J. Seckbach – Algae and Cyanobacteria in Extreme Environments

Автор: J. Seckbach
Название книги: Algae and Cyanobacteria in Extreme Environments
Формат: PDF
Жанр: Альгология
Страницы: 786
Качество: Изначально компьютерное, E-book

This collection of essays is devoted to algae that are unexpectedly found in harsh habitats. The authors explain how these algae thrive in various temperature ranges, extreme pH values, salt solutions, UV radiation, dryness, heavy metals, anaerobic niches, various levels of illumination, and hydrostatic pressure. Not only do the essays provide clues about life on the edges of the Earth, but possibly elsewhere in the universe as well

This volume encompasses a great diversity of subjects as well as authors, most of
whom are well recognized and established in their fields and others who are
younger, upcoming scientists of whom we will hear more in the future. The subject
is extremophiles, and is almost entirely focused on photosynthetic microbes. It is a
complementary and updated version of a volume of similar subject matter
published in 2001 (Algae and Extreme Environments: Ecology and Physiology, eds.
J. Elster, J. Seckbach, W.F. Vincent and O. Lhotsky, Nova Hedwigia, Beiheft 123:
1–602. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart. http://www.
schweizerbart.de/pubs/books/bo/novahedwig-051012300-desc.ht, Berlin/Stuttgart,
602 pp.). Few of the authors of the present volume were involved in the earlier
publication, and many of the subjects are quite different.
The studies of extremophiles have increased almost exponentially in the last
several years, mainly because of the interest in early life on this planet and possible
life (past or present) on others. This has necessitated the creation of a new and
well-received journal, Extremophiles (Volume 1, 1997 to present). Unfortunately,
few of the papers published in this journal to date have focused on phototrophs.
Papers on tolerance to environmental extremes by phototrophic microorganisms
are also frequently found in a number of journals, including Journal of Phycology,
Applied and Environmental Microbiology, Environmental Microbiology, Microbial
Ecology, Archives of Microbiology, and several others, and, of course, in the series
of volumes entitled “Cellular Origin, Life in Extreme Habitats and Astrobiology”
(www.springer.com/series/5775) edited by Joseph Seckbach.
The current volume is divided into nine subject areas in addition to an opening
section: General introductory chapter (1), (2) Phototrophs at high and low
light, (3) Phototrophs in the marine environment, (4) Phototrophs in cold environments,
(5) Phototrophs in hot alkaline and acidic environments and
non-thermal acidic habitats, (6) Phototrophs under water stress: dry and hypersaline
environments, (7) Adaptability to changing environments, (8) Other
microorganisms and extreme habitats, and (9) Outlook.
We need to remember that phototrophs, except for a few, do not generally
extend as far into some extreme environments as do the non-photosynthetic
Bacteria and Archaea. With respect to upper temperature limits, a few
Cyanobacteria and Chloroflexi extend to the lower 70°Cs which demarks the
upper boundary for a few unique organisms of each of these phyla, and consequently
the upper temperature limit for chlorophyll- or bacteriochlorophyll-based
photosynthesis (a limit known and not disputed for many decades). With respect
to upper salinity limits, there too the non-photosynthetic Bacteria and Archaea
dominate the picture, but there are exceptions. Some phototrophic purple bacteria (e.g., Halorhodospira spp.) nearly equal the upper salinity limits of some
of the non-phototrophs, and a few cyanobacteria (e.g., Halothece spp.) and
diatoms come close, as does the green alga, Dunaliella salina. The nonphototrophs
do not have the monopoly on cold or freezing tolerance, although
they may have with respect to true psychrophily, since most of the cyanobacteria
isolated from cold habitats are psychrotolerant rather than psychrophilic.
Cyanobacteria appear to have an edge over eukaryotic microalgae with ability to
withstand freezing in nature.
Acidophily or acidotolerance are well represented by the non-phototrophic
Bacteria and Archaea. The Cyanobacteria and anoxygenic phototrophic Bacteria
are almost excluded from acidic environments below pH 4–4.5 [with the exception
of Acidophilium spp. (α-Proteobacteria) that use Zn in the center of the porphyrin
ring of bacteriochlorophyll instead of Mg].
Periodic and long-term desiccation is another extreme that is well represented
by phototrophs, particularly cyanobacteria – these to a greater extent than
microalgae. It is not clear that non-phototrophic Bacteria or Archaea have the
edge with desiccation as the stressor.
The chapters vary in subject matter from algae and cyanobacteria in subaerial
urban environments, in environments that are exposed to freeze–thaw
cycles and permanent coldness, as well as speculations as to the versatility and
evolution of cyanobacteria in extremes and the role of reduced iron as a reductant
in early photosynthesis. Some papers are more focused on individual species
or natural groups of species and their responses to various extremes. Many habitats
or natural groups of organisms included here have barely been described or
discussed before in the context of extreme environments. There is extensive speculation
in some of the chapters, particularly those that constitute brief reviews of
the subjects. From a scan of abstracts and concluding remarks, I also believe that
portions or conclusions of some of the chapters may be somewhat or highly
controversial, and therefore should stimulate discussion.
With respect to the Eukarya (algae, except for a couple of chapters on
“other” protists or fungi), there is a chapter that emphasizes the uniqueness of the
dinoflagellates as a distinct group, and another, their probable propensity to
acquire endosymbionts in crowded benthic environments. Diatoms are also discussed,
particularly with respect to the mysteries of their intricate and rigidly
controlled wall patterns, and in another chapter, their remarkable ability to thrive
in cold seawater and even to endure freeze–thaw episodes.
Acidic environments are involved in a few chapters. The acidic Rio Tinto of
southwestern Spain drains ancient mine pits. There are descriptions of other acid
mine drainways that are also very rich in heavy metals that would be toxic for
most algae, but these habitats nevertheless sustain a mixed assemblage of a few
specialized species of algae, including diatoms, euglenoids, and green algae. The
biomineral deposits of the extreme Rio Tinto are suggested as possible analogs in
the search for remnants of possible microbial life in ancient deposits on Mars.
Primitive photosynthetic, unicellular red algae of the order Cyanidiales inhabit acid environments from pH 0.0 to 4.0 with temperatures from about 40 to 56°C.
A few species of acidophilic eukaryotic algae also inhabit waters of this pH range,
but at lower temperature. Thermoacidic environments such as the sulfurous vents
near Naples, Italy, are discussed with respect to the Cyanidiales that reside and
thrive there. Another chapter delves into understanding the biology of these algae
based on knowledge of their genomes.
Other chapters describe the distribution of various algae and cyanobacteria
in varied habitats (e.g., under UV stress, in dim light, on man-made substrates,
high diversity in urban environments, soils, and in other periodically xeric or
aero-terrestrial environments).
About half of the chapters focus on prokaryotes (namely cyanobacteria or
mixed communities). Some are reviews, such as that on the diversity, versatility,
and specialization of cyanobacteria that may help to explain their extraordinary
ability to inhabit various extreme environments. Three other chapters examine the
tolerances of cyanobacteria in polar and other cold environments. Some authors
have focused on specific cyanobacteria, such as Acaryochloris, the only cyanobacterium
utilizing chlorophyll d instead of chlorophyll a, the extreme
desiccation-tolerant cyanobacterium, Chroococ-cidiopsis, that may represent a
type close to a form that may have inhabited a terrain like that of Mars in happier
times past, and Mastigocladus cf. laminosus, a common global, thermophilic
cyanobacterium that exhibits somewhat dissimilar genotypes in different portions
of the thermal gradient of the same alkaline, geothermal stream as well as in near
and distant geographic locations.
There are, of course, other topics that I have not mentioned. However, this
compilation has such a remarkable variety of subject matter that I believe that it
should not be treated simply as a reference volume for specialists who would concentrate
on one habitat or on a few species, but should be read in full by every
biologist interested in extreme or unusual environments and their organisms.