153
where C i ChI is the intracellular ChI a concentration and a* is the ChI-specific absorption
coefficient, here taken equal to 20.7 m 2 (g ChI at', as in Bricaud et al., (1983).
To the extent that the initial part of the Qa curve can be regarded as almost linear, the package
effect no longer exists. When expanded, Eq. 10 becomes
Q A - (2/3) p'- (1/4) p'2
(10')
and can be reduced to its first term (within an accuracy of 10 %) as long as p' does not
exceed 0.26. This value delimits the quasi-linear domain with no package effect. This linear
approximation is relevant not only for very small algal cells, but also for bacteria or
heterotrophic flagellates (even at their absorption maximum, 415 nm). Some larger species,
like diatoms, may nevertheless belong to this domain, because of their very low pigment per
cell content (see Table 1 and Fig. 11 in Morel and Bricaud, 1986). Conversely small species
with high C i ChI values can experience an important package effect, in particular those species
able to develop a dense pigmentation in response to low radiative level, (see e.g. Isochrysis
galbana, Table 1 in the above reference). The absorptive properties of tiny recently
discovered marine prochlorophytes (Chisholm et al., 1988; Vaulot et al., 1990) remain to be
studied.
Scattering cross section
The approximation of the van de Hulst anomalous diffraction also provides the efficiency
factor for scattering, Qb, as a function of p (Eq. 7), when the (spherical) particle is nonabsorbing, or as a function of p and p' when absorption occurs within the particle.
Considering the first case (P' = 0 and thence Qa = 0), Qb is expressed as
(11)
graphically represented in Fig. 2 (note that Qb = Qc in this case). The successive maxima and
minima are progressively reduced in amplitude and the limiting value is 2 (when p
approaches 00). For such large particles, twice the amount of energy geometrically
where C i ChI is the intracellular ChI a concentration and a* is the ChI-specific absorption
coefficient, here taken equal to 20.7 m 2 (g ChI at', as in Bricaud et al., (1983).
To the extent that the initial part of the Qa curve can be regarded as almost linear, the package
effect no longer exists. When expanded, Eq. 10 becomes
Q A - (2/3) p'- (1/4) p'2
(10')
and can be reduced to its first term (within an accuracy of 10 %) as long as p' does not
exceed 0.26. This value delimits the quasi-linear domain with no package effect. This linear
approximation is relevant not only for very small algal cells, but also for bacteria or
heterotrophic flagellates (even at their absorption maximum, 415 nm). Some larger species,
like diatoms, may nevertheless belong to this domain, because of their very low pigment per
cell content (see Table 1 and Fig. 11 in Morel and Bricaud, 1986). Conversely small species
with high C i ChI values can experience an important package effect, in particular those species
able to develop a dense pigmentation in response to low radiative level, (see e.g. Isochrysis
galbana, Table 1 in the above reference). The absorptive properties of tiny recently
discovered marine prochlorophytes (Chisholm et al., 1988; Vaulot et al., 1990) remain to be
studied.
Scattering cross section
The approximation of the van de Hulst anomalous diffraction also provides the efficiency
factor for scattering, Qb, as a function of p (Eq. 7), when the (spherical) particle is nonabsorbing, or as a function of p and p' when absorption occurs within the particle.
Considering the first case (P' = 0 and thence Qa = 0), Qb is expressed as
(11)
graphically represented in Fig. 2 (note that Qb = Qc in this case). The successive maxima and
minima are progressively reduced in amplitude and the limiting value is 2 (when p
approaches 00). For such large particles, twice the amount of energy geometrically