Organic compounds drive growth in phytoplankton taxa from different functional groups

Phytoplankton are usually considered autotrophs, but an increasing number
of studies shows that many taxa are able to also utilise organic carbon.
Acquiring nutrients and energy from different sources might enable an
efficient uptake of required substances and provide a strategy to deal
with a varying resource availability, especially in highly dynamic
ecosystems such as estuaries. In our study we investigated the effects of
31 organic carbon sources on the growth (proxied by differences in cell
counts after 24 h exposure) of 17 phytoplankton strains from the Elbe
estuary spanning four functional groups. All of our strains were able to
make use of at least 1 and up to 26 organic compounds for growth.
Pico-sized green algae such as Mychonastes, as well as the nano-sized
green alga Monoraphidium in particular were positively affected by a high
variety of substances. Reduced light availability, typically appearing in
turbid estuaries and similar habitats, resulted in an overall poorer
ability to utilise organic substances for growth, indicating that organic
carbon acquisition was not primarily a strategy to deal with darkness. Our
results give further evidence for mixotrophy being an ubiquitous ability
of phytoplankton and highlight the importance to consider this trophic
strategy in research.