Resumen de Response of the coccolithophore emiliania huxleyi to future ocean acidification conditions: from gene expression to physiological rates
After the Industrial Revolution, the CO2 produced by human population and emitted to the atmosphere has increased exponentially. Around a quarter of this anthropogenic CO2 is sequestered by the ocean, driving the process known as ocean acidification (OA). OA impacts the calcifying marine phytoplankton group of coccolithophores, which are amongst the most important marine primary producers and contribute significantly to the oceanic calcium carbonate precipitation. Since their importance at biogeochemical level, a lot of studies have assessed their response to OA using as model species Emiliania huxleyi, the most abundant coccolithophore species of the present ocean. These studies indicate that while decreases in calcification seem to be a general response under future OA conditions, photosynthetic rates show more variable responses. This variability has been attributed to culture conditions, inter- and intra-species variability and adaptive processes of clones maintained for a long time in the laboratory, though, until now any physiological mechanism has been described to understand this variability of responses.
A better understanding of the responses of marine phytoplankton to OA conditions has been achieved thanks to the introduction of molecular techniques, indicating a generalized down-regulation of the carbon concentrating mechanisms (CCMs) of phytoplankton under OA conditions. Other studies also reported a broader cell impact to OA, not only constrained to CCMs. These metabolic rearrangements under future OA conditions have been explained in diatoms and chlorophytes by conceptual models such as the one from Sobrino et al. (2014). The model proposes that under acclimated future OA conditions, phytoplankton down-regulate their CCMs promoting a cascading cellular response that triggers a decrease of cellular energy-consuming pathways finally translating into an optimization of growth rate. Based on this, in this thesis we hypothesized that similar to other taxonomic groups of phytoplankton, in future OA conditions, coccolithophores will operate with a down-regulated cellular metabolism. In order to test this hypothesis, the response of E. huxleyi metabolic pathways to future OA conditions from gene expression to physiological rates was studied.
The results of this thesis demonstrated that the metabolic responses of coccolithophorids under OA conditions can also be explained by the conceptual model of Sobrino et al. (2014), similar to other taxonomic groups of marine phytoplankton. The results show that during acclimated OA conditions E. huxleyi carbon metabolism down-regulates. This down-regulation ultimately increases cell efficiency, translating into an increase in biomass. On the contrary, in this thesis it is also shown that when E. huxleyi cells acclimated to OA conditions are disturbed, carbon metabolism is up-regulated, negatively impacting total biomass. However, the calcification response of this species always decreases regardless of metabolic conditions, this being reflected in cellular carbon allocation. This down- and up-regulation of metabolism as a function of CCM activity contradicts the commonly assumed "fertilizing effect" of CO2, and demonstrates the relevance of considering CCM activity to predict the response of phytoplankton to future climate conditions at biogeochemical scale. Moreover, this thesis also demonstrates that the cellular response to elevated CO2 implies a broad reprogramming of the cell, even including cell signaling, transcription and genes assumed to be invariable, such as the actin gene. The variability of actin gene expression observed in this thesis conducted in E. huxleyi under OA conditions is also important from a methodological perspective since this gene is widely used as a reference gene in phytoplankton studies under OA conditions evaluating gene expression quantification using relative methods.
