Temperature effects on the fatty acid (FA) profiles of phytoplankton, major primary producers in the ocean, have been widely studied due to their importance as industrial feedstocks and to their indispensable role as global producers of long-chain, polyunsaturated FA (PUFA), including omega-3 (omega3) FA required by organisms at higher trophic levels. The latter is of global ecological concern for marine food webs, as some evidence suggests an ongoing decline in global marine-derived omega3 FA due to both a global decline in phytoplankton abundance and to a physiological reduction in omega3 production by phytoplankton as temperatures rise. Here, we examined both short-term (physiological) and long-term (evolutionary) responses of FA profiles to temperature by comparing FA thermal reaction norms of the marine diatom Thalassiosira pseudonana after ~500 generations (ca. 2.5 years) of experimental evolution at low (16°C) and high (31°C) temperatures. We showed that thermal reaction norms for some key FA classes evolved rapidly in response to temperature selection, often in ways contrary to our predictions based on prior research. Notably, 31°C-selected populations showed higher PUFA percentages (including omega3 FA) than 16°C-selected populations at the highest assay temperature (31°C, above T. pseudonanas optimum temperature for population growth), suggesting that high-temperature selection led to an evolved ability to sustain high PUFA production at high temperatures. Rapid evolution may therefore mitigate some of the decline in global phytoplankton-derived omega3 FA production predicted by recent studies. Beyond its implications for marine food webs, knowledge of the effects of temperature on fatty acid profiles is of fundamental importance to our understanding of the mechanistic causes and consequences of thermal adaptation. 1-FattyAcids_total.measured_molar_05182-FattyAcids_mol.biovolume_raw_05183-FattyAcids_percentages_raw_11184-FattyAcids_MCL_WUnSat_raw_0518 Copyright: CC0 1.0 Universal (CC0 1.0) Public Domain Dedication

A recent theory of the vertical distribution of phytoplankton considers how interacting niche construction processes such as resource depletion, behavior, and population dynamics contribute to spatial heterogeneity in the aquatic environment. In poorly mixed water columns with opposing resource gradients of nutrients and light, theory predicts that a species should aggregate at a single depth. This depth of aggregation, or biomass maximum, should change through time due to depletion of available resources. In addition, the depth of the aggregation should be deeper under low amounts of nutrient loading and shallower under higher amounts of nutrient loading. Theory predicts total biomass to exhibit a saturating relationship with nutrient supply. A surface biomass maximum limited by light and a deep biomass maximum limited by nutrients or co-limited by nutrients and light is also predicted by theory. To test this theory, we used a motile phytoplankton species (Chlamydomonas reinhardtii) growing in cylindrical plankton towers. In our experiment, the resource environment was strongly modified by the movement, self-shading, nutrient uptake, and growth of the phytoplankton. Supporting predictions, we routinely observed a single biomass maximum at the surface throughout the course of the experiment and at equilibrium under higher nutrient loading. However, at equilibrium, low nutrient loading led to a non-distinct biomass maximum with the population distributed over most of the water column instead of the distinct subsurface peak predicted by theory. Also supporting predictions, total biomass across water columns was positively related to nutrient supply but saturating at high nutrient supply conditions. Further supporting predictions, we also found evidence of light limitation for a surface biomass maximum and nutrient limitation for the deep biomass when no surface maximum was present. In addition, the light level leaving the bottom of the water column declined through time as the phytoplankton grew and was negatively related to nutrient loading. Nutrients were strongly depleted where biomass was present by the end of the experiment. This experimental study shows that the vertical distribution of phytoplankton may be driven by intraspecific resource competition in space.

Temperature effects on the fatty acid (FA) profiles of phytoplankton, major primary producers in the ocean, have been widely studied due to their importance as industrial feedstocks and to their indispensable role as global producers of long-chain, polyunsaturated FA (PUFA), including omega-3 (omega 3) FA required by organisms at higher trophic levels. The latter is of global ecological concern for marine food webs, as some evidence suggests an ongoing decline in global marine-derived omega 3 FA due to both a global decline in phytoplankton abundance and to a physiological reduction in omega 3 production by phytoplankton as temperatures rise. Here, we examined both short-term (physiological) and long-term (evolutionary) responses of FA profiles to temperature by comparing FA thermal reaction norms of the marine diatom Thalassiosira pseudonana after similar to 500 generations (ca. 2.5 years) of experimental evolution at low (16 degrees C) and high (31 degrees C) temperatures. We showed that thermal reaction norms for some key FA classes evolved rapidly in response to temperature selection, often in ways contrary to our predictions based on prior research. Notably, 31 degrees C-selected populations showed higher PUFA percentages (including omega 3 FA) than 16 degrees C-selected populations at the highest assay temperature (31 degrees C, above T. pseudonana's optimum temperature for population growth), suggesting that high-temperature selection led to an evolved ability to sustain high PUFA production at high temperatures. Rapid evolution may therefore mitigate some of the decline in global phytoplankton-derived omega 3 FA production predicted by recent studies. Beyond its implications for marine food webs, knowledge of the effects of temperature on fatty acid profiles is of fundamental importance to our understanding of the mechanistic causes and consequences of thermal adaptation.

Ongoing climate change is shifting species distributions and increasing extinction risks globally. It is generally thought that large population sizes and short generation times of marine phytoplankton may allow them to adapt rapidly to global change, including warming, thus limiting losses of biodiversity and ecosystem function. Here, we show that a marine diatom survives high, previously lethal, temperatures after adapting to above-optimal temperatures under nitrogen (N)-replete conditions. N limitation, however, precludes thermal adaptation, leaving the diatom vulnerable to high temperatures. A trade-off between high-temperature tolerance and increased N requirements may explain why N limitation inhibited adaptation. Because oceanic N limitation is common and likely to intensify in the future, the assumption that phytoplankton will readily adapt to rising temperatures may need to be reevaluated.

C:N ratios of the two 34C‐tolerant populations of Chaetoceros simplex and control and ancestral populations, at different temperatures. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/778926 Copyright: https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0

Daily growth rates of 8 populations of Chaetoceros simplex grown at 31C and control population at 25C, in regular L1 medium (884 mum NO3) or nitrogen‐reduced L1 medium (5 mum NO3). For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/778869 Copyright: https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0

Daily growth rates for Thermal Performance Curve (TPC) of Chaetoceros simplex in nitrogen-replete evolved populations after about 200 generations of evolution at eight temperatures, 10-35 degrees C. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/778779 Copyright: https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0