Oceanic Insights

MARINE BIOGEOCHEMISTRY

The vitality of the world's oceans depends on an intricate set of chemical, biological, and physical processes. Several studies published last week provide new information to gauge how natural and human-induced perturbations can affect these processes.

Computer modeling of global ocean data predicts that the expected increase in atmospheric CO₂ concentration could lead to a significant drop in ocean carbonate levels, reports an international team led by James C. Orr of the Laboratory of Climate Sciences & the Environment, in Gif-sur-Yvette, France (Nature 2005, 437, 681). The researchers project that some ocean regions could be adversely affected by these changes within decades, rather than centuries, as previously thought.

They tested their predictions by placing winged snails (pteropods) in seawater having a pH and CO₃^2- concentration expected in the oceans by 2100. The snails' thin CaCO₃ exoskeletons began to dissolve noticeably within two days. The researchers conclude that the snails and other calcifying marine creatures might not be able to adapt quickly enough to survive these changing conditions, which could alter the structure and diversity of some marine ecosystems.

In another study, Antonio Dell'Anno and Roberto Danovaro of Polytechnic University of Marche, in Italy, have shown that DNA leaking out of the ruptured cells of dead phytoplankton plays an important role in the oceanic phosphorus cycle (Science 2005, 309, 2179). Although DNA is a phosphorus-rich molecule, its role in phosphorus cycling generally has been ignored until now, the researchers note.

The team's sampling of global deep-ocean sediments indicates that millions of tons of extracellular DNA annually settles on the ocean floor, where microbes convert the organic carbon, nitrogen, and phosphorus into inorganic carbonate, nitrate, and phosphate. These nutrients subsequently are transported back to the ocean surface by conveyor-belt-like currents and reutilized by phytoplankton.

In other work, Carles Pelejero and Eva Calvo at the Institute of Marine Sciences in Barcelona and their coworkers at Australian National University, Canberra, report that cyclic changes in ocean currents and related changes in the flushing rate of a southwestern Pacific coral reef are responsible for a long-term fluctuation in pH that is important for the livelihood of corals that grow there (Science 2005, 309, 2204). The researchers measured changes in boron isotope levels in a long-lived coral to provide a continuous record of ocean pH extending back 300 years.

The pH has cycled from 7.9 to 8.2 about every 50 years, a variability that is large enough to moderate or exacerbate the ecological impact of the expected gradual lowering of ocean pH if atmospheric CO₂ levels continue to rise as projected, they note.

In a Nature commentary, Marina Lévy of Pierre Simon Laplace Institute, in Paris, writes that achieving a better understanding of the variability of nutrient cycling in the oceans "is one of the next challenges in marine biogeochemistry."