The global explosion of eukaryotic algae in Cryogenian Oceans: the potential role of phosphorus?

Approximately 650 million years ago there arose one of the most important ecological transitions in Earth’s history, seen as a rapid shift from a predominantly bacterial dominated world to a more complex system governed by eukaryotic primary productivity. The resulting ‘Rise of the algae’, led to dramatically altered food webs that were much more efficient in terms of nutrient and energy transfer. This, in turn, allowed for larger and more complex organisms to evolve. The major question is, what caused this ecological shift? As cyanobacteria are known to outcompete eukaryotes under phosphate-deplete conditions, phosphorus (P) may have been a key factor limiting algal growth in pre-Cryogenian oceans. In this study we examined the theory that it was the alleviation of P deficiency which gave eukaryotic algae the prime opportunity to flourish. We performed laboratory experiments on cyanobacteria and eukaryotic algae and examined their ability to compete for P. The strains were cultured in mono- and mixed cultures in chemostats across a range of P conditions to reflect modern and ancient oceanic conditions. Measurements of cell densities, biomass, chlorophyll, intracellular and extracellular phosphorus concentrations, alkaline phosphatase activity and carbon and nitrogen were taken. We also performed short term uptake experiments. Our results show that cyanobacteria were able to outcompete the algae at low (0.2 μM) concentrations, yet the eukaryotic algae were not completely excluded and remained a constant yet low background component. As P concentrations increased towards modern-day levels (2 μM), the eukaryotic algae were able to effectively compete against the cyanobacteria in terms of total biomass. The results suggest that competitive outcome of cyanobacteria and eukaryotic algae are heavily influenced by phosphorus concentrations and that the availability of phosphorus could have been one of the factors influencing the global expansion of eukaryotic algae, but not the apparent complete absence of eukaryotic algae in the marine ecosystem more than 650 million years ago.