Researchers Uncover Genetic Payload Of Ancient Plankton In Black Sea Sediments
May 7, 2013

Researchers Uncover Genetic Payload Of Ancient Plankton In Black Sea Sediments

April Flowers for - Your Universe Online

Sorting through the vast amounts of genetic data from the Black Sea sediment record, Woods Hole Oceanographic Institution (WHOI) marine paleoecologist Marco Coolen was astounded by the variety of past plankton species that left behind their genetic makeup. This vast amount of data is called the plankton paleome.

The Black Sea is semi-isolated from other bodies of water, and highly sensitive to climate driven environmental changes. The underlying sediments contain a high resolution archive of past continental climate and concurrent hydrologic changes in the basin. The Black Sea is a brackish body of water, with salt water of the Mediterranean supplied through the narrow Strait of Bosphorus and freshwater supplied by rivers and precipitation.

"However, during glacial sea level lowstands, the marine connection was hindered, and the Black Sea functioned as a giant lake," says WHOI geologist Liviu Giosan.

He added that "the dynamics of the environmental changes from the Late Glacial into the Holocene (last 10,000 years) remain a matter of debate, and information on how these changes affected the plankton ecology of the Black Sea is sparse."

Coolen and Giosan, along with their colleagues, used a combination of advanced ancient DNA techniques and tools to reconstruct the past climate. This allowed the team to determine how communities of plankton have responded to changes in climate and the influence of humans over the last 11,400 years.

Traditionally, researchers use a microscope to count the fossil skeletons found in sediment cores in order to reconstruct the makeup of plankton. This method is limited, however, because most plankton leave no fossils. The WHOI team looked for sedimentary genomic remains of the past inhabitants of the Black Sea water column instead. The findings of this study will be published in an upcoming issue of Proceedings of the National Academy of Sciences (PNAS).

"DNA offers the best opportunity to learn the past ecology of the Black Sea," says Coolen. "For example, calcareous and organic-walled dinocysts are frequently used to reconstruct past environmental conditions, but 90 percent of the dinoflagellate species do not produce such diagnostic resting stages, yet their DNA remains in the fossil record."

So far, ancient DNA signatures in marine sediments have been used for targeted reconstruction of specific plankton groups. Those studies were based on very small clone libraries of DNA. The WHOI researchers used a high throughput next generation DNA sequencing approach called pyrosequencing to look for the overall plankton changes in the Back Sea from the deglaciation to recent times instead of the small clone libraries of previous studies. Additionally, to understand probable causes for plankton community shifts, they reconstructed past changes in salinity and temperature as well.

The research team analyzed sediments containing highly resistant organic compounds called alkenones to reconstruct the salinity. Alkenones are uniquely produced by Emiliania huxleyi - the same photosynthetic organism oceanographers study to determine past sea surface temperatures. They were able to map the salinity trend in the Black Sea over the last 6,500 years by examining the ratio of two hydrogen isotopes in the alkenones.

"One of the isotopes, deuterium, is not very common in nature," explains Coolen, "And it doesn't evaporate as easily as other isotopes. Higher ratios of deuterium are indicative of higher salinity."

The WHOI team collaborated with Chris Quince and his postdoc Keith Harris from the Computational Microbial Genomics Group at the University of Glasgow, and with micropaloentologist Mariana Filipova-Marinova from the Natural History Museum in Varna, Bulgaria to complete this study of the plankton paleome. Their findings revealed that of 2,710 plankton identified, 150 showed a statistically significant response to four environmental stages since the deglacial.

They found that freshwater green algae were the best indicator species for lake conditions more than 9,000 years ago. The presence of previously unidentified marine plankton species, however, indicates that the Black Sea was influenced to some extent by the Mediterranean Sea over at least the past 9,600 years.

After the latest marine reconnection and during the warm and moist mid-Holocene climatic optimum, dinoflagellates, cercozoa, eustigmatophytes, and haptophyte algae responded most dramatically to the gradual increase in salinity. With the onset of the dry Subboreal climate stage after ca. 5,200 years ago, salinity increased rapidly. This led to an increase in marine fungi and the first occurrence of marine copepods. During the refreshening of the Black Sea with the onset of the cool and wet Subatlantic climate around 2,500 years ago, a gradual succession of phytoplankton such as dinoflagellates, diatoms, and golden algae occurred. In the last century, the most dramatic changes in plankton have occurred. These are associated with human disturbances in the region.

The study findings highlight the sensitivity of marine ecosystems to climate and human impact, and the high throughput sequencing of ancient DNA signatures allow us to reconstruct a large portion of ancient oceanic life, including organisms that are not preserved as fossils.

Even in the oldest analyzed Black Sea lake sediments, when the entire water column was most likely well-mixed and oxygenated, ancient plankton DNA was being preserved. This indicates that such DNA might be widely preserved in sediments and can be used to reconstruct past life in the majority of lake and oceanic environments.