Description: Arctic phytoplankton and their response to future conditions shape one of the most rapidly changing ecosystems on the planet. We tested how much the phenotypic responses of strains from an Arctic diatom population diverge and whether the physiology and intraspecific composition of multi-strain populations differ from expectations based on single strain traits. To this end, we conducted incubation experiments with the diatom Thalassiosira hyalina under present-day and future temperature and pCO2 treatments. Six fresh isolates from the same Svalbard population were incubated as mono- and multi-strain cultures. For the first time, we were able to closely follow intraspecific selection within an artificial population using microsatellites and allele-specific quantitative PCR. Our results show not only that there is substantial variation in how strains of the same species cope with the tested environments, but also that changes in genotype composition, production rates and cellular quotas in the multi-strain cultures are not predictable from monoculture performance. Nevertheless, the physiological responses as well as strain composition of the artificial populations were highly reproducible within each environment. Interestingly, we only detected significant strain sorting in those populations exposed to the future treatment. This study illustrates that the genetic composition of populations can change on very short timescales through selection from the intraspecific standing stock, indicating the potential for rapid population level adaptation to climate change. We further show that individuals adjust their phenotype not only in response to their physico-chemical, but also to their biological surroundings. Such intraspecific interactions need to be understood in order to realistically predict ecosystem responses to global change.

Description: Global change will affect multiple physico-chemical parameters of the oceans, amongst them also the abundances of macronutrients like phosphorus and nitrogen that are critical for phytoplankton growth. Here, we assessed the transcriptomic responses to phosphorus (P) depletion in the haploid and diploid life-cycle stage of the coccolithophore Emiliania huxleyi (RCC1217/1216) and compared the results with an existing dataset on nitrogen (N) depletion. The responses to the two depletion scenarios within one particular life-cycle stage were more similar at the transcriptome level than the responses of the two stages toward only one particular depletion scenario, emphasizing the tripartite nature of the coccolithophore genome. When cells senesced in both scenarios, they applied functionally similar programs to shut down cell-cycling, re-adjust biochemical pathways, and increase metabolic turnover to efficiently recycle elements. Those genes that exclusively responded to either P- or N-depletion modulated the general response to enhance scavenging, uptake, and attempted storage of the limiting nutrient. The metabolic adjustments during senescence involved conserved and ancient pathways (e.g., proline oxidation or the glycolytic bypass) that prolong survival on the one hand, but on the other hand give rise to toxic messengers (e.g., reactive oxygen species or methylglyoxal). Continued senescence thus promotes various processes that lead to cell death, which can be delayed only for a limited time. As a consequence, the interplay of the involved processes determines how long cells can endure severe nutrient depletion before they lyse and provide their constituent nutrients to the more viable competitors in their environment. These responses to nutrient depletion are observable in other phytoplankton, but it appears that E. huxleyi's outstanding endurance under nutrient deficiency is due to its versatile high-affinity uptake systems and an efficient, NAD-independent malate oxidation that is absent from most other taxa.

Description: - There is increasing evidence that different light intensities strongly modulate the effects of ocean acidification (OA) on marine phytoplankton. The aim of the present study was to investigate interactive effects of OA and dynamic light, mimicking natural mixing regimes. - The Antarctic diatom Chaetoceros debilis was grown under two pCO2 (390 and 1000 latm) and light conditions (constant and dynamic), the latter yielding the same integrated irradiance over the day. To characterize interactive effects between treatments, growth, elemental composition, primary production and photophysiology were investigated. - Dynamic light reduced growth and strongly altered the effects of OA on primary production, being unaffected by elevated pCO2 under constant light, yet significantly reduced under dynamic light. Interactive effects between OA and light were also observed for Chl production and particulate organic carbon quotas. - Response patterns can be explained by changes in the cellular energetic balance. While the energy transfer efficiency from photochemistry to biomass production (Φe,C) was not affected by OA under constant light, it was drastically reduced under dynamic light. Contrasting responses under different light conditions need to be considered when making predictions regarding a more stratified and acidified future ocean.