So, I decided to have a look around, and see if there were indeed any studies linking mood disorders (depression) and serotonin. I've found a few, however, I'm pretty sure linking them directly won't work as I've accessed them from my uni database. I'll give the complete references however, and if you're REALLY keen you can have a look for them yourself
O'Rourke, Howard; Fudge, Julie L.
Distribution of Serotonin Transporter Labeled Fibers in Amygdaloid Subregions: Implications for Mood Disorders. [Journal; Peer Reviewed Journal] Biological Psychiatry. Vol 60(5) Sep 2006, 479-490.
This one I couldn't find the full text, however the abstract looked quite promising...
Nutt, David J; Stein, Dan J.
Understanding the Neurobiology of Comorbidity in Anxiety Disorders. [Journal; Peer Reviewed Journal] CNS Spectrums. Vol 11(10,Suppl12) Oct 2006, 13-20.
Hariri, Ahmad R; Holmes, Andrew.
Genetics of emotional regulation: The role of the serotonin transporter in neural function.. [Journal; Peer Reviewed Journal] Trends in Cognitive Sciences. Vol 10(4) Apr 2006, 182-191.
Tsuji, Minoru; Takeda, Hiroshi; Matsumiya, Teruhiko.
Brain 5-HT-sub(1A) Receptors as Important Mediators in the Development of Stress Adaptation. [Journal; Peer Reviewed Journal] Current Neuropharmacology. Vol 1(4) Dec 2003, 315-324.
There are heaps more results, but i'm too tired to look at the rest.
For anyone interested, I used the PsycINFO database for my searches.
EDIT: In all fairness, I realised that 3/4 studies listed here are from 2006, after the article mentioned by the OP was published.