The importance of phytoplankton in the global carbon cycle seems
to be gaining recognition resulting in many studies being performed to better
understand factors that affect the size and productivity of phytoplankton
communities. This trend is not surprising as the need to sequester carbon is
only increasing. Anthropogenic effects continue to raise the level of carbon
dioxide in the atmosphere year by year, in fact global CO2 hit a
multi-million year high of 398.35 parts per million in April this year (up from
396.45ppm in 2012), and exceeded 400ppm for the first time in recorded history,
on the 9th of May. With this in mind it is clear that an
understanding of the organisms responsible for nearly half of global primary
productivity is paramount. Primary productivity, in this case, is the
production of organic compounds from CO2 through the process of
photosynthesis. The modelling of phytoplankton communities is of interest to me
as I will be building a phytoplankton biomass model as one of my honours
projects. Many of the processes determining the growth and life cycle of these
organisms are well understood through decades of empirical research, and the
extension into mathematical modelling is allowing for fascinating predictions
and insights into the future of the marine environment.
Phytoplankton are at the bottom of the oceans very
complicated food web and can be thought of as micro-plants of the ocean which
absorb CO2 through the process of photosynthesis. The productivity
of this reaction is dependent on nutrient concentrations and environmental
conditions such as light availability, mixed layer depth and temperature.
Temperature plays an important role in marine and estuarine systems as it
affects water stratification, most current work involves the modelling of this
relationship. A recent paper by Thomas et
al (2012), titled “A global Pattern of Thermal Adaption in MarinePhytoplankton” published in Science magazine, investigates the direct impact of
temperature on marine phytoplankton. The authors point out that not only is
phytoplankton sensitive to temperature, but this sensitivity is skewed, with higher
temperatures having significantly worse consequences for many phytoplankton
species. With global ocean temperatures set to rise, the future of
phytoplankton appears rather dark. Given that a large portion of global
atmospheric carbon is sequestered by phytoplankton, it is useful to understand
how predicted changes in ocean water temperatures will affect the productivity
and community structure of phytoplankton populations.
In order to understand how ocean warming will affect
phytoplankton productivity and distribution the authors attempted to understand
the current relationships between productivity and temperature and the global
distribution of phytoplankton. The analysis of 194 strains of phytoplankton
from information gleamed from over 80 publications from 1935 - 2011 revealed a
strong trend in the latitudinal distribution of phytoplankton and the optimum
temperature for productivity (the authors sneakily point out that this suggests
a global trend in a microbial trait,
which is currently unidentified). An even stronger trend between that optimum
and the mean annual temperature at the population’s location was found. This
suggests that phytoplankton populations are highly adapted to local
temperatures. Interestingly polar and
temperate optimum temperatures were found to be higher than the annual
temperatures at these points, which got me thinking, if oceanic temperatures are
set to rise (which models predict to be the case in many regions) then surely
phytoplankton with optima above current average temperatures will thrive? Unfortunately
the predicted overall increase in temperature is not globally uniform and some
regions around the poles and temperate regions are expected to remain constant
and even possibly decrease.
The collaboration between optimum temperatures and location
was not enough for the authors to confirm an adaptive relationship (one where
strains possess the characteristics they do as a result of their environment)
and so an eco-evolutionary model was run on the strains of phytoplankton in
question. The model worked by “forcing” differences in strains of phytoplankton
to be the same so that only temperature tolerance could be compared,
essentially simplifying the system enough that the strains are comparable. For
each location the optimum temperature was allowed to evolve, based on an
evolutionary algorithm. This gives an output of the best strategy at each
location based on temperature tolerance. The results showed that optimum
temperature for phytoplankton primary production should in fact increase with increasing
local mean temperatures. I consider this result great news for phytoplankton,
as it indicates that they are in fact able to adapt to changes in temperature. However
there is no current understanding of the rate at which strains are able to
adapt to changing temperatures.
The paper goes on to a species distribution model which
matches the current requirements of phytoplankton communities with the
predicted available environments for 2091-2100. From this point of view large
decreases in diversity are expected as regions become more or less favourable,
with a dominant shift pole-ward for most species and a drastic decrease in
diversity at the tropics. Without an understanding of the rate at which
phytoplankton can adapt to environmental conditions, a species distribution
model seems unimportant. Most biologists will agree that high diversity is an imperative
characteristic of a healthy ecosystem and in this regard the movement of
plankton strains is of interest, however these predictions carry little weight
when it is known that phytoplankton are able to adapt to changing temperatures,
additionally, given their rapid rate of reproduction and short lifespan, they
are increasingly likely to evolve under strong temperature pressures. What
would be of greater interest to me would be if the increasing temperatures
where likely to lead to decreased population sizes, and not just decreased
diversity.
