The metabolic rates of seagrass communities were synthesized on the basis of a data set on seagrass community metabolism containing 403 individual estimates derived from a total of 155 different sites. Gross primary production (GPP) rates (mean ± SE = 224.9 ± 11.1 mmol O2 m−2 d−1) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O2 m−2 d−1), indicating that seagrass meadows tend to be autotrophic ecosystems, reflected in a positive mean net community production (NCP 27.2 ± 5.8 mmol O2 m−2 d−1) and a mean P/R ratio above 1 (1.55 ± 0.13). Tropical seagrass meadows tended to support higher metabolic rates and somewhat lower NCP than temperate ones. The P/R ratio tended to increase with increasing GPP, exceeding, on average, the value of 1 indicative of metabolic balance for communities supporting a GPP greater than 186 mmol O2 m−2 d−1, on average. The global NCP of seagrass meadows ranged (95% confidence limits of mean values) from 20.73 to 50.69 Tg C yr−1 considering a low global seagrass area of 300,000 km2 and 41.47 to 101.39 Tg C yr−1 when a high estimate of global seagrass area of 600,000 km2 was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.