173
10.3 History
N 2 fixation has long been known in the terrestrial environment, from the early recognition of the unique characteristics of self-feeding of N by legumes in agriculture
to its formal discovery by Beijerinck and Winogradsky in the late nineteenth century. The recognition of the importance of N 2 fixation in the marine environment
began with early speculations as to its importance (or unimportance) in the sea and
the subsequent isolation of diazotrophic heterotrophic bacteria from marine samples. A fuller understanding of its significance, distribution and microorganisms
involved took decades of research. Some diazotrophic cyanobacteria, such as
Trichodesmium, were recorded in early phycological surveys such as the German
Plankton Expedition in the nineteenth century, although their role as diazotrophs
was unknown at the time. N 2 fixation was largely ignored during the beginnings of
advanced marine microbiology in the mid twentieth century as it was believed that
P was a more important limiting nutrient than N, and that the role of N 2 -fixers was
to balance N needs limited by P.
Appreciation of the importance of open ocean N 2 fixation began in the 1960s,
with the discovery of N 2 fixation by Trichodesmium. Subsequently, as biogeochemical data accumulated and models developed there appeared to be an imbalance
between global ocean inputs and outputs, which drove greater focus on marine N 2
fixation. The interest in N 2 fixation led to numerous studies in a variety of habitats
from the water column to the deep benthos.
10.4 Microorganisms
N 2 fixation occurs in a wide variety of marine habitats catalyzed by diverse microorganisms. Diazotrophs are found free-living or as symbionts in the water column
and numerous benthic environments. In the water column, N 2 fixation is a characteristic of low-nutrient oligotrophic waters typically found in tropical and subtropical
regions including the major gyres. Trichodesmium, a filamentous nonheterocystforming cyanobacterium has long been known to be a major N 2 -fixer. Richelia,
heterocyst-forming cyanobacteria that live in or on diatoms, have also been known
to be important. Both can form dense blooms. In recent decades, more diazotrophs
have been discovered using molecular biological techniques. Crocosphaera, a unicellular cyanobacterium is common in oligotrophic waters.
A symbiotic cyanobacterium was discovered that is a symbiont with a haptophyte planktonic algae related to Braarudosphaera bigelowii, has an extremely
reduced genome and is metabolically streamlined. Bacterial nifH sequences, also
detected by genomic techniques, are ubiquitous. UCYN-A and heterotrophic diazotrophs are more widely distributed geographically than Trichodesmium, Richelia or
Crocosphaera, being found in nearshore coastal and high latitude waters.
10.4 Microorganisms
10.3 History
N 2 fixation has long been known in the terrestrial environment, from the early recognition of the unique characteristics of self-feeding of N by legumes in agriculture
to its formal discovery by Beijerinck and Winogradsky in the late nineteenth century. The recognition of the importance of N 2 fixation in the marine environment
began with early speculations as to its importance (or unimportance) in the sea and
the subsequent isolation of diazotrophic heterotrophic bacteria from marine samples. A fuller understanding of its significance, distribution and microorganisms
involved took decades of research. Some diazotrophic cyanobacteria, such as
Trichodesmium, were recorded in early phycological surveys such as the German
Plankton Expedition in the nineteenth century, although their role as diazotrophs
was unknown at the time. N 2 fixation was largely ignored during the beginnings of
advanced marine microbiology in the mid twentieth century as it was believed that
P was a more important limiting nutrient than N, and that the role of N 2 -fixers was
to balance N needs limited by P.
Appreciation of the importance of open ocean N 2 fixation began in the 1960s,
with the discovery of N 2 fixation by Trichodesmium. Subsequently, as biogeochemical data accumulated and models developed there appeared to be an imbalance
between global ocean inputs and outputs, which drove greater focus on marine N 2
fixation. The interest in N 2 fixation led to numerous studies in a variety of habitats
from the water column to the deep benthos.
10.4 Microorganisms
N 2 fixation occurs in a wide variety of marine habitats catalyzed by diverse microorganisms. Diazotrophs are found free-living or as symbionts in the water column
and numerous benthic environments. In the water column, N 2 fixation is a characteristic of low-nutrient oligotrophic waters typically found in tropical and subtropical
regions including the major gyres. Trichodesmium, a filamentous nonheterocystforming cyanobacterium has long been known to be a major N 2 -fixer. Richelia,
heterocyst-forming cyanobacteria that live in or on diatoms, have also been known
to be important. Both can form dense blooms. In recent decades, more diazotrophs
have been discovered using molecular biological techniques. Crocosphaera, a unicellular cyanobacterium is common in oligotrophic waters.
A symbiotic cyanobacterium was discovered that is a symbiont with a haptophyte planktonic algae related to Braarudosphaera bigelowii, has an extremely
reduced genome and is metabolically streamlined. Bacterial nifH sequences, also
detected by genomic techniques, are ubiquitous. UCYN-A and heterotrophic diazotrophs are more widely distributed geographically than Trichodesmium, Richelia or
Crocosphaera, being found in nearshore coastal and high latitude waters.
10.4 Microorganisms