The null hypothesis of the first hypothesis is that the
environment does not affect the thermal traits of phytoplankton.
The fact that the optimal, minimal and
maximal growth temperatures are highly correlated with latitude (Fig. 2) rejects the null hypothesis and suggests
that the environmental temperature of the isolation
locale should be an important factor shaping phytoplankton
physiology. It is noteworthy that this evident pattern
exists for phytoplankton taxa that have been possibly cultivated
in the laboratory after multiple generations, suggesting
that the thermal characteristics can be inherited
for a relatively long period. We need to recognize that the
thermal limits obtained from temperature tolerance curves
represent the most extreme temperatures that phytoplankton
species can tolerate; while in nature, phytoplankton experience
high mortality due to grazing, sinking and viral
attack (Lehman and Sandgren, 1985; Calbet and Landry,
2004),which suggests that, even at the lowest or highest environmental
temperatures, the intrinsic growth rate of
phytoplankton should be positive to balance mortality.
This can well explain why we observe that the thermal
breadths tend to encompass the environmental temperature
range (Fig. 3).