Diversity of Marine Phototrophs 3.7 Diversity of Light Energy Transformation Systems and Reconsideration of Photosynthesis 31
Part A | 3.7
Haptophyta accounted for 50% of the total amount of
the picophytoplankton fraction [3.29]. A similar estimate was also reported; picohaptophytes formed 5% of
the global picophytoplankton biomass and contribute
significantly to primary production in the marine environment [3.30].
Furthermore, it was also reported that a significant
amount of microalgae in unknown phylum (unisolated
phylum to date) also exists in marine environments.
In 2007, phytoplankton which did not belong to any
known phyla of algae were discovered by metagenomic analysis of a plankton fraction of less than
3 m in size [3.31]. Its DNA sequence was close to
the cryptophycean linage. It was tentatively named picobiliphytes. The picobiliphytes distribute widely in
coastal areas of Europe and the North Atlantic, and
the cell number was reaches up to 1:6% of the total
eukaryotic phytoplankton. In 2011, the new candidate
phylum was also detected by metagenomic analysis
targeting plastid DNA. Rappemonads, the name tentatively given to the algae in the candidate phylum,
were close to haptophytes, but they did not belong to
any known phyla phylogenetically [3.32]. Rappemonads also widely distribute in coastal areas of Europe,
and the Atlantic and Pacific oceans. Since algae in
both candidate phyla have not been isolated and cultivated, the characteristics in morphology, physiology,
life cycle, photosynthesis, and productivity are not yet
known. It might possible to find a further new phylum
of photoplankton which has not yet been discovered.
The accumulation of knowledge on the diversity of
phytoplankton including unknowns might contribute to
a better understanding of primary production in marine
environments.
3.6 Strange Phototrophic(?) Microorganisms (1990–2011)
The cyanobacterium-like particle UCYN-A was detected by metagenomic analysis of nitrogen-fixing
bacteria in marine waters. It was first detected as
a cyanobacterium which had a nitrogenase gene for
nitrogen fixation in pelagic waters. The wide distribution of UCYN-A indicated that the particle contributes to the nitrogen cycle in pelagic waters. Usual
cyanobacteria grow by an oxygenic photosynthesis process photosystems, a respiration system, and genes for
a carbon fixation system. However, UCYN-A lacks
photosystem II (one of the two photosystems that split
water into protons, electrons, and molecular oxygen),
RuBisCo (ribulose-1,5-bisphosphate carboxylase-oxygenase), the primary enzyme for fixing CO 2 and the
tricarboxylic acid (TCA) cycle for respiration [3.33].
The lacking of these essential genes for free living
suggests that UCYN-A might be a symbiont in some
organisms. Since it possesses a complete gene set for
photosystem I and contains chlorophyll, it must use
light energy for some activity of the particle. While
it is unclear how the light energy absorbed by photosystem I is used in the particle, it must be contribute
to the transformation of light energy into chemical
energy.
3.7 Diversity of Light Energy Transformation Systems
and Reconsideration of Photosynthesis
Organisms that can grow using light as its energy
source are called phototrophs. Representatives of phototrophs are photosynthetic organisms that can perform
photosynthesis. Photosynthesis is classically the reaction that can reduce carbon dioxide into organic
compounds such as sugars using sunlight as a energy source. Primary producers are organisms that
synthesize organic compounds using inorganic carbon as their carbon source. Their representatives are
photosynthetic organisms (such as land plants, algae,
cyanobacteria, and some photosynthetic bacteria) and
chemoautotrophs. Since the productivity of chemoautotrophs in a general environment is considered to
be much smaller than that of photoautotrophs. Primary producers has been used synonymously with
photosynthetic organisms. Similarly, it has been considered that phototrophs and photosynthetic organisms
are almost the same so far. However, the recent discoveries of new marine microbes which utilize light
energy suggest that the conventional understanding of
the terms phototrophs, photosynthetic organisms and
primary producers may not necessarily be suitable for
Part A | 3.7
Haptophyta accounted for 50% of the total amount of
the picophytoplankton fraction [3.29]. A similar estimate was also reported; picohaptophytes formed 5% of
the global picophytoplankton biomass and contribute
significantly to primary production in the marine environment [3.30].
Furthermore, it was also reported that a significant
amount of microalgae in unknown phylum (unisolated
phylum to date) also exists in marine environments.
In 2007, phytoplankton which did not belong to any
known phyla of algae were discovered by metagenomic analysis of a plankton fraction of less than
3 m in size [3.31]. Its DNA sequence was close to
the cryptophycean linage. It was tentatively named picobiliphytes. The picobiliphytes distribute widely in
coastal areas of Europe and the North Atlantic, and
the cell number was reaches up to 1:6% of the total
eukaryotic phytoplankton. In 2011, the new candidate
phylum was also detected by metagenomic analysis
targeting plastid DNA. Rappemonads, the name tentatively given to the algae in the candidate phylum,
were close to haptophytes, but they did not belong to
any known phyla phylogenetically [3.32]. Rappemonads also widely distribute in coastal areas of Europe,
and the Atlantic and Pacific oceans. Since algae in
both candidate phyla have not been isolated and cultivated, the characteristics in morphology, physiology,
life cycle, photosynthesis, and productivity are not yet
known. It might possible to find a further new phylum
of photoplankton which has not yet been discovered.
The accumulation of knowledge on the diversity of
phytoplankton including unknowns might contribute to
a better understanding of primary production in marine
environments.
3.6 Strange Phototrophic(?) Microorganisms (1990–2011)
The cyanobacterium-like particle UCYN-A was detected by metagenomic analysis of nitrogen-fixing
bacteria in marine waters. It was first detected as
a cyanobacterium which had a nitrogenase gene for
nitrogen fixation in pelagic waters. The wide distribution of UCYN-A indicated that the particle contributes to the nitrogen cycle in pelagic waters. Usual
cyanobacteria grow by an oxygenic photosynthesis process photosystems, a respiration system, and genes for
a carbon fixation system. However, UCYN-A lacks
photosystem II (one of the two photosystems that split
water into protons, electrons, and molecular oxygen),
RuBisCo (ribulose-1,5-bisphosphate carboxylase-oxygenase), the primary enzyme for fixing CO 2 and the
tricarboxylic acid (TCA) cycle for respiration [3.33].
The lacking of these essential genes for free living
suggests that UCYN-A might be a symbiont in some
organisms. Since it possesses a complete gene set for
photosystem I and contains chlorophyll, it must use
light energy for some activity of the particle. While
it is unclear how the light energy absorbed by photosystem I is used in the particle, it must be contribute
to the transformation of light energy into chemical
energy.
3.7 Diversity of Light Energy Transformation Systems
and Reconsideration of Photosynthesis
Organisms that can grow using light as its energy
source are called phototrophs. Representatives of phototrophs are photosynthetic organisms that can perform
photosynthesis. Photosynthesis is classically the reaction that can reduce carbon dioxide into organic
compounds such as sugars using sunlight as a energy source. Primary producers are organisms that
synthesize organic compounds using inorganic carbon as their carbon source. Their representatives are
photosynthetic organisms (such as land plants, algae,
cyanobacteria, and some photosynthetic bacteria) and
chemoautotrophs. Since the productivity of chemoautotrophs in a general environment is considered to
be much smaller than that of photoautotrophs. Primary producers has been used synonymously with
photosynthetic organisms. Similarly, it has been considered that phototrophs and photosynthetic organisms
are almost the same so far. However, the recent discoveries of new marine microbes which utilize light
energy suggest that the conventional understanding of
the terms phototrophs, photosynthetic organisms and
primary producers may not necessarily be suitable for