Session 5

Mixotrophy & Macromolecular Models

Papers to Read Before!

This week, we'll explore the shake-up of mixotrophy in the phytoplankton and its ecological consequences.
Stoecker 2017: why mixotrophy is important in phytoplankton
Caron 2016: a review of marine mixotrophy

Mixotrophy as a life history strategy

Mixotrophy is a key strategy among the plankton that could greatly increase primary productivity and trophic transfer. Under upwelling conditions when nutrients are abundant, the ability to use photosynthesis to supplement ordinary heterotrophic nutrient uptake could represent a major advantage.

Mixotrophy itself is defined as the ability to take up alternative nutrients (i.e., that a phototroph uses some other means than normal dissolved uptake to acquire nutritions). The other characterization point for mixotrophs is that the definition of photoautotroph requires that, under normal conditions, they fix carbon using light. Phototrophy, by contrast, simply requires chemical energy (ATP/photoreductant NADPH) to be produced using a light source. All of the fixed carbon could potentially be coming from prey ingestion.

Productivity is enhanced in particulate organic carbon-ingesting mixotrophs because of an increase in gross growth efficiency alongside flexibility in trophic strategy. Gross growth efficiency is given by:

\[ \textrm{GGE} = \frac{\textrm{Growth}}{\textrm{Ingestion}} \times 100 = \frac{\textrm{G}}{\textrm{I}} \times 100 \]

Mixotrophs can be divided into two categories based on the necessity that they behave as mixotrophs:

  • Facultative mixotrophy: the mixotroph can survive without one or the other strategy

  • Obligate mixotrophy: the mixotroph must engage in mixotrophy/have light availability to survive

Further, mixotrophs can be classified by whether or not they have to actively take up photosynthetic machinery from prey in order to use light:

  • Constitutive mixotrophs: they have the photosynthetic machinery all built in

  • Non-constitutive mixotrophs: the photosynthetic machinery is transient/needs to be replenished

Alternatively, download the PDF to view it: Download PDF.

Photoheterotrophy

Recently, evidence has come to light that some phytoplankton may engage in photoheterotrophy, or light-enhanced consumption of organic matter. This 2017 paper by John Casey sheds some light on that process and its mechanics:

Alternatively, download the PDF to view it: Download PDF.