Wofford College, Spartanburg, South Carolina, USA.
mBio. 2024 Sep 11;15(9):e0105924. doi: 10.1128/mbio.01059-24. Epub 2024 Aug 6.
Reef-building corals depend on symbiosis with photosynthetic algae that reside within their cells. As important as this relationship is for maintaining healthy reefs, it is strikingly delicate. When ocean temperatures briefly exceed the average summer maximum, corals can bleach, losing their endosymbionts. Although the mechanisms governing bleaching are unknown, studies implicate uncoupling of coral and algal cell divisions at high temperatures. Still, little is known regarding the coordination of host and algal cell divisions. Control of nutrient exchange is one likely mechanism. Both nitrogen and phosphate are necessary for dividing cells, and although nitrogen enrichment is known to increase symbiont density in the host, the consequences of phosphate enrichment are poorly understood. Here, we examined the effects of phosphate depletion on symbiont growth in culture and compared the physiology of phosphate-starved symbionts in culture to symbionts that were freshly isolated from a host. We found that available phosphate is as low in freshly isolated symbionts as it is in phosphate-starved cultures. Furthermore, RNAseq revealed that phosphate-limited and freshly isolated symbionts have similar patterns of gene expression for phosphate-dependent genes, most notably upregulation of phosphatases, which is consistent with phosphate recycling. Similarly, lipid profiling revealed a substantial decrease in phospholipid abundance in both phosphate-starved cultures and freshly isolated symbionts. These findings are important because they suggest that limited access to phosphate controls algal cell divisions within a host.
The corals responsible for building tropical reefs are disappearing at an alarming rate as elevated sea temperatures cause them to bleach and lose the algal symbionts they rely on. Without these symbionts, corals are unable to harvest energy from sunlight and, therefore, struggle to thrive or even survive in the nutrient-poor waters of the tropics. To devise solutions to address the threat to coral reefs, it is necessary to understand the cellular events underpinning the bleaching process. One model for bleaching proposes that heat stress impairs algal photosynthesis and transfer of sugar to the host. Consequently, the host's demands for nitrogen decrease, increasing nitrogen availability to the symbionts, which leads to an increase in algal proliferation that overwhelms the host. Our work suggests that phosphate may play a similar role to nitrogen in this feedback loop.
造礁珊瑚依赖于与其细胞内共生的光合作用藻类。这种关系对维持健康的珊瑚礁至关重要，但却极其脆弱。当海洋温度短暂超过夏季平均最高温度时，珊瑚会白化，失去共生藻类。虽然导致白化的机制尚不清楚，但研究表明高温会导致珊瑚和藻类细胞分裂解偶联。尽管如此，人们对宿主和藻类细胞分裂的协调知之甚少。控制养分交换是一种可能的机制。氮和磷都是分裂细胞所必需的，虽然氮的富集已知会增加宿主中共生体的密度，但磷富集的后果却知之甚少。在这里，我们研究了磷酸盐耗竭对培养中共生体生长的影响，并比较了培养中磷酸盐饥饿的共生体与刚从宿主中分离出的共生体的生理学。我们发现，刚从宿主中分离出的共生体中的可利用磷酸盐与磷酸盐饥饿培养物中的磷酸盐一样低。此外，RNA 测序显示，磷酸盐限制和刚从宿主中分离出的共生体的磷酸盐依赖基因的表达模式相似，最显著的是上调了磷酸酶，这与磷酸盐的再循环一致。同样，脂质分析显示，在磷酸盐饥饿培养物和刚从宿主中分离出的共生体中，磷脂的丰度都显著降低。这些发现很重要，因为它们表明，宿主内有限的磷酸盐会控制藻类细胞的分裂。
由于海水温度升高导致珊瑚白化并失去依赖的藻类共生体，负责建造热带珊瑚礁的珊瑚正以惊人的速度消失。没有这些共生体，珊瑚就无法从阳光中获取能量，因此，在热带贫营养水域中，它们难以茁壮成长，甚至难以生存。为了解决珊瑚礁面临的威胁，有必要了解导致白化的细胞事件。白化的一个模型提出，热应激会损害藻类的光合作用和糖向宿主的转移。因此，宿主对氮的需求减少，共生体中氮的可用性增加，导致藻类过度繁殖，从而使宿主不堪重负。我们的工作表明，磷酸盐在这种反馈环中可能起到与氮相似的作用。
