The Trichodesmium consortium: the marine N-cycle at the microscale?

地球上光合作用中固定的碳大约有一半发生在海洋中。这种情况大多发生在远离陆地的地区，这些地区通常缺乏促进生长所需的植物营养。在热带和亚热带海洋中最像沙漠的地区，一些细菌利用阳光进行光合作用，就像陆地上的植物一样，也能够利用上覆大气中丰富的氮。这种固氮过程不仅支持这些细菌在这些恶劣的、营养贫乏的条件下生长，而且还为海洋增加了额外的氮源，然后为其他海洋生物提供了肥料。固氮剂通常存在于缺乏其他形式氮的海洋地区，因此在这些地区具有特别的优势。氮缺乏是由于其他类型的细菌在一个被称为反硝化的过程中从海洋中去除氮。通常，这些反硝化菌只在较深的水域才有活性，那里氧气的可用性降低，使它们能够在其他生命通常相当稀少的条件下茁壮成长。然而，我们发现，反硝化菌和固氮菌似乎能够在离海面更近的地方共同生活，而且，它们在海洋中的氮循环中都很活跃。在这项提案中进行的研究首先集中在为这些反硝化菌命名，这样我们就可以识别它们在海洋中的位置，其次，试图找出这些细菌在这种新发现的关联中可能在做什么。这一点很重要，因为海洋有助于从大气中吸收二氧化碳，而吸收二氧化碳的速度取决于海洋的肥沃程度。如果反硝化剂从地表水中去除大量的氮，那么这将影响海洋的整体生产力。我们依靠在海洋中发现的微小植物来锁定化石燃料燃烧释放到大气中的一些额外的二氧化碳。如果它们至关重要的氮供应因反硝化而减少，那么这些未固定的二氧化碳将留在大气中，从而加剧未来的全球变暖。这项研究的另一个重要方面是，在反硝化过程中，强大的温室气体一氧化二氮可以被释放出来，也可以被消耗。这些气体被排放到大气中，不仅使地球变暖，而且还破坏了一些臭氧层，臭氧层可以过滤掉来自太阳的有害紫外线。因此，我们还需要知道，在这个细菌联合体中，氧化亚氮循环的影响可能有多大，以及它将如何受到未来气候变化的影响。通过提高我们对新发现的细菌关联中可能发生的氮输入和损失规模的理解，我们应该能够更好地了解联盟中的合作伙伴在影响陆地和海洋肥力的更大规模的全球氮循环中的重要性。

About half of all of the carbon fixed in photosynthesis on Earth takes place in the oceans. Most of this occurs in those regions which are far from land and which are usually quite poorly provided with the plant nutrients required to fuel growth. In the most desert-like regions found in the tropical and subtropical oceans, some bacteria use sunlight for photosynthesis in just same way as the plants on land and are also able to exploit the abundant nitrogen present in the overlying atmosphere. This nitrogen-fixing process not only supports the growth of these bacteria under these hostile, nutrient-poor conditions but also adds this additional source of nitrogen to the oceans that then acts a fertilizer for other marine life.The nitrogen-fixers are usually found in ocean regions where there is a deficit in the availability of other forms of nitrogen and so are at a particular advantage in these areas. The nitrogen deficit is due to other types of bacteria that remove nitrogen from the ocean in a process known as denitrification. Usually, these denitrifiers, as they are called, are only found to be active in deeper waters where the reduced availability of oxygen allows them to thrive under conditions where other life is usually quite sparse. We have found, though, that the denitrifiers and the nitrogen fixers seem to be able to live together nearer to the surface and, what is more, that they are both active in the cycling of nitrogen in the oceans.The research to be carried out in this proposal is focussed on firstly putting names to these denitrifiers, so that we can recognise where they are found in the oceans, and secondly, on trying to find out what these bacteria might be doing within this newly discovered association. This is important because the oceans help to drawdown carbon dioxide from the atmosphere and the rates at which they do so depend on how fertile they are. If the denitrifiers are removing large quantities of nitrogen from the surface waters then this will affect the overall productivity of the oceans. We rely on the microscopic plants found in the ocean to lock away some of the extra carbon dioxide that is released to the atmosphere by the burning of fossil fuels. If their vital nitrogen supply is reduced by denitrification then this unfixed carbon dioxide will be left in the atmosphere to contribute to future global warming.Another important facet of this research is that the powerful greenhouse gas nitrous oxide can be released in the denitrification process as well as being consumed. Not only does this gas warm the planet when it is vented to the atmosphere but it is also responsible for breaking down some of the ozone layer that filters out hazardous ultraviolet rays from the sun. We need to know also, therefore, what the likely scale of the impact of nitrous oxide cycling within this consortium of bacteria is and how it might be affected by future climate change.By improving our understanding of the scale of nitrogen inputs and losses that might occur in the newly discovered bacterial association we should be in a better informed position to assess how significant the partners in the consortium are in the larger scale global nitrogen cycle that influences the fertility of the land as well as the oceans.