The ocean interior contains vast regions where oxygen concentrations are low or nil. These regions, known as oxygen minimum zones, support a great diversity or microbes, some of which mediate critical chemical transformations that are needed to maintain a balance in the cycle of chemical matter through nature. These cycles – also termed biogeochemical cycles – shape the environment in which we live. Over the last 20 years researchers have observed an expansion of the ocean’s oxygen minimum zones and, as a consequence, for the first time we recorded low oxygen events in the coastal waters off Oregon during summer months. These seasonal events have important direct and immediate consequences at a local scale by impacting fisheries. In addition, they can have broader long term consequences at regional and global scales by affecting biogeochemical cycles. In order to understand and characterize how an expansion of seasonal oxygen minimum zones in coastal regions affects microbes and the chemical transformations they mediate, this project compared the temporal changes in microbial diversity and biological activity as oxygen concentration change in two distinct oceanic coastal regions that experience seasonal decreases in oxygen concentration with different intensity and duration. While the Oregon coast experiences periods of low oxygen during summer months, these events are brief – no more than a few weeks – and anoxia (the total removal of oxygen) is not observed. In contrast, the coastal waters of Central Chile have low oxygen concentration with significant periods of anoxia during most part of the year, becoming oxygenated only during winter storm events that mix rich oxygenated water into the deep low oxygen regions. The difference in the temporal dynamics and intensity of the seasonal depletion in oxygen allows us to ask the following questions: Do we observe in both regions the same microbial diversity and chemical matter transformation as oxygen concentration change? And, if so, can we use our observations off Central Chile to predict what would happen off the Oregon Coast if the seasonal depletion of oxygen intensifies? And, if not, what drives the differences between both regions? Finally, because changes in oxygen concentration in the water can produce distinct chemical signatures in the sediments, looking at the record preserved over time in the sediments can help us explore how important these low oxygen events may have been in the past few hundred years. For this reason, the project includes a paleo-oceanographic effort that analyzes sediment records in these two regions to study how the seasonal variability and intensity in oxygen depletion may have varied in the past.
Although the key microbial processes taking place in oceanic suboxic and anoxic environments are well documented, we still have a rudimentary understanding of the microbial diversity responsible for these processes and the scales of adaptation of microbial assemblages to the physical and chemical dynamics of the marine environment in which they reside. Increasing this understanding is critical in order to improve our model predictions of declining oceanic oxygen associated with anthropogenic warming and of its effects in coastal ecosystems. Accordingly, the objectives of this proposal were to identify similarities and distinctions between the microbial assemblages inhabiting the water column of two distinct continental shelves that display distinct seasonal oxygen dynamics, as well as different patterns of physical and chemical variability; one of these regions is located in the United States off Oregon and the other in Chile off Arauco. In addition, our field effort characterized the seasonal patterns in the development, maintenance and intensity of hypoxic zones over both shelves over three seasonal cycles and determined the long term changes in seasonal fluctuations of oxygen concentration over the shelves through the use of paleo-proxies. While at present the Chilean shelf reaches euxinia every summer and the one off Oregon experiences hypoxia intermittently, paleo-oceanographic records indicate that both systems have varied between more extreme intervals of oxic and anoxic conditions. Furthermore, the present seasonal patterns of hypoxia and anoxia are the result of a delicate balance of biogenic input and physical advection and mixing. Metagenomic and metatranscriptomic analyses indicate that while microbial communities over both shelves are distinct, the expression of key microbial processes, such as archaeal ammonium oxidation and the distribution of key nitrogen species display similar distribution along the oxygen gradients becomes prominent; off Chile pelagic disimilatory nitrate reduction coupled to sulfur oxidation occurs when the system becomes anoxic; Our results imply the high sensitivity of hypoxia on these shelf systems to climate change, and the potential for growth of hypoxia under future climate warming.
Return to the main Marine Microbiology Initiative page