Evaluation of nutrient limitation of CO2 and N2 fixation in marine microbial mats

Photosynthetic CO2 fixation and N2 fixation are fundamentally important in mediating production dynamics of intertidal and subtidal marine microbial mat communities. We examined nutrient [N, P, Fe, MO, dissolved organic carbon (DOC)] limitation of CO, and NZ fixation in geographically and physiologically diverse mats. Nitrogen enrichment (as NO3-) infrequently stimulated CO2 fixation. Phosphorus, Fe and MO enrichment generally failed to stimulate CO2 fixation. The frequent absence of N limitation appears linked to the ability of mat microbial communities to fix N2 and effectively recycle fixed N. Nitrogen fixation was enhanced by DOC, while no P, Fe or trace element stimulation was observed. The lack of nutrient stimulation of CO2 fixation appeared related to low net growth rates of some mats. Slow growing mats, including hypersaline, stromatolitic (Storrs Lake, Bahamas) and certain hypersaline, lagoonal mats (Guerrero Negro, Baja California, Mexico) exhibited virtually no nutrient stimulation of either CO2 or N2 fixation. More productive coastal (North Carolina, USA) and estuarine (Tomales Bay, California, USA) mats showed higher frequencies of nutrient limitation of either process. Seasonally, N and DOC stimulation were most profound during periods of maximum growth. Mats are able to minimize C and N limitation by metabolically coupling CO2 and N2 fixation as sources of 'new' C and N inputs respectively. Phototrophic-heterotrophic microbial consortia appear to mediate coupling, which minimizes losses of fixed C and N to overlying waters.