Reef Chemistry Question of the Day #139 Organic Molecules

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #139
Which statement about dissolved organic molecules in the ocean is true?

A. Certain classes of dissolved organic molecules have been shown to circulate in the ocean largely unchanged for thousands of years.
B. The surface ocean concentration of dissolved organic molecules is typically about 20-30 ppm.
C. The nature of organic molecules in the ocean is well understood.
D. The ratio of carbon to phosphorus to nitrogen in dissolved organic molecules is roughly 100:5:1.

Good luck!



















.

 

[human]

B

 

[twilliard]

I am going to have to go with A from my current readings.

 

[chefjpaul]

My guessing techniques.

A. Final answer. Earths formation of molecules doesn't disappear, maybe changing composition but always a constant. Hard to explain?

B-(is particulate biomass be included?)
Also when the atmosphere co2 increases / decreases it stays in equilibrium with waters surface.

C- we would like to think so, but we may know a lot of carbon and formation, not all the details.

D-160-16-1 Redfield Ratio

 

[JimWelsh]

I know D is wrong for the Redfield ratio, but otherwise don't know. Guessing A.

 

[beaslbob]

a.

something about hormones not being reduced in anyway from what I remember.

 

[beaslbob]

could more then one be wrong?

 

[beaslbob]

I say all of them!!!!!

 

[twilliard]

Alright @Randy Holmes-Farley what is it!
Lets not keep the students waiting too long to hear from the professor

 

[Randy Holmes-Farley]

OK.

The answer is A.

This article explains that A is correct and the others are not:

Organic Compounds in the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com


This part relates to answer A:
One interesting aspect of organic compounds in the ocean is that some have been there for thousands of years.1,3 Many organic compounds, especially in surface waters, are rapidly cycling between living organisms that consume and modify them, and the dissolved forms that are just floating about. Acetate, for example, can have a turnover rate as high as once per day on average in the water column, and once per hour or two in pore water inside sediments.3

With each turn of this cycle, some of these organic materials become more and more refractory. That is, they become less and less palatable to organisms, and are turned over more and more slowly. Eventually, some remain that are largely resistant to further biodegradation and processing, and these can then stay as DOM for many thousands of years. Essentially, they are the waste that is left after every organism has had its shot at using them.

The pathways for degradation of such refractory molecules are not well known, but likely reflect some rare biological events (rare bacteria encounter them, they encounter a rare enzyme, or they are acted upon by an enzyme that does not normally process them, etc.). The long term degradation likely also includes physical and chemical processes, such as oxidation by oxygen, ozone, or other oxidizing agents, and being hit by appropriate radiation (UV, x-rays, gamma rays, etc.).


And the other, wrong answers:

B:
Dissolved organic material in the oceans is often measured in terms of its carbon content, and is referred to as dissolved organic carbon (DOC) and particulate organic carbon (POC). Surface ocean water typically has about 60-90 uM DOC. That range corresponds to 0.7-1.1 ppm. Table 1 shows the breakdown of this DOC into a variety of chemical classes.

C:
Organics in the Ocean
The nature of the organic matter in the ocean is poorly understood. Part of the reason for this lack of understanding stems from the tremendous variety of organic material that exists. There is essentially no limit to the number of different organic compounds that are theoretically possible, and the fact is that many millions of organic compounds have been synthesized or identified. Identifying and quantifying every possible organic material in seawater is just not possible, at least with present day technology. Consequently, identifying the form organic materials take in the ocean most often involves grouping them into classes by a functional test, such as whether they can be extracted from the water with a hydrophobic solvent, whether they contain nitrogen or phosphorus, etc.

A small number of organic compounds have been individually identified and quantified in seawater, but they represent only a small percentage of the total mass of organic material. Those examined in detail include simple sugars and amino acids, and the very simplest organic molecules, such as derivatives of methane (CH4) or ethane (CH3CH3), including acetate (CH3CO2-).

D:
Organics also are often measured in terms of their nitrogen content, such as dissolved organic nitrogen (DON) and particulate organic nitrogen (PON). The same is true for phosphorus, using the terms dissolved organic phosphorus (DOP) and particulate organic phosphorus (POP). Table 2 shows the relative concentrations of C, N, and P in dissolved organic material. In dissolved organic material, nitrogen is about ten-fold less prevalent than carbon, and phosphorus is several hundred-fold lower in concentration than carbon.

Table 2. Elemental composition of dissolved organic material.

Component: Concentration:
DOC................60-90 uM

DON...................3.5-7.5 uM

DOP..................0.1-0.4 uM

So C to P to N is roughly about 100:0.2:5 (on a molar basis)

 

[beaslbob]

gee I was right the first time.

BTW I didn't realize that chemists and scientists like dr randy holmes-Farley had studied this things for thousands of year.

 

[Randy Holmes-Farley]

gee I was right the first time.

BTW I didn't realize that chemists and scientists like dr randy holmes-Farley had studied this things for thousands of year.

Oh sure. These guys did some of the early work:

 

[Kungpaoshizi]

What's the source of this info? Seems plausible but certain polymers are completely "new" compounds in the ocean and those are being "changed" in the ocean, didn't even take 100 years.

Nevermind, I found it in the original article

I always thought it was funny how much we can tell, but were never around to observe.