2.3.2. Sediment organic matter labeling
Ž . 14 Sediment organic matter OM was non-specifically radiolabeled with C-formaldehyde,
following Lopez and Crenshaw Ž1982.. Five milliliter of settled surface
sediment were centrifuged at 1600=g for 6 min, after which overlying water was
replaced with 30 ml of 30% NaCl solution Žby weight., to reversibly inhibit microbial
activity ŽLopez and Elmgren, 1989.. A total of 0.366 MBq of 14C formaldehyde ŽNEN,
Boston, MA; specific activity 1.9 MBqrmmol. were added to the vial and shaken
vigorously, resulting in a spike of approximately 0.073 MBqrml sediment Ž0.15
MBqrg dry weight.. This addition corresponded to a maximum calculated formaldehyde-
sediment concentration of approximately 38 nmolrml Ž79 nmolrg.. The sediment
was left to equilibrate at room temperature Ž24 8C. for 5 days Žshaken once per
day. after which it was centrifuged Ž8 min at 1100=g. and rinsed five times Žeach
cycle with 30 ml Flax Pond sea water. to remove unbound formaldehyde. Formaldehyde
activity associated with the particulate phase at the end of the labeling period was 12%
of the activity originally added, corresponding to a formaldehyde concentration of
approximately 10 nmolrg. The maximal water concentration resulting from feeding
approximately 100–500 mg of this material to worms was several orders of magnitude
lower than the concentration at which formaldehyde exerts toxic effects to marine
invertebrates ŽOffice of Pesticide Programs, 1995..