66
The Radiocarbon Method to Estimate Primary Production
on filtration losses in 14C assimilates, the working solution should be prepared
de novo from the concentrated batches.
2. The possibility of fixing samples before filtration. Such a possibility
obviously is convenient for field work and ship conditions, because it is not
always possible to filter all the samples instantly, while their storage in the dark
at the end of exposure for some time can result III respiratory losses of 14C
assimilates, etc. Evidence of a drastic rupture of phytoplankton cells by
fixatives and subsequent losses of assimilated 14C-radioactivity in the fixed
samples after 14C-experiments was published (Ilmavirta and Jones 1977; Silver
and DavolI 1978), but is insufficient to convince us to reject fixation completely. The above authors used either formalin or a rather strong Lugol iodine
solution, which was unacceptable. In any case, in tropical oceanic waters, any
fixation at all should be avoided. Samples should be filtered within 1 h after
the end of incubation. However, samples from more productive regions can
safely survive a weak Lugol fixation for some hours without detectable losses;
but to be sure that these losses do not take place, it is better to test it first
experimentally with samples from the water body under investigation.
3. The use of the filtration procedure as the basic way to separate 14C
assimilated by algae for radioassaying. This was also a point of discussion
(Berman 1973; Gachter et al. 1984), some authors claiming, that normally the
use of 0.45-mm membrane filters was not suitably, as their use entailed drastic
losses of 14C-radioactivity by phytoplankton as a result of ruptured algal cells.
They suggested either using ultrafine glass fiber filters, or reject the filtration
procedure totally and count radioactivity in a volume of 5-8 ml of a sample
previously acidified and bubbled for some 1-2 h with air or with CO2 gas to
avoid radioactive bicarbonate and 14C02 from the sample (so-called bubbling
method; Shindler et al. 1972; Larson and Hagstrom 1982). This bubbling
method in any case found no practical use because of its high time costs and
the poor reproducibility of results. Concerning the use of glass fiber filters for
filtration, these appeared to be practically useful, mostly for filtration of
samples taken in basins with blooming mucoid cyanobacteria. In other cases,
no difference was discovered when comparing glass fiber and ordinary membrane filters, just as no extra gain in radioactivity was proved later by using
the bubbling method instead of filtration to estimate 14C assimilation rate by
phytoplankton (Sondergaard 1985).
4. Loss of assimilated 14C by planktonic algae after filtration (Fogg 1966;
Sharp 1977; Peterson 1978; Harris 1980; Goldman and Dennett 1985) can be
due to the exudation of fixed 14C-carbon as dissolved organic matter by algal
cells stressed by the excess of light, confinement, and nutrient deficiency (Watt
1965; Nalevajko et al. 1976; Mague et al. 1980). They can lose 14C assimilates
as glycollates and other primary products of photosynthesis due to the regulatory glycollate biochemical mechanism functioning in them (Harris 1986;
Bjornsen 1988; Wood and Van Valen 1990). Loss of assimilated 14C during
filtration occurs also as the result of the rupture of algal cells during their collision with the solid surface of the filter during acid rinsing or fixation. Also
The Radiocarbon Method to Estimate Primary Production
on filtration losses in 14C assimilates, the working solution should be prepared
de novo from the concentrated batches.
2. The possibility of fixing samples before filtration. Such a possibility
obviously is convenient for field work and ship conditions, because it is not
always possible to filter all the samples instantly, while their storage in the dark
at the end of exposure for some time can result III respiratory losses of 14C
assimilates, etc. Evidence of a drastic rupture of phytoplankton cells by
fixatives and subsequent losses of assimilated 14C-radioactivity in the fixed
samples after 14C-experiments was published (Ilmavirta and Jones 1977; Silver
and DavolI 1978), but is insufficient to convince us to reject fixation completely. The above authors used either formalin or a rather strong Lugol iodine
solution, which was unacceptable. In any case, in tropical oceanic waters, any
fixation at all should be avoided. Samples should be filtered within 1 h after
the end of incubation. However, samples from more productive regions can
safely survive a weak Lugol fixation for some hours without detectable losses;
but to be sure that these losses do not take place, it is better to test it first
experimentally with samples from the water body under investigation.
3. The use of the filtration procedure as the basic way to separate 14C
assimilated by algae for radioassaying. This was also a point of discussion
(Berman 1973; Gachter et al. 1984), some authors claiming, that normally the
use of 0.45-mm membrane filters was not suitably, as their use entailed drastic
losses of 14C-radioactivity by phytoplankton as a result of ruptured algal cells.
They suggested either using ultrafine glass fiber filters, or reject the filtration
procedure totally and count radioactivity in a volume of 5-8 ml of a sample
previously acidified and bubbled for some 1-2 h with air or with CO2 gas to
avoid radioactive bicarbonate and 14C02 from the sample (so-called bubbling
method; Shindler et al. 1972; Larson and Hagstrom 1982). This bubbling
method in any case found no practical use because of its high time costs and
the poor reproducibility of results. Concerning the use of glass fiber filters for
filtration, these appeared to be practically useful, mostly for filtration of
samples taken in basins with blooming mucoid cyanobacteria. In other cases,
no difference was discovered when comparing glass fiber and ordinary membrane filters, just as no extra gain in radioactivity was proved later by using
the bubbling method instead of filtration to estimate 14C assimilation rate by
phytoplankton (Sondergaard 1985).
4. Loss of assimilated 14C by planktonic algae after filtration (Fogg 1966;
Sharp 1977; Peterson 1978; Harris 1980; Goldman and Dennett 1985) can be
due to the exudation of fixed 14C-carbon as dissolved organic matter by algal
cells stressed by the excess of light, confinement, and nutrient deficiency (Watt
1965; Nalevajko et al. 1976; Mague et al. 1980). They can lose 14C assimilates
as glycollates and other primary products of photosynthesis due to the regulatory glycollate biochemical mechanism functioning in them (Harris 1986;
Bjornsen 1988; Wood and Van Valen 1990). Loss of assimilated 14C during
filtration occurs also as the result of the rupture of algal cells during their collision with the solid surface of the filter during acid rinsing or fixation. Also