Marine phytoplankton are a taxonomically and functionally diverse group of organisms that are key players in the most important biogeochemical cycles. Phytoplankton taxa show different resource utilization strategies (e.g. nutrient-uptake rates and cellular allocation) and traits. Therefore, acknowledging this diversity is crucial to understanding how elemental cycles operate, including the origin and dynamics of elemental ratios. In this paper, we focus on trait-based models as tools to study the role of phytoplankton diversity in the stoichiometric phenomenology observed in the laboratory and in the open ocean. We offer a compilation of known empirical results on stoichiometry and summarize how trait-based approaches have attempted to replicate these results. By contrasting the different ecological and evolutionary approaches available in the literature, we explore the strengths and limitations of the existing models. We thus try to identify existing gaps and challenges, and point to potential new directions that can be explored to fill these gaps. We aim to highlight the potential of including diversity explicitly in our modeling approaches, which can help us gain important knowledge about changes in local and global stoichiometric patterns.

The theories developed in ecological stoichiometry (ES) are fundamentally based on traits. Traits directly linked to cell/body stoichiometry, such as nutrient uptake and storage, as well as the associated trade-offs, have the potential to shape ecological interactions such as competition and predation within ecosystems. Further, traits that indirectly influence and are influenced by nutritional requirements, such as cell/body size and growth rate, are tightly linked to organismal stoichiometry. Despite their physiological and ecological relevance, traits are rarely explicitly integrated in the framework of ES and, currently, the major challenge is to more closely inter-connect ES with trait-based ecology (TBE). Here, we highlight four interconnected nutrient trait groups, i.e., acquisition, body stoichiometry, storage, and excretion, which alter interspecific competition in autotrophs and heterotrophs. We also identify key differences between producer-consumer interactions in aquatic and terrestrial ecosystems. For instance, our synthesis shows that, in contrast to aquatic ecosystems, traits directly influencing herbivore stoichiometry in forested ecosystems should play only a minor role in the cycling of nutrients. We furthermore describe how linking ES and TBE can help predict the ecosystem consequences of global change. The concepts we highlight here allow us to predict that increasing N:P ratios in ecosystems should shift trait dominances in communities toward species with higher optimal N:P ratios and higher P uptake affinity, while decreasing N retention and increasing P storage.

Temperature strongly affects phytoplankton growth rates, but its effect on communities and ecosystem processes is debated. Because phytoplankton are often limited by light, temperature should change community structure if it affects the traits that determine competition for light. Furthermore, the aggregate response of phytoplankton communities to temperature will depend on how changes in community structure scale up to bulk rates. Here, we synthesize experiments on 57 phytoplankton species to analyze how the growth-irradiance relationship changes with temperature. We find that light-limited growth, light-saturated growth, and the optimal irradiance for growth are all highly sensitive to temperature. Within a species, these traits are co-adapted to similar temperature optima, but light-limitation reduces a species' temperature optimum by similar to 5 degrees C, which may be an adaptation to how light and temperature covary with depth or reflect underlying physiological correlations. Importantly, the maximum achievable growth rate increases with temperature under light saturation, but not under strong light limitation. This implies that light limitation diminishes the temperature sensitivity of bulk phytoplankton growth, even though community structure will be temperature-sensitive. Using a database of primary production incubations, we show that this prediction is consistent with estimates of bulk phytoplankton growth across gradients of temperature and irradiance in the ocean. These results indicate that interactions between temperature and resource limitation will be fundamental for explaining how phytoplankton communities and biogeochemical processes vary across temperature gradients and respond to global change.

Dolichospermum flos-aquae and Cylindrospermopsis raciborskii are two cyanobacteria species which cause harmful blooms around the world. Both these species share the capacity to fix atmospheric nitrogen in heterocytes (cell where fixation occurs). While Dolichospermum can express heterocytes at rather regular intervals across the filament, Cylindrospermopsis can only express heterocytes at the end of the filament. The aim of this study was to experimentally assess the role of heterocyte position in the eco-physiological responses of these bloom forming cyanobacteria. Replicated monocultures of each species were grown at different eutrophication scenarios (limiting and sufficient nitrogen and phosphorus concentrations, in factorial design). Dolichospermum reached high biomass regardless of the nitrogen (and phosphorus) provided, suggesting that this species could bloom in situations with and without nitrogen limitation. In contrast, Cylindrospermopsis reached high biomass only when nitrogen supply was high; its biomass was 15-20 times lower when relying on nitrogen fixation. Hence, despite its ability to fix nitrogen, blooms of Cylindrospermopsis would be expected only under high total nitrogen availability. In Dolichospermum heterocytes occurred only in the scenarios without supplied nitrogen while in Cylindrospermopsis heterocytes occurred regardless of nitrogen availability. Yet, in both species nitrogen fixation occurred (heterocytes were functional) only when nitrogen was limiting, and nitrogen fixation increased significantly at higher phosphorus concentration. Finally, in the absence of supplied nitrogen, filament length in Dolichospermum was the longest, while filaments in Cylindrospermopsis were the shortest (up to 13 times shorter than at nitrogen sufficiency). Therefore, heterocyte expression in Dolichospermum, and filament length in Cylindrospermopsis seem good proxies of nitrogen fixation. The eco-physiological responses recorded here help understand the distribution of these species along nutrient gradients in nature. (C) 2016 Elsevier B.V. All rights reserved.

"It takes a village to finish (marine) science these days" Paraphrased from Curtis Huttenhower (the Human Microbiome project) The rapidity and complexity of climate change and its potential effects on ocean biota are challenging how ocean scientists conduct research. One way in which we can begin to better tackle these challenges is to conduct community-wide scientific studies. This study provides physiological datasets fundamental to understanding functional responses of phytoplankton growth rates to temperature. While physiological experiments are not new, our experiments were conducted in many laboratories using agreed upon protocols and 25 strains of eukaryotic and prokaryotic phytoplankton isolated across a wide range of marine environments from polar to tropical, and from nearshore waters to the open ocean. This community-wide approach provides both comprehensive and internally consistent datasets produced over considerably shorter time scales than conventional individual and often uncoordinated lab efforts. Such datasets can be used to parameterise global ocean model projections of environmental change and to provide initial insights into the magnitude of regional biogeographic change in ocean biota in the coming decades. Here, we compare our datasets with a compilation of literature data on phytoplankton growth responses to temperature. A comparison with prior published data suggests that the optimal temperatures of individual species and, to a lesser degree, thermal niches were similar across studies. However, a comparison of the maximum growth rate across studies revealed significant departures between this and previously collected datasets, which may be due to differences in the cultured isolates, temporal changes in the clonal isolates in cultures, and/or differences in culture conditions. Such methodological differences mean that using particular trait measurements from the prior literature might introduce unknown errors and bias into modelling projections. Using our community-wide approach we can reduce such protocol-driven variability in culture studies, and can begin to address more complex issues such as the effect of multiple environmental drivers on ocean biota.

File List EPAdata.txt Description The EPAdata.txt file is a tab-delimited ASCII file. The file contains data on phytoplankton species richness and environmental variables of 540 lakes and reservoirs distributed across the continental USA. The lakes and reservoirs were sampled from 1973-1975 as part of the National Eutrophication Survey conducted by the U.S. EPA. Column definitions and checksum values: Name of waterbody State Type of waterbody (lake or reservoir) Latitude, SUM = 20585.85475 Longitude, SUM = -51798.3929 Altitude (in km), SUM = 304.5463 Lake area (in km2), SUM = 25453.51 Lake depth (in m), SUM = 4945.149999 Number of sampling days, SUM = 1592 Chlorophyll concentration (in mug/L), SUM = 11718.22239 Total nitrogen (in mg/L), SUM = 705.6529752 Total phosphorus (in mg/L), SUM = 70.3631407 Temperature (in °C), SUM = 10188.35764 Secchi depth (in m), SUM = 885.1552689 Species richness (number of phytoplankton species), SUM = 15875 There are no missing values. Copyright: CC-0

Supplementary statistical methods. Copyright: CC-0

Supplemental figures showing environmental covariation and species-specific responses to environmental predictors. Copyright: CC-0

Coefficient estimates for the 25-species occurrence model, the 13-species abundance model, and the 13-species occurrence model. Copyright: CC-0

File List NutrientTraits.zip (MD5: a65e300844868266cb224be40443174e) Table1.csv (MD5: 5028869fa21cba2757c9b93bb6c4cd17) Table2.csv (MD5: 0f3dfa04fb46fa6bd69e146f4c636c3a) Table3.csv (MD5: e2913589bfda17bb35fb85fb612b68dc) Description This paper presents a compilation of nutrient utilization traits of marine and freshwater phytoplankton. The literature was comprehensively searched for culture experiments using nitrate, ammonium, or phosphate as the limiting nutrient. The following traits were extracted: the response of growth to nutrient supply (maximum growth rate under unlimited nutrient supply and the nutrient concentration at which growth is half-saturated); the response of internal nutrient content to nutrient supply (the minimum subsistence quota at which growth ceases and the maximum nutrient quota under unlimited nutrient supply); and nutrient uptake kinetics for nutrient-limited cells (maximum uptake rate and the nutrient concentration at which uptake is half-saturated). The resulting data set includes 1319 measurements on 129 species from 138 publications. Potential uses of these data include studies of community structure and trait evolution, parameterization of ecosystem models, and biofuel development. Key words: algae; allometry; competition; Droop; Monod; nitrogen; phosphorus; physiology; stoichiometry; uptake kinetics. Copyright: CC-0