Testing the drivers of the temperature-size covariance using artificial selection

Area of Science:

• Ecology
• Evolutionary Biology
• Climate Change Biology

Background:

• Body size in many organisms, particularly phytoplankton, tends to decrease as ambient temperature rises.
• While this size reduction is often considered adaptive, the underlying mechanisms—whether intrinsic properties of smaller size or correlated traits like metabolism—remain debated.

Purpose of the Study:

• To experimentally determine if body size itself, independent of other traits, mediates the performance of phytoplankton under varying temperatures.
• To investigate the specific mechanisms driving temperature-dependent performance in phytoplankton, testing hypotheses related to resource limitation and oxidative stress.

Main Methods:

• Utilized 290 generations of artificial selection on a unicellular phytoplankton species to create distinct populations with a 13-fold volume difference (small-selected vs. large-selected).
• Assessed the fitness of these selected populations across three temperature conditions: 18°C, 22°C (control), and 26°C.

Main Results:

• Warmer temperatures (26°C) enhanced the fitness of small-selected phytoplankton but reduced the fitness of large-selected phytoplankton.
• These findings indicate that cell size alone is a sufficient factor in mediating temperature-dependent performance.
• Results contradict the resource limitation hypothesis and support the idea that larger cells are more susceptible to reactive oxygen species damage at higher temperatures.

Conclusions:

• Smaller phytoplankton species exhibit pre-adaptation to higher temperatures, a crucial factor for predicting ecological responses to climate warming.
• The study highlights the significant implications of size-dependent thermal performance for phytoplankton communities and their role in global biogeochemical cycles, such as the biological carbon pump.