Carbon isotopic compositions of estuarine bacteria

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Abstract
A bioassay was developed to assess the stable carbon isotopic compositions of planktonic bacteria from the Parker River estuary, Massachusetts. A small inoculum of natural bacteria was added to filtered estuarine water, then incubated for 24–48 h until bacteria reached the end of log‐phase growth. Bacteria harvested at the end of these bioassays exhibited a wide range of δ13C values from −11.5% (near the −13% value of Spartina) to −27.4‰ (near the −29‰ value of upland C‐3 plants). This wide range of δ13C values suggests that bacteria in the estuary use substrates from a variety of primary producers. Experiments with glucose and dissolved organic carbon leached from oak and Spartina leaves showed that bacteria had δ13C values within ±2‰ of their growth substrates. The results suggest that carbon isotopic measurements are useful for tracing the linkage between bacteria and the plant sources of substrates that support bacterial growth.