THE DIVERSITY OF AQUATIC ENVIRONMENTS
This course is called botanical limnology, emphasizing lakes/reservoirs. However, aquatic plants, broadly defined, span everything from rain
puddles to the Great Lakes, and intermittent prairie creeks to roaring rivers
(and excluding marine habitats).
Although these will not be systematically addressed in this course, the
basic principles ought to apply to most cases.
We can explore these other environments through selection of primary
literature.
Indirect factors affecting aquatic habitats and the resident
flora:
1.
local
watershed geology - soil/rock type, slope, percolation, etc.
2.
local
climate - seasonal (not diel) temperature cycles, rainfall
amt./distribution
3.
human land
use practices - storm runoff, sewage & industrial effluent, etc.
4.
bathymetry
& geometry of basin - affects localized conditions within a body of
water
Direct
factors (controlled by indirect) influencing aquatic plants:
1.
water flow -
velocity, regularity, depth fluctuations
2.
temperature
3.
light -
turbidity & dissolved organics, daylength & solar angle
4.
water
chemistry
hardness
pH
ionic composition
nutrients
toxins
5.
sediment
properties (for benthic forms)
depth
grain size
organic content
redox properties
THE
DIVERSITY OF AQUATIC “PLANTS”
Review of botanical taxonomic classification and typical
suffixes:
Domain
Kingdom
Division
or Phylum (-phyta)
Class
(-phyceae, -cae, -ae)
Order (-ales)
Family
(-aceae)
Genus-species
Most people, scientists included, tend to think of
aquatic plants as "nuisance" organisms. While there are certainly instances where that is true, it is not
the whole story. Just as in any
terrestrial ecosystem, "plants" (here broadly defined to include all
photoautotrophic organisms) constitute the critical primary producer trophic
level, the foundation of food webs.
Aquatic plants, using the broadest
possible definition (excluding only animals and heterotrophic protists), are
very diverse and encompass the kingdoms Bacteria, Archaea, Protista, Fungi, and
Plantae. [Algae are technically in the
Protista, but their ecological role more nearly resembles plants than
heterotrophic protists.] This course
will cover only the Plantae, algal protists and cyanobacteria.
Following is a coarse breakdown of life forms in general and
aquatic “plants” in particular (see Raven
et al. 1999 Fig. 13-8):
DOMAIN BACTERIA – prokaryotes (lack a
nucleus); most heterotrophic, some chemo- or photoautotrophic
(bacteriochlorophyll a); also includes photoautotrophic, chl a-containing Cyanobacteria (or Chloroxybacteria; a.k.a.
blue-green algae); cyanobacteria in
turn include a few chl b, divinyl chl a-containing species known
as prochlorophytes (initially placed in a separate Division, now
scattered among cyanobacterial taxa)
DOMAIN ARCHAEA – porokaryotes genetically, biochemically
and metabolically distinct from bacteria; mainly from extreme environments,
often anaerobic (however, new species from a variety of habitats are routinely
discovered)
DOMAIN
EUKARYA – eukaryotes having nucleus and organelles
Kingdom Fungi – decomposers
(except lichens, which are photosynthetic via symbiotic algae)
Kingdom Protista –
eukaryotes including (among other groups) the mainly photoautotrophic, chl a-containing
algae
Algae - not an official taxon; actually a polyphyletic
assemblage (multiple independent evolutionary lines) w/ some common features;
may be micro- (most FW) or macroscopic
(mostly marine); includes several Divisions
Chlorophyta - green algae; mostly FW
& terrestrial, 20% marine
Charophyta - often considered green
algae; predominantly FW
Chrysophtya - yellow-green/golden
brown algae, incl. diatoms & coccolithophorids; FW, marine, terrestrial
Dinophyta - dinoflagellates, some
heterotrophic; mostly marine, <10% FW
Euglenophyta - euglenoids, mostly
heterotrophic; mostly FW & terrestrial, few (3%) marine
Cryptophyta - cryptomonads; small
group of FW & marine flagellates
Haptophyta
– haptophytes; mostly marine, biogeochemically important
Rhodophyta - red algae; predominantly
marine but a few FW
Phaeophyta - brown algae; virtually
all marine w/ a very few exceptions
The vast majority of FW algae (all except
Charophyta) are microscopic or small filamentous forms, whereas larger red,
brown & green algae dominate the benthos in the ocean.
At the Division level, algal taxonomy is based
largely on phylogenetically conservative biochemical properties, e.g. pigments,
chemistry of storage polymers & cell walls, flagellation.
Despite the convenient nominal color-coding of
the green, red and brown algae, there are many exceptions that fool the novice.
Kingdom Plantae –
photosynthetic eukaryotes with chl a, b
1.
Nonvascular "higher" plants
Hepatophyta - liverworts
Anthocerotophyta -
hornworts
Bryophyta – mosses
2.
Vascular plants (tracheophytes)
Microphyllophyta
Arthrophyta - "jointed"
plants
Pteridophyta - ferns
Coniferophyta -
Gymnosperms; relatively minor in aquatic habitats
Anthophyta - angiosperms = flowering
plants; most believed to be 2° adapted to aquatic life;
dominant macrophytes in lacustrine systems
INTRODUCTION TO ALGAE AND CYANOBACTERIA
"Algae" is not an official taxon, rather refers to
a convenient polyphyletic grouping of relatively simple (uni- or
multicellular), chl a-containing,
oxygenic photosynthetic organisms having several features in common (w/
exceptions, of course):
reproductive structures are unicellular (most),
superficial (epidermal derivatives) and unprotected by sterile cells; where
multicelluler, every cell is fertile
reprod. cells often differentiate from
individual cells or a change in the organism's shape, NOT by tissue or organ
development
mainly aquatic, or protected by thick cell
walls/mucilage from desiccation
most (all in FW) are nonvascular
relatively nondifferentiated cellular level of
organization
A major distinction between “algal” groups is pro-
(Cyanobacteria) and eukaryotic (all others) cells:
|
Cyanobacteria |
Eukaryotic
algae |
|
Cells typically small <1 to 10 um |
Cells typically (not always) larger (>10 um) |
|
DNA single, circular molecule lacking histones (basic
proteins) |
DNA organized into one or more chromosomes, incl.
histones, in nucleus |
|
no memb. bound organelles: Ps/R on lamellae in cytosol; R
also on invaginations of plasmalemma |
various memb. bound organelles: Ps on thylakoids in
chloroplast; R in mitochondria |
|
70 S ribosomes |
80 S ribosomes in cytoplasm, 70 S in chloroplasts, 55-80 S
in mitochondria |
|
no flagella |
complex flagellar structure comprising microtubules
(tubulin), covered by plasmalemma |
|
cell wall = Gram (-) bacterial type: peptidoglycan +
lipopolysaccharide |
cell wall = structural components such as cellulose, other
complex polysaccharides |
|
Reprod. only by binary fission; transformation and
transduction possible at least in lab |
Variable and complex life cycles, often involving sexual
reproduction (meiosis) |
Algal
Evolution
In 1971 Margulis proposed the endosymbiont theory to explain how eukaryotes evolved from
prokaryotes. This explains a number of
observations (e.g. no intermediate forms) for which the classical theory of
gradual pro- eukaryotic evolution was
inadequate. Different types of
endosymbionts could lead to the various algal divisions, but much subsequent
evolution is still required to explain flagella, nuclei, nucleus-organelle
genetic control, etc.
Red algae are generally considered the most primitive
eukaryotic algae, and along w/ Cryptophyta are only distantly related to other
groups. Cyanobacterial fossils
(stromatolites) go back >3 billion years (Precambrian). Eukaryotic algal fossils date back nearly 1
billion years, and calcified/siliceous forms are best represented in the fossil
record, which is inadequate to help much w/ reconstructing phylogeny.
Distinctive
Features of Algae
Eukaryotic algae have much in common w/ higher plants; some
distinctive features include:
pyrenoids - protein bodies either
within or associated w/ plastids that are probably involved in starch
synthesis; presence/absence seemingly random (no phylogenetic pattern) among
algae
pigments - algae have much more diverse
photosynthetic pigment systems than higher plants, including chlorophylls,
carotenoids
and phycobilins;
these are taxonomically important at the division level (see Dawes
Table 3-2 and Figs. 3-8,
9 & 10);
"all" contain chl a and -carotene
reserve polymers - algae
also contain varied and taxonomically important photosynthate storage
carbohydrates, which differ in types of monomers and how they are joined (see Dawes
Fig. 3-11); starch (alpha-1,4 and 1,6, as in higher plants) and/or various beta-1,3 polymers (e.g. laminarin), also mannitol (a polyol monomer) and oils in some cases
cell wall structure/composition -
cyanobacteria have a typical gram-negative bacterial cell wall; most eukaryotic
algae with a cell wall posess cellulose (beta-1,4 glucan, as in higher
plants) plus various other phylogenetically distinct polymers, e.g. xylan &
mannan (green algae; beta-1,3 or 1,4 glucans), alginic acid (Phaeophyta; Dawes
Fig. 2-6), carrageenan & agar (Rhodophyta; Dawes
Figs. 2-7, 9).
flagella - cyanobacteria, unlike bacteria,
lack flagella; eukaryotic algae have a variety of phylogenetically significant
flagellar morphologies, number, insertion points (Dawes
Table 3-3)
Algal
Reproduction & Life Histories
1.
Asexual
reproduction - progeny formed by cell division not involving genetic
recombination; see Bold & Wynne 1978 Fig.
1.3(a-k)
repeated bipartition or binary
fission - in colonial, muticellular forms, leads to growth
fragmentation -
vegetative fragments of colonial & multicellular algae develop into new
individuals/colonies; called hormogonia in the case of
filamentous cyanobacteria, propagules (special
"buds") in some multicellular forms
coenobic colonies (fixed #cells)
form autocolonies
within parent cells
spores - unicellular, produced
by/within ordinary vegetative cells or specialized sporangia
akinetes = thick-walled, resistant
vegetative cell in many chloro- & cyanophytes
autospores =
nonmotile miniature versions of parental cell
zoospores = flagellate, motile (or
potentially motile = aplanospores) cells common in
chloro-, chryso- and phaeophtes, but NOT rhodophytes
2.
Sexual
reproduction - involves plasmogomy (union of cells), karyogamy
(union of nuclei), chromosome/gene association, and meiosis, resulting in
genetic recombination; known to occur in at least some species from all
divisions except Euglenophyta; see B
& W Fig. 1.3 (l-n)
gametes may sometimes develop parthenogenetically
(without fusion), or more commonly fuse (syngamy) w/ one of three patterns:
isogamous = morphologically
indistinguishable
anisogamous = heterogamous w/ slightly
unequal size
oogamous = large, nonmotile
"egg" and small motile "sperm" (nonmotile spermatia
in reds)
gametes may be morphologically identical to
vegetative cells or, more typically in multicellular algae, markedly different
from " "
gametes develop from variously specialized gametangia,
known as oogonia (carpogonia w/ receptive trichogyne
in reds) & antheridia (spermatangia in reds) in oogamous
algae
monoecious spp. produce both male
& female gametes on the same individual; dioecious spp. have
distinct male & female plants
life
histories of algae are diverse (B
& W Fig. 1.4), often complex and frequently known only from culture
haplontic = one free-living haploid
stage; only zygote is diploid: zygotic meiosis; common in
unicellular greens; Graham & Wilcox 2000 Fig.
1-22
diplontic = one free-living diploid
stage; only gametes are haploid: gametic meiosis; common in
coenocytic greens, fucoids, and diatoms; Graham & Wilcox Fig.
1-23
haplodiplontic = both
haploid (gametophyte) & diploid (sporophyte) free-living
stages (also called alternation of generations), either morphologically similar
(isomorphic)
or different (heteromorphic); sporic meiosis; Graham &
Wilcox Fig.
1-24
“DIVISION”
CYANOBACTERIA (= CHLOROXYBACTERIA, formerly “blue-green algae”)
Cyanobacteria (technically bacteria, not algae) comprise a
single class, Cyanophyceae. Properties
include:
most ancient (~3.5 billion years) oxygenic
photoautotrophs - responsible for initial free O2 in
atmosphere
~2000 spp.?, ubiquitous in marine, FW &
terrestrial habitats; planktonic, benthic (often in mats or endo-/epiphytic),
endolithic
usually extremely small cells (typically <
10 um), either solitary, colonial or filamentous (a few are branched)
bacterial type (Gram -) cell wall w/ variably
thick gelatinous sheath; filament
= trichome
+ sheath
nonflagellate, but may be motile via gliding
peripheral thylakoids
(photosynthetic lamellae) around a central bacteria-like DNA region
chl a,
-carotene, myxoxanthin, zeaxanthin,
phycobilins (organized in phycobilisomes on unstacked
thlakoids): C-phycocyanin (dominant, blueish color), C-phycoerythrin, allophycocyanin;
a few also lack phycobilins, have paired thylakoids, and contain chl b
and divinyl chl a (these “prochlorophytes”, initially placed in a separate Division,
are actually polyphyletic based on DNA sequencing)
only asexual reprod. known, but genetic
evidence suggests conjugation; susceptible to viruses
minimal differentiation, except for
exo-/endospores, akinetes (resting spores) and, in some forms, heterocysts
= thick-walled cells responsible for N2-fixation
very important in biogeochemical N-cycling
Three
orders (apparently contentious taxonomy at lower levels; only DNA sequencing
can resolve):
1. Chroococcales
- unicellular (binary fission) or noncoenobic colonies (fragmentation);
includes common benthic (Gloeocapsa)
and planktonic (Synechococcus,
Prochlorococcus) forms
2. Chamaesiphonales
- unicellular, filamentous or colonial; produce endo-/exospores
endospores ä 
ãexospores
3. Oscillatoriales
- filamentous, non endo-/exospore-producing; incl. Oscillatoria, Spirulina, Lyngbya, Anabaena, Nostoc (latter 2
heterocyst-formers, which form a distinct genetic clade); also Prochlorothrix?
Paerl (Sandgren Table 7-1a,
b)
recognizes 4 morphotypes despite some environmentally-induced cell size
plasticity; putative ecological significance of the various morphologies is
speculative:
1.
unicellular
picoplankton (0.2-3.0 um): Synechococcus, Cyanodictyon, Prochlorococcus
ubiquitous in lakes & oceans (especially
far offshore) at all depths
often dominant 1 prod., esp. in
oligotrophic conditions (high SA:V to compete for nuts)
often form discrete layers in large
oligotrophic lakes (e.g. Tahoe)
2.
colonial coccoid
nano-/microplankton (2-10 um): Microcystis,
Gomphosphaeria, Croococcus
common in meso-/eutrophic ponds, lakes, rivers
may form thick sfc. scums during calm
stratified cond.
3.
solitary
filamentous microplankton: Lyngbya,
Oscillatoria
often form distinct subsfc. (metalimnetic)
layers
occas. rapidly alter depth (buoyancy
regulation), presumably to optimize light & nuts
occas. dominant in blooms, but usually
subdominant in meso-/eutrophic waters
4.
colonial
filamentous microplankton: Anabaena,
Aphanizomenon, Gloeotrichia, Nostoc
large, buoyant, suspended or benthic
aggregates, sometimes visible w/ naked eye
majority form heterocysts for N2-fixation
under N-deficient, Pand Fe-sufficient conditions Sandgren Figs.
7-6, 8, 10
Summary of Planktonic Cyanobacterial Ecology
require physical stability: turbulence or
fluctuating conditions prevent/eliminate blooms Sandgren Fig.
7-1
common in extreme (but stable) habitats
prefer neutral to acidic water
efficient buoyancy regulation
dominance aided by high organics, high P:N
ratio (due to N2-fixation ability), low metals
"leaky" of photosynthate &
organic N attracts helpful bacteria?
some produce potent hepato-/neurotoxins of
unkown ecological role
not preferred food of microcrustaceans;
protists, rotifers, tropical cichlids (Tilapia)
& flamingos are efficient grazers
akinetes form under unfavorable cond., may be
dormant for years, germinate when favorable
DIVISION
CHRYSOPHYTA
Chl a,
c (most), -carotene, domin. by
xanthophylls: fucoxanthin and/or diato-/diadinoxanthin
mostly small, unicellular FW forms, but two
groups, diatoms and coccolithophorids, are often dominant in marine
phytoplankton or epipelic assemblages
-1,3 linked glucans (e.g.
chrysolaminarin) and oils (esp. diatoms) storage polymers
poorly known life histories; diatoms diplontic,
most others haplontic
chloroplasts have 3-banded thylakoids + girdle
lamella
class-specific flagellation/motility; where present,
flagella are apical
pyrenoids, eyespots may be present, sometimes
conspicuous
cell walls absent (ameboid), or scalelike
(secreted by Golgi vesicles): silica or cellulose
CLASS BACILLARIOPHYCEAE (diatoms) [some authors consider this
a separate Division]
very diverse (10-20,000+ spp., half FW) and abundant,
probably 20-25% global productivity
size range from 2 um to 2 mm (equivalent range
to smallest mosses vs. largest trees)
uninucleate, unicellular, nonflagellate (except
sperm in some - anterior, hairy)