Photosynthesis on a schedule: how can planktonic microalgae physiology remain active after dark?

Planktonic microalgae are major contributors to the planet’s oxygen production and carbon fixation (converting CO₂ into organic matter, photosynthate, through photosynthesis). But they can only collect energy from this process during daylight hours; they need to manage their physiology efficiently over each 24-hour cycle. 

Evidence indicates that microalgae compete best when they can quickly take up resources (nutrients) when available, and store enough to keep their physiology running during lean periods. Carbon is especially tricky in this respect because it can only be fixed during daylight, and light varies with day length (dependent on season and latitude), time of day, cloud cover and depth in the water.  

Computer models of microalgae typically assume that resource uptake and growth happen in lockstep, with microalgae growing actively during the daylight hours and de facto shutting down in darkness. This doesn’t reflect reality. In practice, algae adjust uptake depending on what’s available and what they need, and this varies by nutrient type and environmental conditions. So, when it comes to C-fixation, microalgae ‘over’ photosynthesise when they can do so, and can thus compensate for changing availability of photosynthate during daily changes in light.  

This is evidenced by experiments showing that real microalgae can grow much faster than expected with shorter day lengths, compensating for limited daylight. The explanation is that during light hours, they must fix carbon at a rate higher than their daily average requirement. To deal with this, it appears that microalgae channel surplus daytime carbon into an “intermediate pool” of metabolites, which then fuels core growth processes throughout the 24-hour cycle. However, they can’t simply keep pushing growth rates higher under continuous light – something inside the cell limits how quickly resources can be processed, likely linked to DNA replication and other cell-cycle events.  

To explore this topic, PML’s Professor Kevin Flynn, alongside Dr Andrew Yu Morozov of the University of Leicester, developed a simulation model that includes feedback loops to mimic how real algae regulate uptake and growth, showing how these factors interact under different day–night cycles. The model described a metabolite pool that splits the algae’s acquired carbon into two parts: 

• An intermediate metabolite pool (temporary storage of carbon from photosynthesis) 

• A core structural pool (used for building the main parts of the cell) 

Unlike typical simulation models of microalgae, this new model can describe the results of experiments with real organisms. Analysis of the model shows that its behaviour depends on the organism’s maximum possible growth rate (which sets the required day-average C-fixation rate), the size of the intermediate metabolite pool, and the length of the daily light period.  

Faster-growing algae need to photosynthesise more intensely and need larger storage pools to sustain growth in short day-length conditions. Algae such as diatoms, which are more vacuolated, naturally have more storage space inside their cells, and thus may have a particular advantage in this regard.  

However, results from the study also suggest that while storing extra resources is useful, it also comes with costs – such as having to build and maintain the carbon storage functionalities and risks of leaking the metabolites which may then attract predators. These trade-offs likely help explain why some species thrive better at certain latitudes or seasons, and why climate-driven shifts in the location of a given species, affecting day length and light availability, could help select for which planktonic algae dominate in different regions. In other words, it affects biodiversity. 

Professor Kevin Flynn said: 

“By accounting for both the metabolite pool and the core biomass, we can get a better idea of how plankton really manage resources across the diel cycle – that is the day–night cycle. This step forward in modelling helps us better understand not just their growth, but also their competitive edge in different environments – vital for predicting how primary production underpins food webs and biogeochemical processes across most of our planet.”  

Access the full paper, ‘Resource acquisition in diel cycles and the cost of growing quickly’ on PLOS Computational Biology here >>