Long Term Adaptation of a Marine Chlamydomonas sp. to Phosphorus Limitation

Increased vertical stability and reduced mixing caused by climate warming are expected to decrease nutrient supply to phytoplankton in the sea surface and make primary production more nutrient-limited. Short-term physiological acclimation will likely offset some of the impact of increased nutrient stress, but long term adaptive responses of phytoplankton to nutrient limitation are not well known. Here we report the results of a chemostat experiment in which a marine species of Chlamydomonas was maintained for more than 180 generations under phosphorus-limiting conditions to assess how growth and elemental stoichiometry were affected. Selected cell lines grew significantly faster in phosphorus-deplete seawater, but showed reduced fitness under phosphorus-replete conditions compared to the ancestral culture. All isolates derived from the chemostat replicates had higher biomass to phosphorus ratios during exponential growth suggesting that cellular requirements for phosphorus decreased in response to prolonged phosphorus deficiency. Our results show that adaptation may enable phytoplankton to sustain fast rates of biomass production in spite of reduced nutrient supply, partially offsetting the impact of increased nutrient limitation on productivity of the future ocean.