See Vila-Costa et al. (2010) for detailed methodology, which is paraphrased below:

"Sampling was carried out on 15 April 2008 at the Bermuda Atlantic Time-series Study (BATS) station at a depth of 10m using 10-L Niskin bottles. A 2 L subsample of unfiltered water was collected to measure in situ concentrations of Chl-a and total DMSP. The remaining seawater was prefiltered by gravity through 3 mm pore-size polycarbonate filters (142mm, Millipore, Billerica, MA, USA) to exclude eukaryotes and large particles. Water was dispensed into 20 L polycarbonate carboys and maintained in a temperature-controlled room in the dark at in situ temperature for 3 h before beginning the experiment to allow time for adaptation to any DMSP released from the cells during filtration.

DMSP was added to experimental carboys to a final concentration of 25 nM. Control carboys remained untreated. Bacterial cells were collected after 30 minutes by filtering the water through 0.2 um pore-size polycarbonate filter. Filters were placed in 15 ml RNAse-free tubes containing 2 ml of Buffer RLT (RNeasy kit, Qiagen, Valencia, CA, USA) plus 10 ml of b-mercaptoethanol per ml. Messenger RNA (mRNA) extraction, enrichment, amplification, and conversion to complementary DNA was performed as described by Poretsky et al. (2009) with few modifications (see Supplementary Material in Vila-Costa et al. 2010). Only one replicate from the experimental and control treatments yielded RNA of sufficient quality for processing.

Complementary DNA libraries were sequenced with Roche GS FLX sequencing (Branford, CT, USA), yielding 606 286 reads (209-bp average length). The sequences were deposited in the Community Cyberinfrastructure for Advanced Marine Microbial Ecology Research and Analysis (CAMERA) database with the Genome Project ID CAM_PROJ_SargassoSea."

References:
Poretsky, R.S., Hewson, I., Sun, S.L., Allen, A.E., Zehr, J.P., and Moran, M.A. (2009) Comparative day/night metatranscriptomic analysis of microbial communities in the North Pacific subtropical gyre. Environ Microbiol 11: 1358–1375. DOI: 10.1002/9781118010518.ch63