Tiny Organisms with a Massive Impact

Photo: Julien Crespin, SEM facility of the Weizmann Institute

Diatoms consume nutrients

Researchers can monitor the mass reproduction of these organisms by examining the depth profiles of zinc and silicon concentration in the seawater of different oceans: the profiles are identical for both elements, with a significant depletion in the top kilometre of the water column. The conclusion of the study is that that this is caused by the the biological activity of the diatoms in the surface layer around Antarctica, followed by the transport of the resultant nutrient-depleted water masses to other parts of the ocean in currents.

Depleted of nutrients, the uppermost layer of the water flows in the direction of the Equator. As it reaches a latitude of around 45-50 degrees, it sinks below a warmer surface layer. This middle water layer extends well into the northern oceans and does not mix completely with other layers, and therefore remains starved of nutrients.

Not all the surface water around Antarctica flows towards the Equator. Very near the Antarctic continent itself surface waters become very salty and dense due to the formation of sea ice. This dense water sinks into the abyss. The algae that bloom at the surface often end in a type of mass death once all the nutrients have been exhausted. They then sink into this deep ocean. As they sink some diatoms are also “packed” into particles excreted by tiny sea creatures and, in the ocean’s depths, the cells decompose and release zinc and silicon back into the seawater. Deep ocean currents that travel northwards transport the trace elements 5000 metres below the surface. This allows the zinc and silicon to replenish.

The bottom water flows in a wide loop in the direction of the Equator and back towards Antarctica, where the vertical upwelling transports the enriched nutrients to the light-flooded water’s surface, allowing the diatoms to begin a new reproductive cycle.

ETH researchers on board the “Akademik Treshnikov” use a special device to collect seawater samples containing diatoms. 

On an expedition to collect diatoms

To test this hypothesis, one of Vance’s doctoral students and two postdocs are taking part in the Swiss Polar Institute’s current Antarctic Circumpolar Expedition, a venture already reported in ETH News. They will be collecting seawater samples and cultivating the diatoms in the water under different nutrient conditions in the on-board laboratory. The scientists will fertilise some of the diatoms with iron, for example, to investigate the effect this trace element has on cell growth. The chemical analysis of the shell and the cells will only be performed after the expedition, back at ETH Zurich, as a special mass spectrometer is needed for measuring the extremely small amounts of trace elements in the diatom shells.

Understanding how the diatoms affect nutrient cycles in the world’s oceans helps scientists to assess the potential consequences of climate change. “If global warming causes the temperature to rise or salt content of seawater to drop, the ocean currents and the distribution of trace elements and nutrients might change as well, which would in turn affect diatoms and their biological activity” Professor Vance stresses.