> "Here, we show that the seagrass, Posidonia oceanica excretes sugars, mainly sucrose, into its rhizosphere. These sugars accumulate to µM concentrations—nearly 80 times higher than previously observed in marine environments. This finding is unexpected as sugars are readily consumed by microorganisms. Our experiments indicated that under low oxygen conditions, phenolic compounds from P. oceanica inhibited microbial consumption of sucrose."
https://www.nature.com/articles/s41559-022-01740-z
A sugar like glucose is ~180 grams/mol. A µM concentration of glucose is going to be less than a milligram of sugar per liter of ocean water. In contrast, a liter of Coke contains ~120 grams of sugar (~ 0.6 M).
It's kind of interesting because the seagrass appears to be feeding sugar to bacteria, which might be doing nitrogen fixation in return, but it's hardly 'mountains of sugar'.