Effect of global warming on marine phytoplankton bloom timing: photoperiodism, composition and adaptation – Photo-Phyto

Marine phytoplankton plays essential roles in food webs and biogeochemical cycles contributing to nearly half of the global primary production. Ocean warming is supposed to be the main factor contributing to the recently observed changes in global phytoplankton biomass and productivity, phenology (timing of phytoplankton bloom), and community composition.
In temperate oceanic areas, phytoplankton abundance and biomass increase drastically from winter to spring. These spring blooms contribute substantially to the annual primary production and to maintain the entire marine food web. Seasonal blooms have been reported for ecologically important phytoplanktonic groups including diatoms and picoeukaryotes. According to the Sverdup hypothesis, the spring bloom occurs when, upon temperature rise, water stratification of the mixed layer occurs, providing both nutrients and light necessary for growth. However, several examples suggest that the regulation of the spring bloom onset is more complex. Several years of observations in two Mediterranean coastal ecosystems, the Thau Lagoon, a highly productive ecosystem, and the Banyuls Bay which is often nutrient-limited, revealed quite similar patterns during the spring period with nanophytoplankton and picoeukaryotes blooms in February-March. These blooms are likely to arise from a combination of physical (light, temperature) chemical (nutrients) and ecological drivers (interaction with bacteria, differential grazing).
The life of most organisms is strongly influenced by the relative day to night length (photoperiod) which regulates seasonal processes such as flowering time in plants and reproduction in animals. The circadian clock is an internal timer that regulates seasonality as well as cell growth and division. In summary, on the one hand, temperature is a primary driver influencing the physiology and growth of phytoplanktonic communities but on the other hand day length, is likely to influence the blooming time by (1) controlling the amount of light available for photosynthesis, (2) through photoperiod-dependent circadian regulation of physiology. Circadian clocks have evolved so that the physiology is best adapted to photoperiod and temperature along the year. In the context of global warming, it is of particular interest to determine how phytoplanktonic species and communities react and adapt to a loosened coupling between temperature increase and day lengthening.
The objective of the PHOTO-PHYTO project is to investigate the role and hierarchism of fluctuating environmental factors such as temperature and intrinsic cellular processes like the circadian clock (driving photoperiodism) in triggering the phytoplanktonic spring bloom. In this project we will investigate both direct effects of temperature on phytoplankton physiology and growth and indirect effects of warming (i. e. trough mediated effects on biological interactions as bacteria/phytoplankton and predators/phytoplankton interactions). In addition to investigating the impact of temperature increase on selected species and natural and/or artificial communities, the project will also study the adaptation of a key marine blooming species to warming.

We propose to develop a multidisciplinary approach combining unique expertises in oceanography, microbial ecology, functional genomics and experimental evolution to address the following questions
1) What are the main drivers in situ of spring phytoplanktonic blooms?
2) How do temperature and photoperiod interact to trigger phytoplanktonic blooms?
3) Does adaptation to warming affect photoperiodism and trophic interactions?
4) How does warming affect natural microbial communities?

Furthermore, such knowledge should be useful to optimise microalgal communities in high-intensity raceway open tanks under natural conditions as well as to select O. tauri strains with optimal growth at higher temperature for photobioreactors culture in summer.