Article: Lehahn, Y. et al. Decoupling Physical from Biological Processes to Assess the Impact of Viruses on a Mesoscale Algal Bloom.Current Biology, 2014; DOI: 10.1016/j.cub.2014.07.046
Despite their small size, phytoplankton play an incredibly large role in maintaining ocean food webs and can even contribute to global climate. As plants, phytoplankton consume carbon dioxide and fix it into sugar molecules that they (and nearly every animal on earth) use for energy. This process literally removes CO2 from the environment and converts it to a sugar molecule. The amount of CO2 removed is related to how many phytoplankton cells there are and whether or not they are consumed by animals (which reconverts the sugar back into CO2) or sinks to the bottom of the ocean. The amount of phytoplankton that sinks to the deep ocean is a critical component of reducing CO2 concentrations in the atmosphere, removing that CO2 for up to centuries. The more phytoplankton that sinks, the more CO2 is removed from the atmosphere. Thus, it’s really important for scientists to study and understand how phytoplankton live and die.
One topic that still eludes scientists is the role that viruses play in these oceanic carbon movements. Some estimates place virus cells as possibly the most abundant cell on earth (certainly in the oceans as well). Despite this, oceanographers do not have a strong understanding of how these viruses interact with the rest of the ocean. Thankfully, more and more attention is being placed on the importance of viruses in the ocean. A team of scientists, led by Dr. Yoav Lehahn, has released a study showing the important role viruses play in determining the life expectancy of large scale phytoplankton growth (phytoplankton blooms).
Dr. Lehahn and his team used satellite data to find a study site that matched their criteria (learn how satellites are used in oceanography here and here). They ultimately settled on a site in the North Atlantic (Fig. 1). This patch was chosen due to its favorable conditions: a large phytoplankton bloom occurring in a calm, well stratified water column surrounded by surface currents. These conditions contained the phytoplankton bloom in a relatively static location.
Field measurements were taken to confirm satellite data. They took water samples during the phytoplankton bloom to characterize the species of phytoplankton present and measure nutrient concentrations.
Satellite imagery tracked the phytoplankton bloom from beginning to end. They found that the bloom lasted a total of 25 days and followed a typical “boom and bust” pattern with rapid growth in the beginning followed by a quick decline afterwards (Fig. 2). Due to the stratified water column and isolation from currents, the researchers concluded that bloom decline was not caused by dilution or mixing of phytoplankton out of the study site.
The researchers found that the main phytoplankton species was the coccolithophore Emiliana huxleyi which is a common bloom forming phytoplankton characterized by its heavy calcium carbonate plates (called coccoliths). Nutrient abundances werer consistent with typical phytoplankton blooms and thus bloom decline was not caused by nutrient limitation. However, Lehahn and colleagues identified large, viral-like particles in the seawater samples. They found concentrations of up to 2.5 million viral particles per milliliter of seawater (three times as abundant as the number of phytoplankton!). Laboratory analyses of the water samples identified the viral particles as coccolithovirus (Fig. 3) which specifically targets Emiliana huxleyi, causing cell damage and death (through a process called lysis). Thus, the researchers concluded that the phytoplankton bloom decline they had observed was due to viral control of E. huxleyi.
Viral cells are some of the most abundant particles found in the ocean. Despite this abundance, the role of viruses in the ocean is largely unknown. Lehahn et al. showed that viruses have the potential to regulate large scale phytoplankton blooms. Disentangling the influenceof the virus and limiting nutrients on bloom decline decoupled biological and physical controls, respectively, on this phytoplankton bloom. This decoupling stragety will be a useful tool in the future for studying the different components of the ocean that contribute to phytoplankton bloom dynamics.
The fate of the viruses and their dead host phytoplankton are still not known. During virus induced lysis, the host phytoplankton cells break apart and release the newly formed viruses as well as residual nutrients and other chemical elements. Ironically, these nutrients and compounds can be taken up and used to fuel new production, potentially facilitating the growth of other species of phytoplankton. These is great potential for competition between viruses and predators as viruses effectively kill and deplete prey sources (this is known as the viral shunt) which may affect larger consumers like fish and whales. Others hypothesize that some of the lysed host cells remain largely intact and sink to the bottom of the ocean, further increasing carbon sequestration.