adaptation to high CO 2 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 CO(2). 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

Modelling mixotrophic functional diversity and implications for ecosystem function

Mixotrophy is widespread among protist plankton displaying diverse functional forms within a wide range of sizes. However, little is known about the niches of different mixotrophs and how they affect nutrient cycling and trophodynamics in marine ecosystems. Here we built a plankton food web model incorporating mixotrophic functional diversity. A distinction was made between mixotrophs with the innate capacity for photosynthesis (constitutive mixotrophs, CMs) and those which acquire phototrophy from their prey (non-constitutive mixotrophs, NCMs). We present the simulations of ecosystems limited by different light and nutrient regimes. Our simulations show that strict autotrophic and heterotrophic competitors increased in relative importance in the transition from nutrient to light limitation, consistent with observed oceanic biomass ratios. Among CMs, cells <20 μm dominate in nutrient-poor conditions while larger cells dominate in light-limited environments. The specificity of the prey from which NCMs acquire their phototrophic potential affects their success, with forms able to exploit diverse prey dominating under nutrient limitation. Overall, mixotrophy decreases the regeneration of inorganics and boosts the trophic transfer efficiency of carbon. Our results show that mixotrophic functional diversity has the potential to radically change our understanding of the ecosystem functioning in the lower trophic levels of food webs

Mortality from gastrointestinal congenital anomalies at 264 hospitals in 74 low-income, middle-income, and high-income countries: a multicentre, international, prospective cohort study

Summary Background Congenital anomalies are the fifth leading cause of mortality in children younger than 5 years globally. Many gastrointestinal congenital anomalies are fatal without timely access to neonatal surgical care, but few studies have been done on these conditions in low-income and middle-income countries (LMICs). We compared outcomes of the seven most common gastrointestinal congenital anomalies in low-income, middle-income, and high-income countries globally, and identified factors associated with mortality. Methods We did a multicentre, international prospective cohort study of patients younger than 16 years, presenting to hospital for the first time with oesophageal atresia, congenital diaphragmatic hernia, intestinal atresia, gastroschisis, exomphalos, anorectal malformation, and Hirschsprung’s disease. Recruitment was of consecutive patients for a minimum of 1 month between October, 2018, and April, 2019. We collected data on patient demographics, clinical status, interventions, and outcomes using the REDCap platform. Patients were followed up for 30 days after primary intervention, or 30 days after admission if they did not receive an intervention. The primary outcome was all-cause, in-hospital mortality for all conditions combined and each condition individually, stratified by country income status. We did a complete case analysis. Findings We included 3849 patients with 3975 study conditions (560 with oesophageal atresia, 448 with congenital diaphragmatic hernia, 681 with intestinal atresia, 453 with gastroschisis, 325 with exomphalos, 991 with anorectal malformation, and 517 with Hirschsprung’s disease) from 264 hospitals (89 in high-income countries, 166 in middleincome countries, and nine in low-income countries) in 74 countries. Of the 3849 patients, 2231 (58·0%) were male. Median gestational age at birth was 38 weeks (IQR 36–39) and median bodyweight at presentation was 2·8 kg (2·3–3·3). Mortality among all patients was 37 (39·8%) of 93 in low-income countries, 583 (20·4%) of 2860 in middle-income countries, and 50 (5·6%) of 896 in high-income countries (p<0·0001 between all country income groups). Gastroschisis had the greatest difference in mortality between country income strata (nine [90·0%] of ten in lowincome countries, 97 [31·9%] of 304 in middle-income countries, and two [1·4%] of 139 in high-income countries; p≤0·0001 between all country income groups). Factors significantly associated with higher mortality for all patients combined included country income status (low-income vs high-income countries, risk ratio 2·78 [95% CI 1·88–4·11], p<0·0001; middle-income vs high-income countries, 2·11 [1·59–2·79], p<0·0001), sepsis at presentation (1·20 [1·04–1·40], p=0·016), higher American Society of Anesthesiologists (ASA) score at primary intervention (ASA 4–5 vs ASA 1–2, 1·82 [1·40–2·35], p<0·0001; ASA 3 vs ASA 1–2, 1·58, [1·30–1·92], p<0·0001]), surgical safety checklist not used (1·39 [1·02–1·90], p=0·035), and ventilation or parenteral nutrition unavailable when needed (ventilation 1·96, [1·41–2·71], p=0·0001; parenteral nutrition 1·35, [1·05–1·74], p=0·018). Administration of parenteral nutrition (0·61, [0·47–0·79], p=0·0002) and use of a peripherally inserted central catheter (0·65 [0·50–0·86], p=0·0024) or percutaneous central line (0·69 [0·48–1·00], p=0·049) were associated with lower mortality. Interpretation Unacceptable differences in mortality exist for gastrointestinal congenital anomalies between lowincome, middle-income, and high-income countries. Improving access to quality neonatal surgical care in LMICs will be vital to achieve Sustainable Development Goal 3.2 of ending preventable deaths in neonates and children younger than 5 years by 2030

Evaluation of the complexity and performance of marine planktonic trophic models

ABSTRACT Planktonic models represent a powerful tool for creating hypotheses and making predictions about the functioning of marine ecosystems. Their complexity varies according to the number of state variables and the choice of functional forms. We evaluated plankton models during the last 15 years (n =145) with the aims of understanding why they differ in complexity, evaluating model robustness, and describing studies of plankton modelling around the globe. We classified models into four groups: Nutrient-Phytoplankton-Zooplankton (NPZ), Nutrient-Phytoplankton-Zooplankton-Detritus (NPZD), Size-Structured (SS) and Plankton-Functional-Type (PFT). Our results revealed that the number of state variables varied according to the question being addressed: NPZ models were more frequently applied in physical-biological studies, while PFT models were more applied for investigating biogeochemical cycles. Most models were based on simple functional forms which neglect important feedback related to control of plankton dynamics. Modelling studies sometimes failed to describe sensitivity analysis, calibration and validation. The importance of testing different functional forms was commonly overlooked, and the lack of empirical data affected the verification of model robustness. Lastly, we highlight the need to develop modelling studies in the Southern Hemisphere, including Brazil, in order to provide predictions that assist the management of marine ecosystems

Differences in physiology explain succession of mixoplankton functional types and affect carbon fluxes in temperate seas

Different hypotheses have been proposed explaining plankton community assembly and how changes in biodiversity can impact ecosystem function. Mixoplankton (photo-phago-trophs) are important members of the plankton, but science lacks a clear understanding of their role in plankton succession. Here, we used a modelling approach to evaluate the seasonalities of mixoplankton functional types (MFTs) and to test the hypothesis that functional differences affect their roles in key carbon fluxes. Functional differences were modelled based on cell size and whether mixoplankton possess their own, or acquire, photosystems. Ecosystem simulations incorporated realistic environmental variability and were validated against a 9 yr long-term time series of nutrients, chlorophyll-a, and plankton data from a coastal temperate sea. Simulations, consistent with empirical data, show that mixoplankton of different sizes are present throughout the water column and over time, with seasonal population dynamics differing among the different MFTs. Importantly, the partitioning of production among different size-classes depends on how mixoplankton functional diversity is described in the model, and that merging mixoplankton into one functional type can mask their diverse ecological roles in carbon cycling. Mixoplankton thus play an important role in structuring the plankton community and its dynamics in the simulations