Abstract: Introduction: Evidence suggests that baseline 18fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography-derived parameters, such as metabolic tumor volume (MTV) and maximum standardized uptake value (SUVmax), may predict progression-free survival (PFS) in patients (pts) with follicular lymphoma (FL) treated with first-line R-CHOP immunochemotherapy. However, data from pts routinely treated with bendamustine or antibody maintenance are lacking, and several methods have been used to evaluate PET metrics for tumor burden in pts with lymphoma. This prospective exploratory analysis assessed the prognostic value of baseline MTV, total glycolytic activity (TLG), and SUVmax for PFS and overall survival (OS) in pts with FL treated with first-line obinutuzumab (GA101; G) or rituximab (R) plus chemotherapy (chemo) in the Phase III GALLIUM study (NCT01332968; Marcus et al. N Engl J Med 2017), using two published methods for measuring tumor burden. Methods: Pts ≥18 years with previously untreated FL (grade 1-3a) and advanced disease (Stage III/IV or Stage II with tumor diameter ≥7cm) requiring treatment were randomized 1:1 to receive 6-8 cycles of G (1000mg intravenous [IV] on days [D] 1, 8, and 15 of cycle [C] 1 and D1, C2-6 or 8) or R (375mg/m2 IV on D1) plus standard chemo (CHOP, bendamustine, or CVP). Responding pts received the same antibody as maintenance every 2 months for up to 2 years. After an early protocol amendment, PET imaging at baseline and end of induction (EOI) was mandatory in the first 170 pts and optional thereafter. Independent reviewers segmented FDG-avid tumors applying thresholds of I) standardized uptake value (SUV)max ≥2.5, and II) SUVmax ≥41% of lesional maximum SUV and a minimum volume of 1mL, using MIM software. Results were analyzed for the PET intent-to-treat population. MTV, TLG, and SUVmax were split into quartiles: Q1, 〈 25%; Q2, 25-49%; Q3, 50-74%; and Q4, 75-100%, based on their distribution in the available population. Investigator-assessed PFS and OS were estimated using Kaplan-Meier methods (data cut-off, February 12, 2018). Hazard ratios refer to stratified log-rank tests comparing Q2, Q3, and Q4 with Q1, adjusted for the randomization stratification factors FLIPI score and chemo regimen. Multivariable Cox analyses were also undertaken to investigate whether baseline MTV quartiles and other covariates were prognostic for PFS. Statistical significance at 0.05 was determined using the Wald test. Results: Of 1202 enrolled FL pts, 609 had a baseline PET scan and 521 had baseline PET scans available for all quantitative assessments by central review; 303 pts (58%) received bendamustine, 179 (34%) CHOP, and 39 (8%) CVP. After a median follow-up of 57 months, none of the 3 baseline PET parameters (MTV or TLG measured by either method or SUVmax) significantly predicted PFS (Table). Multivariable analysis, which included baseline pt and disease characteristics, confirmed that MTV did not predict PFS. Consistent with the primary analysis, receipt of G-chemo was an independent predictor of improved PFS. Conclusions: Contrary to previous reports, these prospective data from the Phase III GALLIUM study show that baseline quantitative PET metrics do not predict PFS or OS in FL pts receiving first-line immunochemotherapy (of whom the majority received bendamustine) followed by antibody maintenance treatment, irrespective of the measurement method applied. Conversely, a previously reported analysis from GALLIUM suggests that PET-complete metabolic response at EOI assessed using Lugano 2014 criteria is a strong predictor of long-term outcome in these pts. Table. Table. Disclosures Barrington: EPSRC: Research Funding; Department of Health (England): Research Funding; F.Hoffmann-La Roche: Membership on an entity's Board of Directors or advisory committees; National Institute of Health Research: Research Funding; MRC: Research Funding; CRUK: Research Funding. Trotman:Janssen: Other: Unremunerated member of Ad Board, Research Funding; Celgene: Other: Unremunerated member of Ad Board, Research Funding; PCYC: Research Funding; Takeda: Other: Unremunerated member of Ad Board; F. Hoffman-La Roche: Other: Travel to meeting, Unremunerated member of Ad Board, Research Funding; Beigene: Research Funding. Sahin:Roche: Employment, Equity Ownership. Belada:Janssen-Cilag: Consultancy, Research Funding; Gilead Sciences: Consultancy, Research Funding; Seattle Genetics: Consultancy, Research Funding; Celgene: Research Funding; Roche: Consultancy, Research Funding; Takeda: Consultancy, Research Funding. Davies:Janssen: Consultancy, Honoraria; GSK: Research Funding; Karyopharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees; ADC Therapeutics: Research Funding; Acerta Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; F. Hoffman-La Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Kite: Consultancy; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Honoraria, Research Funding; Pfizer: Research Funding. MacEwan:Consultant Radiologist/ Nuc Med Physician: Consultancy. Owen:Pharmacyclics: Research Funding; F. Hoffmann-La Roche Ltd: Honoraria, Research Funding; Celgene: Research Funding; AstraZeneca: Honoraria, Research Funding; Merck: Honoraria; Janssen: Honoraria, Research Funding; Teva: Honoraria; AbbVie: Research Funding. Ptáčník:F. Hoffman-La Roche: Honoraria. Hiddemann:F. Hoffman-La Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Consultancy, Research Funding. Marcus:Roche: Consultancy, Other: Travel support and lecture fees ; Gilead: Consultancy; F. Hoffman-La Roche: Other: Travel support and lecture fees. Nielsen:F. Hoffmann-La Roche Ltd: Employment, Other: Ownership interests PLC. Mattiello:Roche: Employment. Zeuner:F. Hoffman-La Roche: Employment, Equity Ownership. Meignan:F. Hoffman-La Roche Ltd: Honoraria.

Abstract: Mixotrophy usually is considered with respect to the advantages gained and the associated trade‐offs of this form of nutrition, compared to specialized competitors, strict photoautotrophs and heterotrophs. However, we currently have an incomplete understanding of the functional diversity of mixotrophs and the factors controlling niche differentiation in different mixotrophic species. Here we experimentally studied the light‐dependent niche differentiation in two chrysophyte species. We show that the newly isolated Ochromonas sp. is an obligate phototroph and possibly an obligate mixotroph. In contrast, Poterioochromonas malhamensis is a facultative mixotroph; photosynthesis and heterotrophy in this species represent substitutable routes of resource acquisition. We further hypothesize that the variable plasticity in the considered traits of the here tested species may result in different niche differentiation with regard to a vertical light gradient. Ochromonas sp. should perform better in stable stratified surface water layers, where light is available, but prey abundances might be low. However, P . malhamensis should be able to also successfully grow in deeper water layers, benefiting from higher bacterial production. This study represents a first step towards understanding competition between mixotrophs engaging in different physiological strategies, and consequently their potential co‐occurrence due to niche differentiation.

Abstract: Recent observational studies form oligotrophic waters provide ample evidence that mixotrophic flagellates often account for the bulk of bacterivory. However, we lack a general framework that allows a mechanistic understanding of success of mixotrophs in the competition with heterotrophic bacterivores. This is especially needed for integrating mixotrophy in models of the microbial loop. Based on general tradeoffs linked to the combined resource use in mixotrophs (generalist versus specialist), we propose a concept where mixotrophs are favored by conditions of high light – low losses, corresponding to the situation found in the surface waters of oligotrophic oceans. Under such conditions, they can achieve positive net growth at very low resource levels, allowing simultaneous competition with specialized protists. Conversely, heterotrophic bacterivores and photoautotrophs should be especially favored in more productive and low‐light conditions. We show experimentally that the combined effect of light and loss rates (dilution) predicts the success of mixotrophic bacterivorous flagellates. Moreover, our results suggest that total bacterivory, contrary as seen in the traditional microbial loop concept, has a more intricate coupling to light.

Abstract: The importance of mixotrophic algae as key bacterivores in microbial food webs is increasingly acknowledged, but their effects on the next trophic level remain poorly understood. Their high stoichiometric food quality is contrasted by anti‐grazing strategies. We tested the quality of freshwater mixotrophs as prey for zooplankton, using four non‐colonial chrysophyte species and a cryptophyte as a high quality reference food. We (1) analyzed the stoichiometric and biochemical (fatty acid) composition of the mixotrophs, and (2) quantified their dietary effects on Daphnia longispina survival. Survival of D . longispina significantly depended on the identity of species provided as food, ranging from higher to lower as compared to starvation. This was not reflected in differences in cellular stoichiometry or fatty acid profiles of the mixotrophs. We suggest that toxicity may be the driver for the observed differences. Generalization of the dietary effects of mixotrophic chrysophytes does not appear straightforward. Besides fundamental species‐specific differences, potential toxic effects may vary depending on environmental cues or physiological strategies. Notably in our study, Ochromonas tuberculata , a species previously reported to be deleterious, turned out to be a beneficial food source in terms of enabling high survival of D . longispina . We challenge the generality of the assumption that chrysophytes are of low value as food for zooplankton. We recommend that future studies test how environmental conditions and physiological strategies shape the quality of mixotrophs as food for consumers at higher trophic levels, specifically focusing on effects of dietary toxicity.

Abstract: Mixotrophic and heterotrophic protists hold a key position in aquatic microbial food webs. Whereas they can account for the bulk of bacterivory in pelagic systems, the potential structuring effect of these consumers on bacterial communities is far from clear. We conducted short-term grazing experiments to test for the overall impact on bacterial community structure and possible prey preferences of phagotrophic protists. The protist taxa selected for this study include three mixotrophic flagellates, comprising two obligate- and one facultative mixotroph, and one phagoheterotrophic flagellate lacking phototrophic capacity. Bacterioplankton from seven different lakes were enriched and used to represent semi-natural prey communities. Our study demonstrated protist strain specific impacts on bacterial community composition linked to grazing. The three mixotrophs had variable impacts on bacterial communities where the two obligate mixotrophs exhibited lower grazing rates, while showing a tendency to promote higher bacterial diversity. The phagoheterotroph displayed the highest grazing rates and structured the bacterial communities via apparent selective grazing. Consistent selectivity trends were observed throughout the experiments, such as the apparent avoidance of all flagellates of Actinobacteria , and high grazing on dominant Burkholderiales taxa. However, there was no consistent “fingerprint” of mixotrophic grazing on prey communities, but the structuring impact rather seemed to depend on the trophic mode of the individual protist taxa, i.e. their dependence on phototrophy vs. phagotrophy. Our findings highlight the differential structuring impact of protist taxa on bacterial communities which may have important ecological implications, for example during periodic dominance of obligate mixotrophic bacterivores in changing lake ecosystems.

Abstract: Biodiversity losses in marine, terrestrial, and freshwater ecosystems have raised concerns about the maintenance of sustainable ecosystem functions and services (“biodiversity crisis”). A positive diversity—productivity relationship has previously been supported by theoretical models, and by laboratory and field experiments in a variety of ecosystems including unicellular microbial communities. Here we show an increasing biomass yield of aquatic primary producers at the ecosystem scale, paralleled by a long‐term positive biodiversity change, which contrasts with the trend of global biodiversity loss. The implied direct long‐term biodiversity effect on ecosystem functioning was an increase of phytoplankton biomass per unit limiting nutrient by a factor of 1.2 to 1.4. Changes in diversity of microorganisms may have immediate implications for essential ecosystem processes like productivity and biomass yield. Diversity‐driven enhancement of resource use in primary production can lead to increased food web yields, but they also can cause a stoichiometric mismatch between autotrophs and primary consumers. Unveiling the functional roles of planktonic biodiversity therefore has essential implications both for global change and for harvestable marine resources.