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Carbon, the major nutrient element of all plants, is available in seawater chiefly as HCO3-. There are small amounts of CO2, the proportion depending on pH and salinity. Carbon is fixed into organic compounds in the “dark” reactions of photosynthesis. Parasitic algae rely on carbon from their hosts, but other seaweeds have little or no ability to use exogenous organic carbon, particularly at environmental concentrations. Most seaweeds are thus photoauxotrophic and are not facultatively heterotrophic. The relationships between some seaweeds and symbiotic fungi pose interesting nutritional and hormonal questions.

CO2 is fixed by RuBP carboxylase in the Calvin eyele HCO3 is converted to CO2 with the aid of carbonic anhydrase. In the brown algae, especially in young tissue of kelps and fucoids, light-independent carbon fixation occurs, in light and dark, via PEP catboxykimase, which also uses free CO2. RuBP carboxylaes also acts as an oxygenaes, leading to the photoresporatory or glycolate cycle.

Various principal products of photosynthetic carbon fixation are found among the seaweeds. Green algae form chiefly sucrose and starch; brown algae form mannitol and laminaran ; red algae form several low molecular weight compounds and floridean starch. In addition, all groups accumunds some amino acids.

In the kelps, the low molecular weight compounds can be translocated from mature regions tj meristematic regions. The driving force is probably a water potential gradient ste up by the loading of organic molecules into the sieve elements in the source and their unloading in the sink.

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  • 1 0 年前
    最佳解答

    Carbon, the major nutrient element of all plants, is available in seawater chiefly as HCO3-. There are small amounts of CO2, the proportion depending on pH and salinity. Carbon is fixed into organic compounds in the “dark” reactions of photosynthesis. Parasitic algae rely on carbon from their hosts, but other seaweeds have little or no ability to use exogenous organic carbon, particularly at environmental concentrations. Most seaweeds are thus photoauxotrophic and are not facultatively heterotrophic. The relationships between some seaweeds and symbiotic fungi pose interesting nutritional and hormonal questions.

    碳,為所有植物的主要生成元素,在海水中則大部分以HCO3的成份出現,此外,海水中也含有微量的CO2成份,其比例取決於海水的pH值與鹽度. 對各種有機聚合物來說,在光合作用的"暗反應"中,碳是不可或缺的元素,寄生型的海藻需可自母體獲取碳元素(來進行光合作用),但其他類海藻則幾乎無法以外生的方式來獲得碳元素,尤其是對大叢集聚生的藻類. 因此大部分海藻都是光合作用缺陷體,無法自體完成. 部分海藻與共生蕈類之間的這種相應性形成了有趣的營養學與生長荷爾蒙議題.

  • 5 年前

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  • 1 0 年前

    碳是所有植物的營養元素,其在海中的成份主要以HCO3來呈現。海水中存在少數的二氧化碳,其比例根據酸鹼值(PH)及鹽分而不同。在光合作用的黑化反應發生時,碳就會在有機物質內產生。寄生的藻類依賴從它們的宿主產生的碳,但是其他的藻類很少,或幾乎不能利用外生的有機碳,特別是在集中的環境下。大部分的海藻因此自行光合作用,而且也不能從別的地方獲得新陳代謝需要的碳水元素。一些海藻及其共生的菌纇的關係提示出有趣的營養學上與荷爾蒙的問題。

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