Download Acquired phototrophs as mediators of planktonic community dynamics

Acquired Phototrophs as Mediators of Planktonic Community Dynamics
Holly V. Moeller*, Matthew D. Johnson, and Michael G. Neubert
Biology Department, Woods Hole Oceanographic Institution
Stolen photosynthetic potential Dynamics depend on light and traits Mediators of seasonal blooms?
Acquired phototrophs1 gain
photosynthesis by stealing functional
plastids from phytoplankton.
Omnipresent in planktonic food webs,
acquired phototrophs can have substantial
impacts on community dynamics. For
example, the coastal ciliate Mesodinium
rubrum forms red-water blooms2.
An M. rubrum red tide seen from
space (NASA). Inset: M. rubrum and a
cryptophyte prey cell.
For a strict acquired phototroph (f = 1), the model transitions through four kinds
of dynamics as light increases:
1.  No species (equilibrium point)
2.  Phytoplankter only (equilibrium point)
3.  Stable coexistence (equilibrium point)
4.  Population oscillations (limit cycle)
In isolation, the phytoplankton population tracks light availability, producing
a seasonal bloom. Consumers curtail this bloom. However, only strict
acquired phototrophs exhibit a subsequent bloom themselves.
Acquired phototrophs may be strict (>90%
carbon from photosynthesis) or
mixotrophic (combining photosynthesis
and heterotrophy3).
Modeling acquired phototrophy
We modified the Huisman-Weissing model4 of
phytoplankton competition in a well-mixed water
column.
The acquired phototroph’s traits affect these dynamics.
Higher attack rates and longer plastid retention times reduce the light thresholds
for coexistence and boom-bust population cycles.
Experimental validation
The phytoplankter W grows photosynthetically and is predated.
The acquired phototroph has two states:
CH (heterotrophic state) grazes on W. A fraction f of predation events
lead to acquired phototrophy (transition to CP).
CP (phototrophic state) cannot replicate acquired photosynthetic
machinery, so phototrophy produces CH. Plastids degrade at a rate m.
We co-cultured the
strict acquired
phototroph M.
rubrum and its algal
prey Geminigera
cryophila to confirm
the model’s
predictions about
light sensitivity.
Experimental data
qualitatively
matched model
predictions.
Each organism absorbs light, so the total absorptivity is:
This matches field observations:
• Strict acquired phototrophs M. rubrum and green Noctiluca scintillans5
have been observed blooming.
• No blooms of oligotrich ciliates and other mixotrophic acquired
phototrophs have been recorded.
Summary
We use an experimentally validated mathematical model to explore the
effects of acquired photosynthesis on community dynamics.
Depending on light availability, acquired phototrophs can drive population
cycles: First, they graze down their phytoplankton prey while undergoing a
population boom. However, without available prey to supply photosystems,
their population then crashes, at which point the prey population booms.
These intrinsic cyclic dynamics may explain why strict acquired phototrophs
are known bloom-formers, but mixotrophic acquired phototrophs are not.
References
Johnson, M. D. (2011). Photosynthesis Research, 107, 117–132.
1
2Crawford,
D. W., Purdie, D. A. & Lockwood, A. (1997). Estuarine, Coastal and
Shelf Science, 45, 799-812.
3McManus, G. B., Schoener, D. M. & Haberlandt, K. (2012). Frontiers in
Microbiology, 3, 1–9.
4Huisman, J. & Weissing, F. J. (1994). Ecology, 75, 507–520.
5Hansen, P. J., Miranda, L. & Azanza, R. (2004). Marine Ecology Progress Series,
275, 79–87.
*[email protected]. HVM gratefully acknowledges funding from an NSF
Postdoctoral Research Fellowship in Biology.