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Active heterotrophic bacterial communities exist in all marine environments, and although their growth rates or respiratory rates may be limited by the interaction of low substrate concentrations with temperatures near their lower limit for growth, temperature and substrate con- centrations are rarely considered together as limiting factors. Moreover, attempts to evaluate meta- bolic limits by both temperature and substrate concentration have sometimes led to confusing con- clusions, because, while we can measure dissolved organic carbon (DOC) concentrations in natural waters, much of it is not readily available to heterotrophic bacteria. In spite of this procedural limita- tion, it can be helpful to regard temperature and substrate concentration as potential limiting factors that interact. In temperate ocean surface waters and estuarine waters, where bacterial growth is often reduced in winter, growth and respiration may be increased experimentally either by raising the tem- perature or by increasing organic substrate concentrations, providing indirect evidence that the lim- itation is an effect of temperature on substrate uptake or assimilation. Experimental work with bac- terial isolates also has shown a temperature-substrate interaction. In permanently cold polar waters, most heterotrophic bacteria appear to be living at temperatures well below their optima for growth. Nevertheless, bacteria in permanently cold surface waters can achieve activity rates in summer that are as high as those in temperate waters. In sea ice, rates of bacterial production are most often low, even though concentrations of substrates, including free amino acids, are sometimes much higher than they are in seawater. This suggests that at sea ice temperatures heterotrophic bacteria have low- ered ability to take up or utilize organic substrates.

Temperature and substrates as interactive limiting factors for marine heterotrophic bacteria