Modelling alkaline phosphatase activity in microalgae under orthophosphate limitation: the case of Phaeocystis globosa
J. Plankton Res. (
Modelling alkaline phosphatase activity in microalgae under orthophosphate limitation: the case of Phaeocystis globosa
CAROLINE GHYOOT 0
NATHALIE GYPENS 0
KEVIN J. FLYNN
CHRISTIANE LANCELOT 0
0 UNIVERSIT E ́ LIBRE DE BRUXELLES
Many phytoplankton exploit phosphorus (P) from organic sources when dissolved inorganic P (DIP) is depleted. This process is, however, rarely considered in ecological and biogeochemical models. We present a mechanistic model describing explicitly the ability of phytoplankton to use dissolved organic P (DOP) when DIP is limiting, by synthesizing alkaline phosphatase (AP) that releases DIP from DOP. This model, applicable to any phytoplankton species expressing AP, is here specifically developed for the colony-forming Phaeocystis globosa. It describes the main processes related to P metabolism, including DIP transport, intracellular accumulation and assimilation. Model behaviour is explored in DIP-limiting batch-type conditions for different DOP ranging between 0 and 1.5 mmol P m23. Simulations show that the DOP-derived DIP increases the maximum biomass reached and extends the period of net growth. The magnitude of the enhanced biomass production is controlled by the DOP initially present as well as the released DOP, the latter being recycled by lysis of P. globosa cells. We also present a simplified model version derived from the mechanistic model, which involves fewer state variables and parameters. The latter is directly usable in both variable (quota-type) and fixed stoichiometry descriptions of phytoplankton growth. available online at www.plankt.oxfordjournals.org # The Author 2015. Published by Oxford University Press. All rights reserved. For permissions, please email:
P limitation; alkaline phosphatase activity; modelling; Phaeocystis globosa
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I N T R O D U C T I O N
Phosphorus (P) is fundamental to all life processes and is
therefore an essential element for growth. The preferred
source of P for phytoplankton is dissolved inorganic
P (DIP) (Cembella et al., 1984). In the absence of
sufficient DIP, various metabolic processes are de-repressed
to enable a more efficient use of internal P and/or to
allow acquisition of P from organic sources (Gobler et al.,
Observations Phosphate transport (i) The maximum specific DIP uptake rate is regulated by the internal P content.
Phosphate storage
(ii) Only a modest amount of P accumulation is observed in
the presence of excess P (for P. globosa only two cells
division are allowed with the accumulated P)
(iii) Under DIP depletion, colony and single cells both
decrease their cellular P content
(iv) DIP can be absorbed and accumulated in the colonial
structure
Alkaline phosphatase activity
(v) Under DIP depletion colony and single cells have high
APA
(vi) AP synthesis occurs at the transition between the late
exponential phase and the early stationary phase when the
DIP concentration is very low
(vii) AP responds rapidly to fluctuations in P level in the
environment
(viii) AP synthesis is regulated by an internal pool that is
directly influenced by the influx of DIP into the cell
(xi) AP enzymes are rapidly lost when DIP is added back to
P-stressed cultures
Species
References
Riegman et al. (2000)
Phaeocystis sp.,
E. huxleyi
Jahnke (1989), Riegman et al.
(2000), Shaked et al. (2006)
Veldhuis and Admiraal (1987)
Veldhuis et al. (1991)
Veldhuis and Admiraal (1987)
Kuenzler and Perras (1965),
Dyhrman and Palenik (2003),
Xu et al. (2010)
Dyhrman and Palenik (2003), Xu et al. (2006)
original AQUAPHY is here modified to explicitly
describe the synthesis and the phosphohydrolytic activity of
AP (grey rectangle in Fig. 1A). The model structure
includes 12 state variables (Table II) linked by 19 processes
(Supplementary data, Appendix A and B). State variables
describing P. globosa physiology include: the functional and
structural metabolites (F) with the exception of the AP that
is described by a separate state variable, carbon monomers
(early products of photosynthesis; SC), internal carbon
reserves (i.e. carbohydrate, fatty acids; RC), intracellular
soluble phosphate (SP), polyphosphate reserves (RP) and
the extracellular mucus (M) in which colonial P. globosa
cells are embedded and that provides an extracellular
supplementary carbon reserve (Lancelot and Mathot, 1985).
The total C-cell biomass (mmol C m23) of P. globosa is
given by the sum of F, AP, SC and RC. The F pool is
assumed to have a fixed C:N:P stoichiometry
(Supplementary data, Appendix C) based on biochemical
constraints in agreement with Geider and Laroche
(Geider and Laroche, 2002). Similarly, AP has a fixed C:N
stoichiometry (assumed to be the same as for the F pool).
Variable cellular stoichiometry is enabled by taking
account of the additional C and P accumulated as carbon
monomers (SC), carbohydrates and fatty acids (RC),
soluble inorganic phosphorus (SP) and polyphosphates
(RP). Therefore, the C-biomass-based P-quota (PC) can be
calculated as th (...truncated)