Agusti, S., et al. “Ubiquitous healthy diatoms in the deep sea confirm deep carbon injection by the biological pump.” Nature communications 6 (2015) DOI: 10.1038/ncomms8608
The ocean helps to mediate global climate, especially through the absorption of anthropogenic CO2. The role of the ocean as a sink for CO2 depends on the vertical transport of carbon. If carbon reaches depths below 1,000 m, it cannot easily escape back into the atmosphere. How does this carbon travel thousands of meters below the surface? As it enters the ocean, gaseous CO2 is used by phytoplankton to carry out photosynthesis. Individual phytoplankton cells (which now contain organic carbon) will sink, albeit at extremely slow rates (~ 1.5 m/day). However, sinking rates can be greatly accelerated if phytoplankton cells stick together or become packaged in dense fecal pellets (via grazers). Thus, fast-sinking phytoplankton particles can facilitate the vertical transport of organic carbon. Though shown to reach the deep ocean, it is unclear if this source of organic carbon remains fresh. Are phytoplankton cells capable of survival as they descend into deeper water?
Using a new plankton sampling device called the Bottle-Net, researchers tested for the presence of healthy phytoplankton cells in the deep sea (2,000-4,000 m). The Bottle-Net is ideal for dilute deep sea environments because it can filter large quantities of seawater. This global sampling effort spanned tropical and subtropical areas of the Atlantic, Indian and Pacific Oceans. The health, abundance and diversity of the deep phytoplankton community was assessed and compared to the surface community.
Phytoplankton cells were found in the deep ocean throughout the entire sampling track. Though present, the concentration of phytoplankton cells in the deep ocean was low, averaging 2.5 x 10^5 cells/m^2 (Fig. 1). Overall, diatoms dominated the phytoplankton community in the deep sea (81.5%). Diatoms are silica-rich phytoplankton that have heavy outer coatings known to readily sink in the water column. Other phytoplankton groups such as dinoflagellates and small cyanobacteria were also represented (Fig. 2). Though at lower concentrations, the same type of phytoplankton we see at the ocean’s surface were found at depth.
Phytoplankton were collected in the deep ocean, but were they still alive? On average, 18.24 ± 2.4% of the phytoplankton cells present in the deep ocean had intact plasma membranes. This means that less than a quarter of the phytoplankton community survived the downward trek and were in good living condition. Laboratory experiments were used to estimate the amount of time phytoplankton populations at the surface would survive when exposed to deep ocean conditions (i.e. very cold temperatures, no sunlight and high pressure). Living phytoplankton populations exposed to these conditions experienced high decay rates, with half-lives ranging from 3 to 10 days (Fig. 3). In the laboratory, the time for living cells to decline to 18% of the community (the percentage of living cells found in the deep ocean) ranged between 6.8 and 24.2 days. It is estimated that phytoplankton can survive for only a few weeks at extreme depths!
Discussion and Significance:
Individual phytoplankton cells that sink from the surface at rates of 1.5 m/day would take around 3.8-7.3 years to reach depths of 2,000-4,000 m. At this slow pace, one may expect little organic carbon to ever reach the deep ocean. As a phytoplankton cell descends deeper from the surface, the conditions become more stressful. Microscopic phytoplankton need to travel the length of nearly 32 football fields (i.e. 4,000 m) to reach the deep ocean. Hence, the presence of healthy phytoplankton cells seen in this study can only be explained by fast-sinking rates (few days to a few weeks). The well-preserved phytoplankton observed in the deep sea must have been injected within fast-sinking fecal pellets or cell aggregates.
Results from this global study confirm that fast-sinking mechanisms can transport fresh organic carbon down into the deep ocean. By keeping carbon in the ocean and preventing it from re-entering the atmosphere as CO2, the ocean acts to mediate Earth’s climate. This carbon cycle also impacts organisms living in the deep ocean which rely upon nutrients from sinking phytoplankton particles. The cycling of carbon in the ocean is an important topic and further research is needed to describe the ocean’s role in transferring carbon, especially with increasing CO2 levels in the atmosphere.
How may increasing atmospheric CO2 impact the ocean?