Australasian Science Magazine

Different phytoplankton species, including diatoms and algae. Image courtesy of Richard Kirby, Plymouth University

By Kuldeep More & Marco Coolen

Ancient plankton DNA is revealing how marine ecosystems have responded to long-lasting changes in past climate – and enabling us to predict the future.

Oceans cover 71% of the planet’s surface and contain an estimated 80% of all life on Earth. Free-floating microscopic plankton represent the majority of ocean biodiversity and are a crucial food source for organisms higher in the food chain, like fish, whales and even humans. For example, the green algae Chlorella and Spirulina are popular green superfoods.

Phytoplankton are green algae that perform photosynthesis. They live in the upper layers of the ocean where they can harvest light. We owe half of the oxygen in the atmosphere that we breath to these tiny creatures.

One of the most abundant group of phytoplankton are diatoms. These have external body armour made from silica, and incorporate 6.7 billion metric tonnes of silicon every year.

Other plankton members feed on bacteria and small phytoplankton. For example, radiolarians consume bacteria or small phytoplankton by filtering sea water.

Then there are plankton that can do both. Mixotrophic plankton such as dinoflagellates feed on other plankton but are also capable of photosynthesis. Being able to live both lifestyles, they have a wide distribution in the ocean.

Plankton sit at the bottom of the marine food pyramid, so any devastating impact on their population could result in the collapse of the entire marine ecosystem. This would affect fisheries, food production and recreation, with the global socioeconomic value estimated at US$21 trillion annually.

Thus, to ensure a balanced marine ecosystem it is essential to have a healthy population of plankton. Climate change directly affects the plankton population by altering their environment, such as sea surface temperature, nutrient availability and oxygen supply. The analysis of past changes in climate–plankton interactions is the best way to predict how ongoing climate change will affect marine plankton in the long term.

DNA Unlocks the Past

When they die, marine plankton sink to the ocean floor and become partly buried in the underlying sediments. The thickness of the sediments grows year by year, much like tree rings, and creates an archive of the plankton communities that lived in the ocean in the past. Some plankton species, such as diatoms, leave behind microscopic fossils in marine sediments. These can be used for identification, much like the use of fossil bones to identify the type of dinosaur species.

However, the majority of plankton are soft-bodied and do not leave behind any fossils. Hence, it’s necessary to develop independent methods to study past planktonic communities.

When cellular or large fossils are lacking, chemical fossils such as photosynthetic pigments are often used as a tool to identify past plankton, but these biomarkers are not very specific. In contrast, some genetic markers can provide much more detailed information about past species compositions in a similar way to how DNA helps forensic investigators identify criminals.

Although DNA is considered a labile biomolecule, it is quite well preserved in marine sediments dating back tens of thousands of years. Sediments can be radiocarbon-dated, and the DNA in those sediments can be sequenced to reveal ecosystem changes through time. This relatively young field is called sedimentary palaeogenomics.

Combined with established tools that can reconstruct changes in environmental conditions, palaeogenomics can be used to discover how climate or changes in water column conditions affected past plankton communities. This can then help us to extrapolate the possible effects of anthropogenic changes on marine ecosystems.

The Birthplace of the Oxygen-Minimum Zone

We conducted our studies in the north-east of the Arabian Sea, off the coast of north-west India and southern Pakistan. Widespread occurrence of algal blooms in this region occurs during the intense monsoon season. When these algal blooms die off at the end of the growth season, marine bacteria degrade the sinking dead biomass, consuming oxygen in the process. Hence the oxygen content in the water column decreases, which causes the formation of mid-water oxygen-minimum zones (OMZ).

Our sediment core from the OMZ region of the Arabian Sea revealed long-term shifts in OMZ strength dating back 43,000 years. During warmer climate stages, strong OMZ conditions resulted in the formation of dark organic-rich sediments, indicating high plankton productivity. During colder periods, weak OMZ conditions resulted in light-coloured sediments with low organic content, indicating lower plankton productivity.

We extracted plankton-derived DNA from these sediments to reconstruct how the diversity and relative abundance of past plankton species were affected by past OMZ dynamics.

How Plankton Responded

The DNA of these ancient plankton communities show that plankton adapted in many ways through periods of low oxygen. One of the most significant ways was through symbiotic relationships. For instance, during strong OMZ periods, dinoflagellates would live in a special sac created by a radiolarian host. Radiolarians would provide nutrients to dinoflagellates in this sac, and dinoflagellates would produce a jelly that helped radiolarians capture prey.

During long-term periods of strong OMZ we also observed an increase in parasitic planktons, especially fish parasites.

Yet another way plankton responded to stressful OMZ conditions was by becoming dormant, forming resting cysts and remaining inactive until conditions become favourable again. Such a response to harsh conditions is quite common in nature, like polar bears hibernating in the harsh winter.

We also observed that dinoflagellates have became more dominant than diatoms during the past few thousands of years. This is a general sign of increased nutrient supply, a strengthening of OMZ conditions and a disturbed marine ecosystem.

In summary, our results show that long-term exposure to OMZ conditions will cause a disturbance in the marine food web and will ultimately have negative effects on fisheries and local economies. OMZs are increasing worldwide due to global warming, increased land use and discharge of nutrients into coastal waters.

However, our data also show that the marine ecosystem is resilient enough to return to healthy conditions soon after these OMZs cease to exist.

Nevertheless, prevention is the best cure, so we must control nutrient levels in coastal areas to keep marine ecosystems healthy and minimise economic losses.