CIRCADTAN PERTODIOI'PIES TN PHYTOYLANKTON
329
Counts on ten Exuwiu~Zlrc red ticls (ditioflngellates) showed a rate of 0.3
tlivisioos per tlaty (Endo, 1970). Gentmition times obtained in the same
way by Wood i d Corcoriin ( 1 !M6) were much fwter: 4 to 8 hours. A
freshwtltcr Coruliumi divided 0-2 times daily (Nauwerck, 1903).
Another series of observations is bltstxl on morphological characteristics of the organisms. I)ecreiwe in diutoms cell size allowed Cushing
(1!16ti) to c!stimi~te the ciitiiy division rate for several species as varying
from 0.0114 to 2.910 mid, for another set of date (Cushing, 1963), from
0.48 to 0.93 ; however, the principle of decreasing size in diatoms must,
be iised with much caution (see e.g. Margelef, 1969). On the other
hand, Swift and Ilurbin (1 972) counted the dividing cells of the dinoflttgelltct,e I'yrocplis nt interval8 and, nssuming synchronous division
(see below), wcre itble to eatimate the gcnertltion times as 4-19 days,
wliich are icmong tlw slowest known; Apstein's (191 I ) earlier study
on Cerntium showed a similar range.
B. An order of magnitude
The niitin conclusion wising from the observations reported above
is that generation time of phytoplankhrs varies from u few hours to a
Sew day8 tmtl commonly comes around 24 hours; other conclusions
such as tirxonomicnl or geographical differcnces would be anticipated.
The above applies mainly to optimal conditions : it must he suspected
that, under limitation by light or nutrients, when cells are merely
surviving rather than growing, life span tends to be longer (only a few
of the above r t d t s , however, apply to these conditions).
Bruce (1965) hiis reviewed the subject for a variety of unicellular
and multicclliilur plants end rmimals, rind he concludes that " it is
niore thicn coincidence that generation times in the vicinity of twentyfour hr w e in some sense ' favoured ' by many cell types ". No more is
it i i coincidence that physiologists frequently Hucceed in synchronizing
micro-algae on ic 24-hour period.
Thutl it is quite reasonable to focus attmtion on the circadian period,
which turns out to be the real tinie-scale for the phytoplankton cell
(generation tinies of zoop1a;nktei-s are longer by one or several orders of'
magnitude). It is even s'urpric3ing that much more effort has been
devoted to the study of annual cycles ; points of view differ in the two
cases, in the same way as do, among human beings, the cope^ of
medicine and history.
IIr. CELL DIVISION AND SYNCHRONY
Unicellular nlgee grown in light/dark cycles a t the laboratory, under
good or optiinul growth conditions, generally become more or less
329
Counts on ten Exuwiu~Zlrc red ticls (ditioflngellates) showed a rate of 0.3
tlivisioos per tlaty (Endo, 1970). Gentmition times obtained in the same
way by Wood i d Corcoriin ( 1 !M6) were much fwter: 4 to 8 hours. A
freshwtltcr Coruliumi divided 0-2 times daily (Nauwerck, 1903).
Another series of observations is bltstxl on morphological characteristics of the organisms. I)ecreiwe in diutoms cell size allowed Cushing
(1!16ti) to c!stimi~te the ciitiiy division rate for several species as varying
from 0.0114 to 2.910 mid, for another set of date (Cushing, 1963), from
0.48 to 0.93 ; however, the principle of decreasing size in diatoms must,
be iised with much caution (see e.g. Margelef, 1969). On the other
hand, Swift and Ilurbin (1 972) counted the dividing cells of the dinoflttgelltct,e I'yrocplis nt interval8 and, nssuming synchronous division
(see below), wcre itble to eatimate the gcnertltion times as 4-19 days,
wliich are icmong tlw slowest known; Apstein's (191 I ) earlier study
on Cerntium showed a similar range.
B. An order of magnitude
The niitin conclusion wising from the observations reported above
is that generation time of phytoplankhrs varies from u few hours to a
Sew day8 tmtl commonly comes around 24 hours; other conclusions
such as tirxonomicnl or geographical differcnces would be anticipated.
The above applies mainly to optimal conditions : it must he suspected
that, under limitation by light or nutrients, when cells are merely
surviving rather than growing, life span tends to be longer (only a few
of the above r t d t s , however, apply to these conditions).
Bruce (1965) hiis reviewed the subject for a variety of unicellular
and multicclliilur plants end rmimals, rind he concludes that " it is
niore thicn coincidence that generation times in the vicinity of twentyfour hr w e in some sense ' favoured ' by many cell types ". No more is
it i i coincidence that physiologists frequently Hucceed in synchronizing
micro-algae on ic 24-hour period.
Thutl it is quite reasonable to focus attmtion on the circadian period,
which turns out to be the real tinie-scale for the phytoplankton cell
(generation tinies of zoop1a;nktei-s are longer by one or several orders of'
magnitude). It is even s'urpric3ing that much more effort has been
devoted to the study of annual cycles ; points of view differ in the two
cases, in the same way as do, among human beings, the cope^ of
medicine and history.
IIr. CELL DIVISION AND SYNCHRONY
Unicellular nlgee grown in light/dark cycles a t the laboratory, under
good or optiinul growth conditions, generally become more or less