Microbial growth in the polar oceans - role of temperature and potential impact of climate change

Key Points Heterotrophic bacteria and other heterotrophic microorganisms typically process about half of the primary production in the oceans and therefore are important in determining the response of oceanic ecosystems and the carbon cycle to climate change. Previous studies suggested that heterotrophic bacteria are less active and are less important in the carbon cycle in polar waters because of low temperatures. A synthesis of old and new data confirms that the amount of primary production used by heterotrophic bacteria is in fact lower in the Arctic Ocean and in the Ross Sea, Antarctica, than in several lower-latitude oceans. The low rates are not due, however, to low temperatures, but rather to low supply of labile dissolved organic material. Only about 20% of the variation in bacterial growth rates in polar waters can be explained by temperature alone. These results have several implications for understanding how the Arctic Ocean and Antarctic seas may respond to climate changes already affecting these ecosystems. The decline in sea ice cover, for example, is likely to have large effects on ocean mixing and thus the supply of labile organic matter and nutrients supporting bacteria and other microorganisms at the base of polar food chains. In this Analysis, Kirchman and colleagues compare microbial processes in the western Arctic Ocean and other polar waters with low-latitude oceans to attempt to understand the role of heterotrophic bacteria in oceanic biogeochemical cycles. This may further our understanding of the changes that could occur as these waters warm. Heterotrophic bacteria are the most abundant organisms on the planet and dominate oceanic biogeochemical cycles, including that of carbon. Their role in polar waters has been enigmatic, however, because of conflicting reports about how temperature and the supply of organic carbon control bacterial growth. In this Analysis article, we attempt to resolve this controversy by reviewing previous reports in light of new data on microbial processes in the western Arctic Ocean and by comparing polar waters with low-latitude oceans. Understanding the regulation of in situ microbial activity may help us understand the response of the Arctic Ocean and Antarctic coastal waters over the coming decades as they warm and ice coverage declines.