Hi Lee,

Without knowing too much more about your exact streams it's a bit difficult to pinpoint the exact reasons underlying your observations. . However, Valerie Beaumont and others (2000, Organic Chemistry 31,1075-1085) described depleted 15N (-10 or so) associated with Rhodobacter capsulatus, which they associated with with the biosynthesis of bacteriochlorophyll a in this species. Other species did not show this depletion. There have been a range of other studies since that have shown a similar thing so taking a closer look at your community composition may help clarify what's going on.

There are a range of processes that may be contributing to the 13C values you're observing. The less 13C-depleted signal for the cyanobacterial material suggests

fractionation associated with CO2 concentration mechanisms in cyanobacteria and the diffusion limitations of dissolved inorganic C which leads to relatively less 13C-depletion in the biomass. Do you know anything about the 13C-DIC of your systems?

Have you analyzed the 15N of different nitrate sources? Maybe special N source(s) has so depleted 15N.

Have you analyzed the 15N of different nitrate sources? Maybe special N source(s) has so depleted 15N.

Hello

in fact i have not much experience with the 15N but i studied the variation of the 15 N within the PETM and we found that a sharp negative C isotopic excursion associated with a similar one in 15N and we found at the same intervals presence of cyanobacteria and also could be linked to the presence of different types of nitrogen fixer that could be the main reason of the 15N depletions

Hello

I'm not familiar with algae since I work in deep-sea organisms however there are several papers that would help you, look for example at Chikaraishi et al 2009 "Determination of aquatic food-web structure based on

compound-specific nitrogen isotopic composition of amino acids" ther are several values on d15N of cyanobacteria with high variability. As far as concerns d13C the variability you found is normal, I analyzed some Mediterranean algae (not cyanobacteria) and I found similar isotopic range among brown, green ald red algae, it depends on their metabolism, carbon uptake etc

I hope it might help

Regards

Emanuela

Hi Lee,

I am not a specialist in this mater, but it seems to me that it will totally depend on the deta15N of the N source they are using. Delta of N sources can vary in a very wide range (e.g. sewage and manure ranges from ~+5 till +25, while inorganic fertilizers, and N in precipitation etc... is more depleted (-10 till +5)). So it might be that in the “other studys” they worked in more organically polluted streams, while yours has more depleted sources or low N, and that your cyanobact are doing N fixation (15N air is by deff= 0)) and during the fixation or assimilation processes you will have some fractionation (so going more to the negative side).

Sincerely yours,

Samuel

Hi Lee,

Negative δ15N values can sometimes be found in nitrogen fixing plants. The low δ15N values of the cianobacteria mats you studied likely result from strong discrimination of 15N during nitrogen fixation by cianobacteria. Regarding δ13C values, very low values (-32‰) are indicative of no carbon limitation for the plants, because of high water turbulence (increases the turnover of inorganic carbon around the plant) or low plant growth (because of very low water temperature, low light intensity or low inorganic nutrient availability). The opposite (low water turbulence or fast plant growth) explain high values. However -2‰ is very high and suggests the existence of inorganic carbon in your samples. Photosynthesis may cause the precipitation of inorganic carbon around the plant and inorganic carbon is highly enriched in 13C. Thus, you should first remove inorganic carbon from your samples using 0.5 M HCl, although this may modify the δ15N values and hence forces you to duplicate your analysis.

Hello. May I suggest that you read the Sammarco and Risk, and Risk and Sammarco papers on 13C and 15N in the zooxanthellae of corals on a cross-shelf transect on the Great Barrier Reef. These values can change with respect to nutrient availability in the water column. Also, if you see the Wiklnson and Sammarco papers, you'll see that grazing can cause a change of up to 7X in the N-fixation rates, which will change the balance between dissolved N2 uptake and that taken up from the medium. There are so many things that affect these ratios. I hope this helps. Paul W. Sammarco

I have tried to get an opportunity to work with 15N several times. I was always defeated by the limited access to mass spectrometers. You need to have access to someone to fly the machine for you because you physically cannot do everything yourself. Access to 13N is even worse and makes the politics of getting access to a mass spec costing only a few hundred thousand dollars look like kindergarten. There is too much of the "dog in the manger" mentality around. I finally gave up on N-tracer methods and followed other projects.

There are some things I could have done in a few afternoons if I had had proper access to 15N or 13N but it will never happen - perhaps in my next incarnation.

Hello Lee:

Some good ideas have been expressed above regarding both N and C isotopic ratios. An N fixing organism (cyanos) should be ~0 (plus or minus a few del), so the depleted value you have may be a combination of physiological bias and source(s) bias. Either of both of these are going to be good explanations; best to get source N information to better constrain this question. Using the diffusion technique to describe source nitrate and ammonium from stream water would be ideal, with the caution that N sources in streams can be transient and dynamic.

Regarding the carbon values, and Luis Cardona already mentioned this; some algae become covered with carbonates due to differences in the water chemistry, pH and the pH superficial to the algal mat. The treatment mentioned should remove this problem. I have encountered carbonate covered algae to an extent that it makes the surface rough and the algae brittle. Might want to do a before and after treatment with acid to better understand the possible influence of this factor.

Separating out the possible influence of a community of organisms is a little more complicated.

Cheers,

-Bryan

All,

Thanks so much for the thorough and insightful responses, and for the suggested readings. I know that the primary source of N is atmospheric N deposition on glaciers from auroral activities, as well as N-fixation. Unfortunately, I don't think we have 15N isotopic ratios for this N, so I am in the dark there. The mats do tend to accumulate N, although growth doesn't appear to be limited by N. Low temperatures are likely prevailing in limiting growth.

With regards to DIC, based on a previous study we have stream DIC concentrations between 0.1-18.0 mg C/L, highly variable. Based on my observations there isn't significant carbonate formation on the mats, although an acid treatment would be helpful in ruling out this influence on the isotopic values.

Again, many thanks for your responses!

-Lee

Hi Lee,

The low d15N values are probably from fractionation against 15N on uptake. Similar low values were recorded in arctic algae after fertilizing streams with N allowed such fractionation to be expressed (Peterson BJ, Bahr M, Kling GW. 1997. A tracer investigation of nitrogen cycling in a pristine tundra river. Canadian Journal of Fisheries and Aquatic Sciences 54(10): 2361-2367.). Hoch (1992, 1994) give additional examples.

Hoch MP, Fogel ML, Kirchman DL. 1992. Isotope Fractionation Associated with Ammonium Uptake by a Marine Bacterium. Limnology and Oceanography 37(7): 1447-1459.

Hoch MP, Fogel ML, Kirchman DL. 1994. Isotope Fractionation During Ammonium Uptake by Marine Microbial Assemblages. Geomicrobiology Journal 12(2): 113-127.

In terms of the variable carbon isotope values, I poked around the literature on mats attempting to explain high d13C values in some fungi. So, look at the refs in this paper, I'd suggest.

Hobbie EA, Boyce CK. 2010. Carbon sources for the Palaeozoic giant fungus Prototaxites inferred from modern analogues. Proceedings of the Royal Society B-Biological Sciences 277(1691): 2149-2156.

But values of -2 seem really high, as Bryan Dail suggested, since atmosphere is at -8 per mille or so.

good luck,

Erik

I do not know where are other studies of algae from ? But I am afraid that the depletion of 15N is because of the thin diffusive boundary layer. The algae with thinner diffusive boundary layer lead to depletion of isotope. I do not know N isotope, I do some carbon isotope, please see attachment. I hope it will be useful.

The high velocity of stream results in thinner diffusive boundary layer which could be leading to depletion of isotope. see attachment.

As for d13C variation, look up "carbon concentrating mechanisms in cyanobacteria" (CCM is a common abbreviation in the field). Papers by Murray Badger and Dean Price at the Australian National University will be of particular use, though only a few also look at d13C. The short story is that many cyanobacteria and proteobacteria have carboxysome-based CCMs and many micro algae have pyrenoid-based CCMs. When expressed (usually at atmospherically equilibrated or lower CO2 or HCO3), cells will have d13C values closer to 0 (maybe 0 to -5) because the 13C cannot escape before being used. When the CCM is off (i.e. high CO2, with few studies exploring what high means and just equilibrating with 0.5 to 5%)  

Arg, I accidentally submitted when I attached that paper. To finish:

the d13C values will be much more negative (i.e. -20 to -30) because the cells are relying on passive diffusion of CO2 more and rubisco can discriminate, meaning the 13CO2 can get away before it is assimilated. I could imagine that a benthic environment or lower in a mat could see high enough CO2 to turn a CCM off or having a change is species between ones with and without a CCM. In addition the paper above mentioning some of the carboxysome CCM diversity (references by Raven and others within will help), I have also attached example papers discussing the 13C effect in a microalgae.