Diel cell cycle analysis of Heterosigma akashiwo (Raphidophyceae) with special reference to vertical migration behavior

Abstract

The marine alga, Heterosigma
akashiwo(Raphidophyceae), is known to frequently
form blooms in temperate coastal waters throughout
the world (Horner et al. 1997; Smayda 1998; Taylor
and Haigh 1993; Taylor and Horner 1994). H.
akashiwo populations usually develop rapidly in early
summer (Imai 2000; Shikata et al. 2007), forming
dense blooms in the upper water column at or near the
surface (Handy et al. 2005; Honjo 1993; Yamochi and
Abe 1984). Its blooms can cause massive fish kills
through the production of chemicals such as
neurotoxins, cardiotoxins, and reactive oxygen (Endo
et al. 1992; Khan et al. 1997). In addition to the
significant impacts to fish populations, algal blooms
can cause a decrease in plankton biodiversity and are
sometimes accompanied by low oxygen in deeper
water (Tsutsumi 2006) which may kill shellfish and
other invertebrates. Given these potential impacts, a
better understanding of the conditions leading to an
algal bloom, and the bloom process itself, are
important in helping to protect coastal ecosystems.

The accumulation of algae on the surface
water is caused by biological processes such as growth
and vertical migration (Bearon et al. 2006) and/or
physical processes such as wind-driven advection (Hill
1991). These events should be considered separately in
the analysis of the bloom-development process. Since
cells of H. akashiwo can accumulate in “nutrientdepleted”
surface waters during the daytime and move
downward to “nutrient-rich” bottom waters at night
(Watanabe et al. 1983; Watanabe et al. 1988), growth
rate measurement for this species is quite difficult
when employing in situ incubation experiments using
bottles.