498 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

In the North Sea, herring avoid the ocean areas where there are Phaeocystis blooms because of its unpalatability to the fish (Savage, 1930). Also, for a number of years biologists were puzzled by the near sterility of the intestines of certain Antarctic birds in regard to Protozoa and microorganisms. This condition was shown to be due to the large quantities of Phaeocystis in their diet, with Phaeocystis producing large amounts of acrylic acid which has a strong bactericidal action. Phaeocystis secretes 16% to 64% of the carbon assimilated in photosynthesis as polysaccharides of varying molecular weight. As much as 7 g liter 1 of acrylic acid, and at least 0.3 mg liter 1 of polysaccharides, can be released in the formation of a dense bloom (Guillard and Hellebust, 1971; van Rijssel et al., 1997).

Classification

The Prymnesiophyceae can be divided into two orders (Edvardsen et al., 2000; Fujiwara et al., 2001):

Order 1 Prymnesiales: cells with two equal smooth flagella, no eyespot, scales commonly covering the cell body.

Order 2 Pavlovales: cells with two unequal flagella often covered with hairs and deposits, eyespots may be present.

Prymnesiales

Emiliania huxleyi (Fig. 22.16(b)) is typical of the order with motile cells that have two flagella. The life cycle of E. huxleyi (Fig. 22.19) probably involves a diploid, non-motile phase with coccoliths, which alternates with a haploid phase that has scales but no coccoliths (Paasche, 2002).

Every spring from 1978 to 1983, large areas (10 to 100 square miles) of water off the Atlantic Coast of France and southern England gave a strong reflectance of visible light to the Nimbus space satellite. Samples showed that the water contained mostly the coccolithophorid Emiliania huxleyi (Fig. 22.16(b)). The large coccoliths of E. huxleyi resulted in the high light reflectance which was picked up by the satellite (Holligan et al., 1983).

Fig. 22.19 The probable life history of Emiliania huxleyi.

(Adapted from Green et al., 1996; Klaveness, 1972.)

PRYMNESIOPHYTA 499

The life cycle of the coccolithophorid Hymenomonas carterae is more complex (Fig. 22.20).

H. carterae has a haploid Apistonema stage (21 1 chromosomes) that consists of a filamentous branched system of spherical or elongate cells. Cells of the Apistonema stage can give rise either to asexual swarmers, which form new Apistonema thalli, or to motile gametes. Each asexual swarmer has two long flagella and a cup-shaped chloroplast, and the cell is covered with uncalcified scales. Gametes can be distinguished from asexual swarmers by their smaller size and the reduced number or absence of chloroplasts. After fusion of two gametes, the resulting zygote develops coccoliths. The diploid Hymenomonas stage (421 chromosomes) has an outer layer of coccoliths under which are several layers of organic scales. A

short haptonema is present between the flagella. The coccolith-bearing stage can perpetuate itself asexually by mitotic divisions. Under certain circumstances the protoplasts of Hymenomonas divide meiotically to produce four meiospores, which form haploid Apistonema thalli (von Stosch, 1967; Leadbeater, 1970; Parke, 1971).

The non-motile benthic stages of this order are resistant to fairly wide variations in the environment. The benthic stage of Cricosphaera sp. can survive temperatures of 35 to 40 °C for an hour or more as well as deep freezing for periods up 4 days. The same benthic phase is able to withstand a salinity up to the crystallization of the salt in solution. Normally benthic stages of Cricosphaera sp. grow near the high-water mark where they are presumably exposed to great variations in the

Fig. 22.20 The life history of

Hymenomonas carterae.