<a href=showthread.php?s=&postid=8118551#post8118551 target=_blank>Originally posted</a> by Fredfish
Ninong. Did your information on phosphate limitation come directly from a scientific paper? Did it give any indication at what levels of phosphate limitation started?
Fred
You might say it came from you. :lol:
In a previous post you mentioned the Redfield ratio and that got me to thinking that if P is lacking, nitrogen fixation will be retarded. When nutrients are not limiting, the molar element ratio C:N
is 106:16:1 -- the Redfield ratio. I don't recall this being mentioned in the article that I read some five or six years ago, so I did an online search and came up with the following.
Abstract:
A compilation of data on the elemental composition of marine phytoplankton from published studies was used to determine the range of C:N
. The N
ratio of algae and cyanobacteria is very plastic in nutrient-limited cells, ranging from <5 mol N:mol P when phosphate is available greatly in excess of nitrate or ammonium to <100 mol N:mol P when inorganic N is present greatly in excess of P. Under optimal nutrient-replete growth conditions, the cellular N
ratio is somewhat more constrained, ranging from 5 to 19 mol N:mol P, with most observations below the Redfield ratio of 16. Limited data indicate that the critical N
that marks the transition between N- and P-limitation of phytoplankton growth lies in the range 20-50 mol N:mol P, considerably in excess of the Redfield ratio. Biochemical composition can be used to constrain the critical N
. Although the biochemical data do not preclude the critical N
from being as high as 50,
the typical biochemical composition of nutrient-replete algae and cyanobacteria suggests that the critical N is more likely to lie in the range between 15 and 30. Despite the observation that the overall average N
composition of marine particulate matter closely approximates the Redfield ratio of 16, there are significant local variations with a range from 5 to 34. Consistent with the culture studies, lowest values of N
are associated with nitrate- and phosphate-replete conditions.
The highest values of N are observed in oligotrophic waters and are within the range of critical N observed in cultures, but are not so high as to necessarily invoke P-limitation. The C:N ratio is also plastic. The average C:N ratios of nutrientreplete phytoplankton cultures, oceanic particulate matter and inorganic N and C draw-down are slightly greater than the Redfield ratio of 6.6. Neither the analysis of laboratory C:N
data nor a more theoretical approach based on the relative abundance of the major biochemical molecules in the phytoplankton can support the contention that the Redfield N
reflects a physiological or biochemical constraint on the elemental composition of primary production.
That leads me to believe that at N
ratios well in excess of the Redfield ratio, P-limitation may take over. That was something I hadn't considered previously, before reading your post. I believe the reason I overlooked it was because P-limitation is extremely rare in marine ecosystems. Virtually all marine ecosystems are N-limited. Freshwater ecosystems are typically P-limited.
If the N
ratio is high enough, N-fixation would be impacted. In that sense, P could be limiting. While this may not be something you would come across in a natural ecosystem, it could be the basis for claims that P reduction in a captive marine aquarium could help to control cyanobacteria.
