Marine autotrophs

The carbon fixed by marine autotrophs has several possible fates. Approximately one third is respired within the algal cells to provide energy for growth and maintenance. 

Duarte, C. M., and Cebrian, J. (1996). The fate of marine autotrophic production. Limnol. Oceanogr. 41, 1758-1766. 

Although inorganic N has traditionally been thought to be the primary N source for marine autotrophs, it is now apparent that this is not always true (see Chapter 7 by Mulholland and Lomas, this volume) and that organic N can be a major N source in many marine environments. With that in mind, enzymes involved in the hydrolysis or mobihzation of organic N are examined. 

Haptomonada (Prymnesipophytes) few Mostly marine, autotrophic, calcareous... 

Figure 10.12. A. Carbon isotopic composition of major groups of higher plants and autotrophic microorganisms compared with oxidized carbon (CO2, HCO3, CC>32 )- The triangles are mean values. (After Holser et al 1988 Schidlowski, 1988). B. Sulfur isotopic composition of bacteriogenic sulfide in modern marine anaerobic sediments (open bars, 1-6) and in the Permian Kupferschiefer (black bar, 7) compared with oxidized sulfate of modem (1-6) and Permian seawater (+11 %o). The black triangles are mean values. (After Holser et al., 1988.)...

The only cultivated AOA, Nitrospumilus maritimus, depends on CO2 as its only carbon source and the presence of even low levels of organic carbon were inhibitory to growth. The pathway of CO2 fixation is, however, unknown. Hyperthermophilic Crenarchaeota generally utilize a 3-hydroxypropionate pathway or a reductive TCA cycle for autotrophic carbon fixation. Another cultivated marine Crenarchaeota strain, Cenarchaeum symbiosum, a sponge symbiont, appears to use the 3-hydroxypropionate pathway. It cannot be concluded on this basis which pathway is used by the AOA, but it very likely that is not the Calvin cycle. N. maritimus had a minimal generation time of 21 h, longer but roughly on the same scale as AOB. 

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