How are Limacina helicina affected?

Images of Pteropod shells exposed to low pH: http://www.pmel.noaa.gov/co2/file/show/246


The figure above explains exactly why we ought to be concerned about the effect of ocean acidification on Limacina helicina. This is the result of an experiment performed by scientist at the PMEL carbon program. The experiment was done in situations simulating ocean CO2 by the end of this century. It is very evident from the above figures that it wont take long at all for the shell of this pterpod to undergo complete dissolution.

What exactly is causing this: What is the chemistry behind it?

As mentioned in the introduction of this blog , Pteropods (of which, L.helicina is a member) use a polymorph of calcium carbonate known as aragonite to make their shells. Both the making and dissolution of this shell is dependent upon certain properties, the most important of which is called saturation state (omega) (Munday et al 2009). This saturation state is directly proportional to the concentration of Ca²⁺ and CO₃^2- in the ocean. Well as previously discussed, the increased carbon dioxide concentration in the ocean tends to favor the formation of bicarbonate ion while using up CO₃^2- (Guinotte et al 2008). Consequently, the saturation state of aragonite is going to be lowered which would slow down calcification rate of the shell.Additionally, acidification has an especially significant impact on these species given the relative depth of their habitat in the ocean. Studies have shown that saturation depth has a negative correlation with increasing acidity of the ocean (Cao et al 2010). This makes them increasingly sensitive to increased ocean acidification.

What’s even more concerning is that studies have shown that when this saturation state falls below 1, the aragonite component of these shell are going to start dissolving. Infact, it has been projected that Limacina helicina will lose its calcification ability by the year 2100 because the saturation state of aragonite will fall below 1 by the end of this century per  IPCC report. The lack of shell due to continous dissolution of aragonite could possibly lead to the extinction of these very important species (Comeau et 2012). 

The pteropods use their calcified shells as a defense mechanism because it is harder for some predators to bite on them. An article by Comeau et al sums up the fate of a  possible shell-less pteropod:  They stated that it very unlikely that the pteropods will survive this dissolution of their shell, because it hasn't been shown, historically, that they were ever without a shell at any point during their evolution (Comeau et al 2012).