Background
The authors write that "most of the laboratory studies investigating the effects of climate stressors on phytoplankton have been performed on single strains," and that "the significant effects often found in such experiments are contrasted by the general lack of clear responses in natural populations," citing Engel et al. (2008). Thus, they speculate that the "contradictory responses to changed climate conditions sometimes observed within the same species might be partly attributable to strain variability between or within populations," citing Langer et al. (2009), which observations emphasize "the need to consider variability in studies aiming to understand the effects of climate change on phytoplankton species."

What was done
Kremp et al. studied "the effects of increased temperature and CO2 availability, as predicted consequences of global change, on 16 genetically different isolates of the diatom Skeletonema marinoi from the Adriatic Sea and the Skagerrak (North Sea), and on eight strains of the PST (paralytic shellfish toxin)-producing dinoflagellate Alexandrium ostenfeldii from the Baltic Sea." This they did by assessing maximum growth rates of acclimated isolates grown in batch cultures for five to ten generations in a factorial design at 20 and 24°C, and present-day and next-century atmospheric CO2 concentrations (385 and 750 ppm), respectively.

What was learned
The seven scientists found that strains of one species, and even of a single population, "can be impacted in very different ways by climate stressors," noting that "a particularly wide response range was found in the population of S. marinoi from the NW Adriatic sea, where temperature and CO2 caused positive, negative or no effect at all."

What it means
Kremp et al. conclude that "depending on the strain of choice," experiments using single isolates of the population they studied "could have given opposite response patterns," which would likely have led to "contrasting predictions" about the future. And they indicate that this observation draws attention to the unavoidable fact that "responses observed in single strain experiments may not be representative" of the species or population in question, and that predictions for specific species behavior under future climatic conditions must thus "be treated with caution."

References
Engel, A., Schulz, K., Riebesell, U., Bellerby, R., Delille, B. and Schartau, M. 2008. Effects of CO2 on particle size distribution and phytoplankton abundance during a mesocosm bloom experiment (PeECE II). Biogeosciences 5: 509-521.

Langer, G., Nehrke, G., Probert, I., Ly, J. and Ziveri, P. 2009. Strain-specific responses of Emiliana huxleyi to changing seawater carbonate chemistry. Biogeosciences 6: 2637-2646.