in the ocean. They form three ecological groups with widespread importance in the
ocean: high-light-adapted Prochlorococcus, low-light-adapted Prochlorococcus, and
diversely pigmented strains of Synechococcus. Comparisons of gene families in these
three groups (Zhaxybayeva et al. 2009) suggest frequent gene transfers between
Synechococcus and low-light-adapted Prochlorococcus. Such gene sharing (horizontal
gene transfer) also occurs in bacteria, and can result in specific and presumably rapid
ecological adaptation.
Eukaryotic Picoplankton
(^) In addition to cyanobacteria, the photosynthetic picoplankton often include numerous,
eukaryotic forms smaller than 2 μm in cell diameter. The first report that cells of this
type are abundant was again recent (Johnson & Sieburth 1982). Some of these forms
are coccoid green algae, Chlorophyta, without flagellae but retaining basal bodies or
even a rhizoplast structure showing relationship to a Chlamydomonas-like ancestor.
The genome of one species, Ostreococcus lucimarinus, has been sequenced, showing
a large number of selenoenzymes (Palenik et al. 2007). These enzymes are more
catalytically active than enzymes without selenium and may be an adaptation to the
organism’s small size. Two other divisions, Heterokonta and Haptophyta, have marine
species that are ecologically important eukaryotic picoplankton (Worden & Not
2008).
(^) Liu et al. (2009) used a combination of genetic, pigment, and microscopy data to
elucidate the abundance and diversity of very small haptophytes. These phytoplankton
use 19′hexanoyloxyfucoxanthin (19-Hex) as an accessory photosynthetic pigment,
and although this pigment is pervasive in the photic zone of the world oceans,
haptophyte nuclear rRNA sequences (see Box 2.4) seemed scarce. Classical primers
for small subunit ribosomal RNA (SSU rRNA) genes do not work well for the
haptophytes, and when more specific primers were developed, the diversity and
abundance of the haptophytes became apparent. These tiny haptophytes may
contribute from 30 to 50% of the phytoplankton standing stock across the world
ocean.
Box 2.4 Molecular genetic classifications and
phylogenetic reconstructions
(^) In recent years, classifications of organisms and evaluation of their evolutionary relationships
(phylogeny) have come to be based partly on similarities and differences in their DNA sequences.
Moreover, extended sequences can be examined to determine what developmental and metabolic
genes are present, or even to identify those genes active under different circumstances. To follow the
arguments, you need a rudimentary understanding of molecular genetics. Those rudiments are
provided here. For a more extended introduction, the Wikipedia article
(http://en.wikipedia.org/wiki/DNA) is excellent and well illustrated.
(^) The formulas, genes, for construction of proteins from 20 amino acids are stored in cells as sequences