Spatial Segregation of Co₂ Fixation in Trichodesmium Spp.: Linkage to N₂ Fixation Potential¹

The aggregate‐forming, nonheterocystous, filamentous blue‐green alga (cyanobacteria) Trichodesmium spp. is a widespread and important planktonic N 2 fixer and primary producer in tropical and subtropical oceans. It is unique among nonheterocystous genera because it conducts N 2 and CO 2 fixation (O 2 evolution) simultaneously; a notable achievement, because O 2 is a potent inhibitor of N 2 fixation. Spatial and temporal CO 2 fixation patterns were examined in trichomes and aggregates from natural and cultured populations, utilizing microautoradiographic detection of 14 CO 2 incorporation. Parallel N 2 fixation (acetylene reduction) measurements were also made. Diel N 2 and CO 2 fixation patterns were similar, with co‐optimization of both processes near midday. Microautoradiographs revealed several trichome‐level 14 CO 2 incorporation patterns: 1)uniform, heavy labeling, 2)uniform, light labeling, 3) heavier labeling in distal as opposed, to proximal regions, and 4) virtually no labeling throughout. Similar patterns were observed in natural and cultured populations. Given previous immunochemical findings that N 2 fixation potential is widespread in Trichodesmium spp. trichomes and aggregates, current results suggest a high degree of individuality, and possibly a “division of labor” in terms of CO 2 fixation, among trichomes comprising active N 2 ‐fixing aggregates. Segregation of photosynthesis within and among trichomes facilitates simultaneous N 2 and CO 2 fixation in Trichodesmium spp. trichomes and aggregates.