Chlorophyll to Carbon Ratio Derived From a Global
Ecosystem Model With Photodamage
E. Álvarez
1
, S. Thoms
1
, and C. Völker
1
1
Helmholtz Centre for Polar and Marine Research, Alfred Wegener Institute, Bremerhaven, Germany
Abstract
Phytoplankton harvests light by integrating chlorophyll in protein-pigment complexes
(photosystems) that are variable in number and size. In ecosystem models, the capacity of light harvesting
is described as the pool of chlorophyll. Since most of the variability in phytoplankton chlorophyll content is
driven by acclimation to changing nutrient and light conditions, photoacclimation is generally parameterized
as a regulation of chlorophyll synthesis with changing light. However, photosystems can also be degraded,
and of the few process-based models that have been proposed in the literature for the representation of their
degradation and repair, none of them have been extended to more realistic conditions offered by pelagic
biogeochemical models. We proposed three potential parameterizations to treat the degradation of
photosystems as a function of light intensity and included them as a source of variation in the size of the
chlorophyll pool in Regulated Ecosystem Model, version 2 (REcoM2). These model versions provided
chlorophyll values highly correlated with satellite chlorophyll and accurate patterns of the chlorophyll to
carbon ratio at global scale. The improvement in the prediction of the ratios was remarkable in scenarios
where cells are exposed to periods of low light conditions. By isolating the effects of light and nutrients on the
variability of the chlorophyll to carbon ratio and the growth rate, we observed a potential reduction in
photosynthetic performance under simultaneous light saturation and nutrient stress. This effect, whose
strength depends on the presence of photoprotective mechanisms and the degree of nutrient limitation,
allowed to assess the role of photodamage and photoprotection under stress conditions.
1. Introduction
Most biogeochemical (BGC) models use carbon or nitrogen to quantify model microalgal biomass. However,
they are often assessed using phytoplankton pigment concentration. Chlorophyll a (Chla) is a common input
of real data to models that can be estimated on a global scale since late 1990s. To bridge the gap between
Chla-based observations and carbon-based BGC models, it is now becoming common in global BGC models
to include dynamic Chla to carbon ratios (ϴ; e.g., Arteaga et al., 2016; Behrenfeld et al., 2005; Dunne et al.,
2013; Wang et al., 2009). Little attention has so far been paid to check the assumptions inherent in the used
parameterizations of ϴ (which are often based on steady state assumptions, as, e.g., in Dunne et al., 2013) or
to validate the modeled ϴ with observational data.
The algal growth models that account for the variability in ϴ fall into two categories. Empirical models
describe directly ϴ as a function of growth conditions (Cloern et al., 1995) masking all processes that contri-
bute to variability of ϴ. On the other hand, mechanistic models require the explicit description of all
processes that cause stoichiometry variability. This can be done by modeling the variability of ϴ as a function
of input variables (Pahlow et al., 2013) or, resolving phytoplankton into metabolic pools that are flexible in
size, with separate functions for the acquisition of each resource (Geider et al., 1998). This last model type
does not assume that cells are necessarily acclimated at all points in time.
The phytoplankton growth model proposed by Geider et al. (1998) describes the differential allocation of
resources to two metabolic pools, carbon biomass, and the light harvesting apparatus. The light harvesting
apparatus is formed by functional units that contain proteins and pigments, known as photosystems I and II
(PSI and PSII). The main pigment in these complexes that is present in all photoautotroph organisms is Chla.
Hence, in the model, the metabolic pool that accounts for the size of the light harvesting apparatus is referred
as the Chla pool. So that, when describing changes in the Chla pool, this then refers to changes in the size or
number of the protein pigment complexes. Free pigments are, in this context, nonfunctional pigments.
Most of the variability in ϴ is driven by acclimation to changing nutrient and light conditions (MacIntyre et al.,
2002). The process of adjustment in cellular pigmentation to match the demands for photosynthesis is
ÁLVAREZ ET AL. 799
Global Biogeochemical Cycles
RESEARCH ARTICLE
10.1029/2017GB005850
Key Points:
• Potential parameterizations for the
loss term in the chlorophyll pool are
included in the REcoM2
biogeochemical model
• Balance between the degradation and
synthesis of photosystems generates
phytoplankton pigment content in
agreement with field and laboratory
data
• Nutrient-limited chlorophyll synthesis
cannot compensate the loss of
functional pigmentation under light
stress and thereby limiting
photosynthetic activity
Supporting Information:
• Supporting Information S1
• Data Set S1
Correspondence to:
E. Álvarez,
eva.alvarez@awi.de
Citation:
Álvarez, E., Thoms, S., & Völker, C. (2018).
Chlorophyll to carbon ratio derived
from a global ecosystem model with
photodamage. Global Biogeochemical
Cycles, 32, 799–816. https://doi.org/
10.1029/2017GB005850
Received 27 NOV 2017
Accepted 16 APR 2018
Accepted article online 24 APR 2018
Published online 9 MAY 2018
©2018. American Geophysical Union.
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