5 C release
Dissolved organic N contains C and so it is therefore logical to assume that Trichodesmium also release substantial amounts of photosynthetic products as dissolved organic carbon (DOC). In fact, Shimura at al. (1978) first quantified the extracellular release of photosynthate in 14C incubations and calculated that about 8% of the total photosynthetic products were released during incubation experiments (range 0–18%). Similarly, Gallon et al. (1996) estimated that DOC excretion by Trichodesmium in the western North Atlantic and eastern Caribbean Sea represented 7% of the primary productivity, and, as for N, the amount of C released changed depending on light conditions and the physiological status of cells. More recently, Renaud et al. (2005) estimated a much lower value (1%) for DOC release by Trichodesmium. However, they suggested that tight coupling between organisms in the Trichodesmium consortium might cause underestimates of actual release rates. Thus, the same methodological limitations that make it difficult to estimate N release from tracer studies make it difficult to make estimates of C release; just as 15N2 uptake can underestimate gross N2 fixation, 14C (or 13C) incorporation can underestimate the gross rate of photosynthetic carbon fixation (Gallon et al., 2002).
Cyanobacteria release compounds such as glycolate (Renstr ¨om-Kellner et al., 1989) and amino acids (Capone et al., 1994). Amino acid release as glutamine and glutamate (molar C:N ratios of 5:2 and 5:1, respectively) represented only 3% of the C fixed by Trichodesmium (Capone et al., 1994). However, Trichodesmium have a carbohydrate mucoid matrix, which is colonized by other organisms (Stal, 1995; Sellner, 1997; Sheridan et al., 2002) and so there is a constant production of glucose- and mannose-rich mucilage that could account for more DOC release (e.g., Sellner, 1997). Cyanobacteria in general can exude as much as 80% of the CO2 they fix as extracellular polymeric substances (mainly polysaccharides) (Sellner, 1997).
Production of colored dissolved organic matter (CDOM) by Trichodesmium has recently been observed (Steinberg et al., 2004). Production of DOC ranged from 0.04 to 0.32μg C col−1 h−1. Assuming an average of 11.3μg C col−1 (Mc- Carthy and Carpenter, 1979), this represents between 0.4 and 2.8% h−1 or up to 67% d−1, although it is unclear whether this production is confined to the dark or light periods. The CDOM had absorption spectra similar to microsporinlike amino acids, compounds that Trichodesmium are know to have and serve in photoprotection (Subramaniam et al., 1999).
Although globally we are interested in the fate of new production from diazotrophy as a means to export C, little has been done to quantify or characterize DOC release and its fate from Trichodesmium or other marine N2 fixers. Robust isotopic and molecular tracers of diazotroph-derived DOC have yet to be identified. If we are to extrapolate export from production of N2 fixers, it will be important to determine the primary pathways of C flow through these organisms.
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