190
S.M. Coelho et al.
6.4.4 Diversity and Dynamics of Planktonic Ecosystems
High-throughput sequencing has been applied to only a limited number of samples so far and there is still an enormous potential for identifying new organisms
and understanding new ecosystems. High-throughput sequencing has confirmed that
individual planktonic ecosystems are remarkably diverse, but we also know that
there are significant differences between planktonic ecosystems in different areas of
the marine environment.
The oligotrophic environments of the open ocean differ markedly from
mesotrophic coastal areas (Guillou et al. 1999, Massana et al. 2004, Romari and
Vaulot 2004) and the communities of organisms found in arctic waters differ significantly from those of warmer waters (Lovejoy et al. 2006). One environment that is
attracting a lot of attention is sea-ice (Mock and Thomas 2005). The organisms that
are able to live in the extreme conditions of cold and highly variable salinity, pH
and light conditions found in this ecosystem are of interest for a number of reasons
ranging from biotechnological applications to understanding the origin of life on
Earth. The coming years will almost certainly see a more widespread application of
high-throughput sequencing to these diverse marine ecosystems.
Another important consideration is the dynamics of planktonic ecosystems. The
density and the types of organisms in these ecosystems can be highly variable,
being affected both by physical and chemical constraints (such as light, temperature and nutrient availability) and by interactions with other organisms. Predation by
other phytoplankton such as nanoflagellate protists or cellular lysis caused by viral
infection are two major forces impacting on phytoplankton populations. Predation
may cause nutrients such as carbon to be sequestered into sinking particles or
accumulated in larger animals, whereas viral lysis is likely to release more nutrients back into the environment (Suttle 2005). Collectively, the planet’s oceans are
thought to contain about 4×10 30 virus particles, probably including pathogens of
most, if not all, other marine organisms. These predators are therefore likely to
play a critical role in marine carbon cycling but little is known about their basic
biology. Marine microalgae are infected by a wide range of viruses (Nagasaki
2008) and genomic approaches have started to be used to study some of these,
such as the Coccolithovirus EhV-86, which infects Emiliania huxleyi (Wilson et al.
2005), and OtV5, which infects the prasinophyte Ostreococcus tauri (Derelle et al.
2008). Moreover, the Joint Genome Institute (California, USA) has recently initiated
sequencing of the genomes of heterotrophic nanoflagellates that predate microalgae and some uncultivated viruses, so more information about these important
organisms can be expected in the near future.
Algal blooms are particularly impressive examples of the dynamic nature of
planktonic ecosystems. These blooms, which involve rapid multiplication of one or
a small number of phytoplankton species, can have a significant impact on human
activity, particularly when the blooming alga produces toxins. Many toxic algal
blooms are caused by rapid growth of dinoflagellates in response to favourable
environmental conditions such as high temperatures, high nutrient concentrations
and a stagnant water column during the summer months. These so-called “red
S.M. Coelho et al.
6.4.4 Diversity and Dynamics of Planktonic Ecosystems
High-throughput sequencing has been applied to only a limited number of samples so far and there is still an enormous potential for identifying new organisms
and understanding new ecosystems. High-throughput sequencing has confirmed that
individual planktonic ecosystems are remarkably diverse, but we also know that
there are significant differences between planktonic ecosystems in different areas of
the marine environment.
The oligotrophic environments of the open ocean differ markedly from
mesotrophic coastal areas (Guillou et al. 1999, Massana et al. 2004, Romari and
Vaulot 2004) and the communities of organisms found in arctic waters differ significantly from those of warmer waters (Lovejoy et al. 2006). One environment that is
attracting a lot of attention is sea-ice (Mock and Thomas 2005). The organisms that
are able to live in the extreme conditions of cold and highly variable salinity, pH
and light conditions found in this ecosystem are of interest for a number of reasons
ranging from biotechnological applications to understanding the origin of life on
Earth. The coming years will almost certainly see a more widespread application of
high-throughput sequencing to these diverse marine ecosystems.
Another important consideration is the dynamics of planktonic ecosystems. The
density and the types of organisms in these ecosystems can be highly variable,
being affected both by physical and chemical constraints (such as light, temperature and nutrient availability) and by interactions with other organisms. Predation by
other phytoplankton such as nanoflagellate protists or cellular lysis caused by viral
infection are two major forces impacting on phytoplankton populations. Predation
may cause nutrients such as carbon to be sequestered into sinking particles or
accumulated in larger animals, whereas viral lysis is likely to release more nutrients back into the environment (Suttle 2005). Collectively, the planet’s oceans are
thought to contain about 4×10 30 virus particles, probably including pathogens of
most, if not all, other marine organisms. These predators are therefore likely to
play a critical role in marine carbon cycling but little is known about their basic
biology. Marine microalgae are infected by a wide range of viruses (Nagasaki
2008) and genomic approaches have started to be used to study some of these,
such as the Coccolithovirus EhV-86, which infects Emiliania huxleyi (Wilson et al.
2005), and OtV5, which infects the prasinophyte Ostreococcus tauri (Derelle et al.
2008). Moreover, the Joint Genome Institute (California, USA) has recently initiated
sequencing of the genomes of heterotrophic nanoflagellates that predate microalgae and some uncultivated viruses, so more information about these important
organisms can be expected in the near future.
Algal blooms are particularly impressive examples of the dynamic nature of
planktonic ecosystems. These blooms, which involve rapid multiplication of one or
a small number of phytoplankton species, can have a significant impact on human
activity, particularly when the blooming alga produces toxins. Many toxic algal
blooms are caused by rapid growth of dinoflagellates in response to favourable
environmental conditions such as high temperatures, high nutrient concentrations
and a stagnant water column during the summer months. These so-called “red