Background
The authors write that "intertidal macroalgae live at the interface of marine and terrestrial habitats and are subjected to environmental challenges posed by both aquatic and atmospheric climatic regimes," as a result of which "they experience large temporal changes in temperature, irradiance and nutrient availability, and are faced with frequent desiccation and osmotic stresses." And, therefore, they say that "by virtue of their habitat, these algae are exposed to significant climatic and anthropogenic stresses, and are being increasingly used as an early warning system for climate change impacts," citing Harley et al. (2006) and Lima et al. (2007).

What was done
Clark et al. used "a quantitative genetic breeding design to establish whether there is a heritable variation in thermal sensitivity in two populations of a habitat-forming intertidal macroalga, Hormosira banksii (Turner) Descaisne," wherein "gametes from multiple parents were mixed and growth and photosynthetic performance were measured in the resulting embryos, which were incubated under control and elevated temperatures (20°C and 28°C)."

What was learned
The four researchers report that "significant interactions between male genotype and temperature in one population indicated the presence of genetic variation in thermal sensitivity," such that "selection for more tolerant genotypes thus has the ability to result in the evolution of increased thermal tolerance." In addition, they found that "genetic correlations between embryos grown in the two temperatures were positive, indicating that those genotypes that performed well in elevated temperature also performed well in control temperature."

What it means
Clark et al. write, in the concluding paragraph of their paper, that their "finding of genetic variation in thermal tolerance of H. banksii embryos suggests resilience to thermal stresses." But they nevertheless warn that "the persistence of populations will also depend on tolerance to other environmental stressors and anthropogenic disturbance," such as trampling by visitors to the rocky shore (Keough and Quinn (1998) and sewage discharge (Doblin and Clayton (1995).

References
Doblin, M.A. and Clayton, M.N. 1995. Effects of secondarily treated sewage effluent on the early life-history stages of two species of brown macroalgae: Hormosira banksii and Durvillaea potatorum. Marine Biology 122: 689-698.

Harley, C.D.G., Hughes, A.R., Hultgren, K.M., Miner, B.G., Sorte, C.J.B., Thornber, C.S., Rodriguez, L.F., Tomanek, L. and Williams, S.L. 2006. The impacts of climate change in coastal marine systems. Ecology Letters 9: 228-241.

Keough, M.J. and Quinn, G.P. 1998. Effects of periodic disturbances from trampling on rocky intertidal algal beds. Ecological Applications 8: 141-161.

Lima, F.P., Ribeiro, P.A., Hawkins, S.J. and Santos, A.M. 2007. Do distributional shifts of northern and southern species of algae match the warming pattern? Global Change Biology 13: 2592-2604.