Chapter 2 Marine Primary Producers 第 2 章 海洋初级生产者

Within the sunlit surface layer of the sea, marine primary producers thrive, ranging from extremely small cyano- bacteria （蓝细菌） to tree-sized kelp plants. The divisions of marine algae are characterized by their unique combinations of photosynthetic pigments, cell-wall structures, storage products, and growth forms.

Of these, only five are predominantly marine: the Cyanobacteria, Chrysophyta( 金藻门 ), Bacillariophyta （硅藻门）, Dinophyta （甲藻门）, Phodophyta （红藻门 ), Phaeophyta( 褐藻 门 ). Two other divisions, the Chlorophyta and Anthophyta, are found mostly in fresh water and on land, but they still contribute measurably to some coastal marine communities.

Section 1 Phytoplankton Almost all marine phytoplankton belong to three divisions in the kingdoms Monera （原核生物） and Protista （原 生生物）. They are found dispersed throughout the photic zone of the oceans and account for the major share of primary productivity in the marine environment.

Only in the last few years has it been possible to collect representative samples of the exceptionally small Pico- plankton （微微型浮游生物） ( ＜ 2 µ m) and ultra-plankton （超微型浮游生物） （ 2 － 5 µ m ）.

Presently, it is thought that the most important primary producers in all marine environments, but especially in oceanic waters, are nanoplankton-sized or small. Nanoplankton （微型浮游生物）： 5 － 20 µ m ； Microplankton （小型浮游生物） : µ m.

Cyno-bacteria Marine cynobacteria have been the object of much recent study.Their small cell size (most are less than 5 µ m ) make them very difficult to collect and examine.

Their cellular structure is typical of procaryotes, with only a few of the complex membrane-bound organelles so obviors in larger eucaryotic cells.

Unlike photosythetic bacteria, photosynthesis in cyanobacteria is similar to that in eucaryotic autotrophs in structure and function. It is bases on chlorophyll a and results in the production of oxygen.

Marine cynobacteria are especially abundant in intertidal and estuarine areas, with a lesser role in oceanic waters. Some species of cyanobacteria produce dense blooms in warm-water regions. The red phycobilin( 藻胆素 ) pigment of Oscillatoria （颤藻） is responsible for the color and name of the Red Sea.

Cyanobacteria are not newcomers to marine environments. Modern cyanobacteria descend from some of the earliest forms of life on earth. Fossil stromatolites( 叠层石 ) over three billion years old are remarkably similar to modern ones living at the edges of tropical lagoons in Australia.

Chrysophyta （金藻门） and Bacillariophyta( 硅藻门 ) The division consists of two classes: the Chrysophyceae and the Bacillariophyceae. The marine members of this division are single-celled. Like all other eucaryotic autotrophs, their primary photosynthetic pigment is chlorophyll a.

In addition, Chrysophyta have accessory chlorophyll c and golden or yellow-brown xanthophyll （叶黄素） pigments also characteristic of brown algae. Most have mineralized cell walls or internal skeletons of silica or calcium carbonate.

Some species possess flagella for motility but, like other planktonic organisms, can do very little to counter horizontal transport by water currents.

The most obvious and often abundant members of the phytoplankton are the diatoms (Bacillariophyceae ). Although diatom are unicellular, they may occur in chains or other loose aggregates of cell.

Cell sizes range from less than 15 µ m in length to 1 mm in length. Most diatoms are between 50 and 500 µ m in size.

Frustule( 细胞壁 ) Diatoms have a cell wall, or frustule, composed of pectin with large amounts ( up to 95%) of silica. The frustule consists of two closely fitting halves, the epitheca （上壳） and the hypotheca （下壳）.

chloroplasts Planktonic diatoms usually have many small chloroplasts scattered throughout the cytoplasm, but in low light intensities, the chloroplasts may aggregate near the exposed cell ends.

Centric diatoms( 中心硅藻 ) Diatoms exist in an immense variety of shapes derived from two basic cell shapes. The frustules of most planktonic species appear radially （放射地） symmetrical from an end view..

Circular, triangular, and modified square shapes are common. These are known as centric diatoms

Pennate diatoms （羽纹） Other diatoms, especially benthic forms, display varying types of bilateral symmetry and are termed pennate diatoms.Only pennate diatoms are capable of locomotion.

The mechanism for for locomotion is not fully understood, but it is believed to involve a wavelike motion in the cytoplasmic surface that extends through a groove (the raphe ：纵沟 )in the frustule.

Locomotion This flowing motion is accomplished only when the diatom is in contact with another surface. Diatoms capable of locomotion are generally restricted to shallow water sediments or to the surfaces of larger plants and animals.

Reproduction Diatoms and most other protists reproduce by simple cell division.an individual parent cell divides in half to produce two daughter cells, each daughter cell grows to repeat the process. This method of reproduction can yield a large number of diatoms in a short period of time.

When conditions for growth are favorable, rates of cell division greater than once each day are not uncommon. By dividing every day, a single diatom requires less than three weeks to produce one million daughter cells. Populations of diatoms and other rapidly dividing unicellular plants thus have an exceptional capacity to respond rapidly to improved growth condition.

Restrictions of size and shape imposed upon diatoms by the rigid frustule create a peculiar cellular reproduction problem. During diatom cell division, two new frustule halves are formed inside the original frustule.

One is the same size as the hypotheca of the parent cell and is destined to become the hypotheca of the larger daughter cell. The other newly formed frustule half becomes the new hypotheca for the other daughter cell. Each daughter cell receives its epitheca from the original frustule of the parent cell.

The daughter cells grow and repeat the process. This method of cell division is efficient in the use of silica for new frustules, as the old frustules are not discarded. However, a slight decrease in the average cell size results with each successive cell division.

Reduction of cell size may continue for many months, eventually reaching a minimum of about 25 ％ of the original cell size. This size reduction is not observed in all natural diatom populations, suggesting that continual readjustment of cell diameter occurs in some species.

Auxospore( 复大孢子 ) When the minimum cell size is reached, the small diatom sheds its enclosing frustule, and the naked cell, known as an auxospore, flows out. The auxospore enlarges to the original cell size, forms a new frustule, and begins dividing again to repeat the entire sequence.