The global increase in atmospheric CO2 concentration and the subsquent temperature rise has dramatically altered the marine ecosystem, including planktonic microorganisms. Two new studies describe the long-term shifts in plankton community composition in response to climate change. Corals incorporate essential amino acids, which they cannot synthesize themselves but have to take up from primary producers, into their proteinaceous skeleton. McMahon et al. used carbon isotope fingerprinting of these amino acids from long-lived Hawaiian gold corals to investigate shifts in plankton communities over the past 1,000 years. In general, photoautotrophic cyanobacteria were the main producers of organic matter. However, different climate periods were characterized by substantial shifts in primary producers. The Medieval climate anomaly, a period of warm sea surface temperatures and weak winds, was dominated by non-nitrogen-fixing cyanobacteria, probably in response to nutrient limitation. This was followed by the Little Ice Age, when the supply of inorganic nitrate increased and eukaryotic microalgae provided almost half of the carbon sources. In the past 150 years, an unprecedented shift in the plankton regime was observed. Community composition exhibited a rapid increase in nitrogen-fixing cyanobacteria, which might represent a negative feedback to rising CO2 levels. Rivero-Calle et al. also investigated changes in plankton composition during the recent phase of increasing CO2 and temperatures using Continuous Plankton Recorder surveys, a sampling network based on shipping routes in the North Atlantic. In the 1960s, ∼1% of samples contained coccolithophores, which are unicellular calcifying algae, whereas >20% of samples from the 2000s contained these organisms. Statistical models indicate that CO2 concentrations are the best predictors of this change, a result that corresponds with laboratory data from previous publications on coccolithophore growth under different CO2 levels.