Bacteria and Bathymetry
60°34'S, 165°03'E
The view from the deck of the Palmer has changed little since we left Cape Adare several days ago. All that's visible is an endless plain of wine-dark sea stretching to a featureless horizon beneath a flat grey sky. Aside from a few magnificent albatrosses, the world appears lifeless and monotonous.
But beneath the waves the world is full of life and change. Just to our west lie the Macquarie Ridge and Hjort Trough, where within a span of 60 miles the seafloor plummets from the shallow waters of Macquarie Island to a depth of 6,494 meters (21,305 feet). In this inverted Himalaya young and buoyant seafloor is diving into a deep-sea trench along a complex and poorly understood segment of the Australia-Pacific plate boundary. The geologists onboard have brought us here to study the boundary's eastern side, as it holds important clues to better understanding earthquake dynamics in the South Tasman Sea and the history of New Zealand's Alpine fault zone.
The world beneath the waves of course hides biological surprises as well. One is the recently discovered profusion of bacteria in the marine environment. Recent advances in microscopy and flow cytometry reveal that millions of these single-celled organisms inhabit every milliliter of seawater. A 2003 study by biologist J. Craig Venter, cartographer of the human genome, shows that these organisms are not only abundant but incredibly diverse—a single sample of seawater from the tropical Atlantic contained 1,800 new microbial species and 1.2 million new genes.
One of the goals of the IVARS project is to better understand the role that bacteria play in the Ross Sea ecosystem. Oceanographers didn't fully appreciate the potential ecological significance of bacteria until quite recently. Studies of the Southern Ocean using 1960s technology and instruments suggested that Antarctic waters contained less than 10 bacterial cells per milliliter. But more recent studies, led by the work of VIMS microbiologist Dr. Hugh Ducklow and other researchers, show that bacterial abundances in the Ross Sea can peak at 2 to 3 million cells per milliliter.
What are all these bacteria doing? It appears they make their living by eating organic matter produced by plankton during the spring bloom. Bacterial consumption of this material is crucial to the continued functioning of most marine ecosystems, as it serves to recycle these organic components back into an inorganic form that phytoplankton can use.
Yet there's a wrinkle to this process in the Ross Sea, where the ratio between bacterial biomass and total plankton biomass is much lower than in other, more temperate ecosystems. Outside Antarctica, the ratio between bacterial biomass and that of other plankton is about 50/50. In the Ross Sea, the ratio is only about 5/100. This is despite the fact that peak bacterial abundances in the Ross Sea are roughly equal to those of fertile areas elsewhere in the ocean.
Researchers within IVARS and in the microbiology program at VIMS are working to solve this puzzle by quantifying and clarifying the role of bacteria in the Ross Sea food web. The bacteria samples we've collected aboard the Palmer will be sent back to VIMS for analysis as soon as we make port in Lyttleton, New Zealand. They'll be analyzed by Dr. Ducklow's marine technician Helen Quinby using flow cytometry.
Some earlier studies hinted that cold temperatures are the main constraint on bacterial growth in the Ross Sea and other polar areas. Ducklow's recent work in the Ross Sea challenges this claim. He suggests instead that bacterial growth in the area is limited by insufficient quantities of dissolved organic carbon. That might seem counterintuitive given that the Ross Sea's annual bloom of the alga Phaeocystis antarctica is one of the largest plankton blooms on the planet. The problem, according to Ducklow, is that most of the carbon from this bloom is in a form that bacteria can't use. That's partly because P. antarctica is not very palatable to zooplankton grazers. Thus the bacteria don't benefit from the grazers' ability to metabolize and rupture algal cells, processes that generate the labile types of dissolved organic carbon that bacteria need for growth.
Ducklow also points to the considerable time lag between the onset of the phytoplankton and bacterial blooms in the Ross Sea. Most bloom events can be divided into three consecutive phases: an initial spike in phytoplankton production (which in the Ross Sea roughly coincides with the onset of perpetual summer daylight and melting of sea ice), a consequent increase in zooplankton grazing (with concomitant production of particulate and dissolved organic carbon), and finally an increase in bacteria. In temperate areas, the lag between phytoplankton and bacterial peaks is typically less than 10 days, and often almost nil. In the Ross Sea, the lag is about a month. This may reflect the sluggishness of degradation processes and bacterial growth in the Sea's cold waters.
Taken together, Ducklow's data suggest that the low rates of bacterial recycling in the Ross Sea result primarily from a lack of food, and only indirectly from cold temperatures. He estimates that bacteria recycle only about 30% of photosynthetic carbon back into the ecosystem each year. So how does the system continue to function? The phytoplankton obtain the remaining 70% of their nutrient requirements from seasonal upwelling of nutrient-rich water. Moreover, they consume only a portion of the nutrients available due to limitation by iron.
Further study of the IVARS bacteria samples will help to test these ideas, and to track how bacteria dynamics in the Ross Sea might change from year to year.