Freshwater phytoplankton could tolerate wider temperature
ranges than marine taxa (Fig. 2E). The positive
relationship between maximal growth rate and thermal
breadth did not exist in freshwater phytoplankton
(Fig. 2F). I also examined the relationships between
environmental temperature extremes and thermal limits
of phytoplankton growth to assess to what extent the
thermal limits could be affected by environmental temperature
extremes (Fig. 3). The optimal growth temperatures
of both marine and freshwater phytoplankton
increased with environmental annual mean temperature
(Table I; Fig. 3A) with the trend more pronounced for
marine phytoplankton. The relationship between optimal
temperature of marine phytoplankton and environmental
mean temperature was well fit by a three-order polynomial,
confirming the conclusion that species optimal
growth temperatures could be lower than annual mean
temperature in the tropics (Thomas et al., 2012) (Fig. 3A).
The minimal growth temperatures of marine taxa
increased linearly with, and were usually lower than, environmental
lowest temperatures. The phylogenetic
signal was still evident in the minimal growth temperatures.
When controlling environmental lowest temperatures
constant, the average minimal growth temperatures
of marine chlorophytes, cyanobacteria, dinoflagellates
and raphidophytes were all significantly higher than
those of diatoms (P, 0.01). The maximal growth temperatures
of both marine and freshwater phytoplankton
increased significantly with environmental highest temperature,
with the trend of marine phytoplankton much
more drastic than of freshwater ones (Fig. 3C). The
maximal growth temperatures of freshwater phytoplankton
were significantly higher than marine ones and were
much higher than environmental highest temperatures,
particularly for polar taxa (Fig. 3C). There were no clear
trends of thermal breadths of phytoplankton with environmental
temperature ranges (Fig. 3D). Again, the phylogenetic
differences were pronounced in the thermal
breadth of marine phytoplankton. The thermal breadths
of cyanobacteria, dinoflagellates, and haptophytes were
significantly lower than those of diatoms (P, 0.001).