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> In this example controlling nitrogen supply:
> 
> https://www.the-scientist.com/image-of-the-day/image-of-the-day--symbiotic-algae-66950?utm_campaign=TS_DAILY%20NEWSLETTER_2020&utm_source=hs_email&utm_medium=email&utm_content=81817312&_hsenc=p2ANqtz-9L2qwVbJc31ZF7lkgejAfqtlWQId4PCNgqTzy_Z_duq2cVtJ9Gft3-yyCMZvY3Yf-Ze4oSZ0eWL07n-icvDsde874mBg&_hsmi=81817312
> 
> 
> "Some anemones and corals depend on symbiotic relationships with the photosynthetic algae living inside them for survival. Algae produce sugars that the sea creatures use for food, and in return, they get nutrients such as carbon dioxide, phosphorus, sulfur, and nitrogen. The hosts have to keep their algae populations in check. Too few can lead to starvation and coral bleaching, while too many can overburden the host’s metabolism. Just how they manage algae numbers was unknown until now. A study released in Nature Communications on January 8 has revealed that anemones control their algae by limiting the supply of shared nitrogen.
> 
> "A research team led by Tingting Xiang,...studied the anemone Exaiptasia pallida and found that at high densities, its algae, Breviolum minutum, showed an increased expression of genes associated with taking in and using nitrogen. When algae are starved of the nutrient, these genes are upregulated to compensate. "
> 
> From the paper:
> 
> https://www.nature.com/articles/s41467-019-13963-z#Sec20
> 
> Abstract
> "In cnidarian-Symbiodiniaceae symbioses, algal endosymbiont population control within the host is needed to sustain a symbiotic relationship. However, the molecular mechanisms that underlie such population control are unclear. Here we show that a cnidarian host uses nitrogen limitation as a primary mechanism to control endosymbiont populations. Nitrogen acquisition and assimilation transcripts become elevated in symbiotic Breviolum minutum algae as they reach high-densities within the sea anemone host Exaiptasia pallida. These same transcripts increase in free-living algae deprived of nitrogen. Symbiotic algae also have an elevated carbon-to-nitrogen ratio and shift metabolism towards scavenging nitrogen from purines relative to free-living algae. Exaiptasia glutamine synthetase and glutamate synthase transcripts concomitantly increase with the algal endosymbiont population, suggesting an increased ability of the host to assimilate ammonium. These results suggest algal growth and replication in hospite is controlled by access to nitrogen, which becomes limiting for the algae as their population within the host increases."
> 
> Comment: A complex arrangement between the two organisms. The question, obviously, is how did it evolve? It is difficult to imagine a beginning, since algae cannot live in anemones unless the metabolic arrangement is in place, and how did the  anemone live without the supplied sugars?. Design?