Diversity of the Marine Cyanobacterium Trichodesmium: Characterization of the Woods Hole Culture Collection and Quantification of Field Populations

Annette Hynes, Ph.D., 2009
Scott Doney, John Waterbury, Advisors

Trichodesmium is a colonial, N2-fixing cyanobacterium found in tropical
oceans. Species of Trichodesmium are genetically similar, but several species
coexist. In order to coexist, species may occupy different niche spaces through
differential utilization of resources and responses to physical characteristics.
To investigate niche differentiation in Trichodesmium, I characterized cultured
strains of Trichodesmium, identified and enumerated Trichodesmium clades in
the field, and investigated P stress and N2 fixation in field populations. Species
of Trichodesmium group into two morphologically diverse clades that are coherent
with respect to pigment composition: clade I (Trichodesmium erythraeum
and Trichodesmium contortum), and clade II (Trichodesmium thiebautii, Trichodesmium
tenue, Trichodesmium hildebrandtii, and Trichodesmium pelagicum).
I developed a quantitative polymerase chain reaction (qPCR) method to
distinguish between these clades and applied it to field populations in the Atlantic
and Pacific. Clade II Trichodesmium spp. were more prominent than clade
I in the open ocean. Neither clade correlated with nutrients, but clade I had wider
temperature and depth distributions than clade II. Trichodesmium populations in
the North Atlantic were more P stressed and had higher N2 fixation rates than the
Pacific. Unraveling differences among Trichodesmium spp. enables us to understand
their coexistence and factors controlling global N2 fixation.