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Frequency, abundance and distribution of zoosporic organisms from Las Can˜as
stream (Buenos Aires, Argentina)
Agostina V. Marano
1
Instituto de Bota´nica Spegazzini, calle 53 N 477,
La Plata, Buenos Aires, Argentina
Marcelo D. Barrera
Laboratorio de Investigacio´n en Sistemas Ecolo´gicos y
Ambientales, diagonal 113 N 469, La Plata, Buenos
Aires, Argentina
Mo´nica M. Steciow
Instituto de Bota´nica Spegazzini, calle 53 N 477,
La Plata, Buenos Aires, Argentina
Jorge L. Donadelli
Instituto de Limnologı
´a Dr R.A. Ringuelet, avenida.
Calchaquı
´Km. 23.5, Florencio Varela, Buenos Aires,
Argentina
Mario C.N. Saparrat
Instituto de Fisiologı
´a Vegetal, diagonal 113 esq. 61,
La Plata, Buenos Aires, Argentina, and Instituto de
Bota´nica Spegazzini, calle 53 N 477, La Plata, Buenos
Aires, Argentina
Abstract
:Zoosporic organisms are common inhabi-
tants of aquatic environments; however there are few
ecological studies made for Argentinean streams. In
this contribution the taxonomic composition of
zoosporic organisms from a stream and their abun-
dance, frequency and diversity on cellulosic baits were
analyzed. Samples of water and floating organic
matter (vegetable debris) were taken at four dates
and different environmental variables (temperature,
dissolved oxygen and nutrient concentrations) were
measured. Twenty-one taxa were recovered with the
baiting technique. Physicochemical fluctuations af-
fected the structure of the studied community; in
spring the greatest species richness was related to
high nutrient levels whereas in winter the greatest
abundance and diversity was related to low water
temperature, nutrient levels and well oxygenated
conditions.
Key words:
aquatic environment, cellulosic
substrates, Chytridiomycota, Peronosporomycota
INTRODUCTION
Among the fungal members associated with freshwa-
ter bodies zoosporic organisms comprise phylogenet-
ically unrelated groups of taxa belonging to kingdoms
Fungi (Blastocladiomycota and Chytridiomycota) and
Straminipila (Hyphochytridiomycota and Peronos-
poromycota) and occur mainly as saprotrophic forms
on plant debris (Mu¨eller et al 2004, James et al 2006).
These organisms represent a key group in the organic
matter decomposition into aquatic systems (Czeczuga
2000), revealing an outstanding ability for degrading
cellulose, the major component of vegetable debris
(Park 1974).
Las Can˜as stream is a lotic system in the Rı´o de la
Plata basin and is in the Reserva Natural Integral Selva
Marginal Punta Lara on the northeastern part of
Ensenada and Berazategui districts, Buenos Aires,
Argentina (Cabrera 1960). This water body receives a
considerable amount of up-water from the Rı´o de La
Plata, which contributes to the movement of particle
materials from natural origin, sediments and pollut-
ants (Steciow 1998). Preliminary systematic studies
have shown the existence of a diverse array of
zoosporic organisms in this stream (Marano and
Steciow 2006a, Marano et al 2006, 2007, Steciow and
Marano 2007). However little information is available
on the factors that influence the occurrence and
distribution of zoosporic organisms (Sparrow 1968,
Dick 1976, Dix and Webster 1995). In Argentina,
while several works deal with systematics (Steciow
2000, 2001a, b, 2002a, b, Steciow and Elı´ades 2002),
few regard the occurrence of these organisms in lotic
environments (Steciow 1998, Marano and Steciow
2006b).
The aim of this work was to analyze the composi-
tion of the zoosporic organisms (Chytridiomycota and
Peronosporomycota) on a cellulosic material (corn
leaves) and to assess trends of abundance, frequency
and diversity and their relationship with temperature,
dissolved oxygen and nutrient concentrations in a
lotic system of Buenos Aires province (Argentina).
MATERIALS AND METHODS
Description of the study area.—
The study was carried out in
Las Can˜as stream (34u47958.50S–57u57919.30W, 34u47929.30S–
57u59949.20W), a 600 m long lotic system surrounded by an
alluvial forest (locally known as ‘‘selva marginal’’). The
geomorphology consists of basins and levees (50–500 m
wide) with alluvial to gley humic soils (Cappannini and
Maurin˜o 1966). Human interventions in this area have
greatly modified the original vegetation, so the resulting
landscape is a mixture of native
Allophyllus edulis
(Camb.)
Accepted for publication 1 April 2008.
1
Corresponding author. E-mail: agosvm@hotmail.com
Mycologia,
100(5), 2008, pp. 691–700. DOI: 10.3852/07-198
#2008 by The Mycological Society of America, Lawrence, KS 66044-8897
691
Radlk.,
Citharexylon montevidense
(Spreng.) Mold.,
Loncho-
carpus nitidus
(Vog.) Benth.,
Ocotea acutifolia
(Nees.) Mez.,
Pouteria salicifolia
(Spreng.) Radlk. and exotic
Ligustrum
lucidum
Ait. forest species (Cabrera and Dawson 1944,
Dascanio et al 1994).
Sampling.—
Samples were collected at four dates autumn
2006–summer 2007 (fall 22 Mar 2006; winter 28 Jun 2006;
spring 18 Oct 2006; and summer 28 Feb 2007).
A 200 m transect was established and six sites (M1, M2,
M3, M4, M5 and M6) were sampled from headwaters to
where the stream flows into the Rı´o de la Plata. At each
sampling site the measurement of the morphometric and
hydraulic characteristics and temperature was made. Two
randomly distributed subsamples of surface water (200 mL)
and floating organic matter (200 g) for zoosporic organisms
analysis and for physical and chemical parameters were
collected at each site. In addition a 7 36 cm plastic 1 mm
mesh bag containing 50 disks (5 mm diam) of corn leaves
used as bait was placed in the stream 10 d, fixed to
polyestyrene floating structures attached to the bank. Disks
placed in the stream for colonization are here referred to as
in situ samples.
Water samples for nutrient analysis were filtered imme-
diately through glass fiber filters (Whatman GF/C) and
stored together with the other samples at 4 C until
processed (12–24 h).
Laboratory analysis.—
Dissolved oxygen (DO), biological
(BOD) and chemical oxygen demand (COD) as well as
sulphate, nitrate, nitrite, ammonia and soluble reactive
phosphorus (SRP) concentrations were determined accord-
ing to Mackereth et al (1978) and the pH was measured
(Jenco electronics Co. Ltd., 6091).
Samples for zoosporic organisms analysis were proccessed
with the baiting technique (Sparrow 1960, Stevens 1974).
Aliquots of 30 mL of the sampled water were placed in Petri
dishes and each baited with five corn leaves disks. For the
preparation of organic matter samples 7 g was flooded with
50 mL sterile deionized water and also baited with that
substrate. In situ samples were washed several times in
sterile deionized water and processed first by direct
observation and then baited as the other samples.
Each Petri dish was considered a sample unit. Two sample
units of each subsample obtained were carried out, and a
total of 240 Petri dishes with 1200 cellulosic baits (60 sample
units for each sampled date) were analyzed: 12 sample units
for in situ, 24 for organic matter and 24 for water samples.
Cultures were kept at room temperature (18–22 C) 42 d
(Letcher and Powell 2002) and examined at 4, 7, 10, 15, 20,
30 and 42 d of the incubation period.
Data analysis.—
The species composition of zoosporic
organisms (Chytridiomycota and Peronosporomycota) was
characterized. Taxonomic identifications were made ac-
cording to Coker (1923), Sparrow (1960), Seymour (1970),
Karling (1977) and Johnson et al (2002), with an Olympus
BX 40 microscope (Olympus Optical Co. Ltd., Tokyo).
Presence-absence (occurrence) of the taxa in each Petri
dish was recorded. A species was recorded as present for a
sample if it was observed at any of the observational times
(4–42 d). Frequency and abundance was calculated accord-
ing to Marano and Steciow (2006c): (i) frequency of
colonization (FC) (number of sample units colonized by a
taxon/number of sample units examined) 3100 (Figuer-
eda and Barata 2007), and (ii) abundance (A) (number of
isolations recorded for a taxon/number of substrate units
employed) 3100.
Frequency and abundance were determined for total
collections (a collection consisted of all samples taken from
all sites in each season) and for each type of sample (water,
organic matter or in situ baits). The species were assigned to
five frequency groupings of the Braun-Blanquet scale:
ubiquitous 100–80.1%occurrence; common 80–60.1%
occurrence; often present 60–40.1%occurrence; scarce
40–20.1%occurrence; and rare 20–0.1%occurrence (Ker-
shaw 1973, Letcher and Powell 2001, 2002).
Community structure was analyzed by (i) species richness
(S), (ii) Shannon’s diversity index H952S
S
i51
p
i
.log
2
(
p
i
),
where
pi
is the abundance of the species
i
that contributes
to total diversity, (iii) eveness E 5H9/H9
max
, where H9
max
is
the maximum value of the diversity for the number of
species that are present (Magurran 1988), (iv) Simpson’s
dominance index D 512S
S
i51
(
p
i
)
2
, which was calculated
for each season based on species abundance and (v)
Sorensen’s similarity index SI 52
j
/(
a
+
b
), where
j
is the
number of species common to both seasons/sample types
and
a
is the number of species in season/sample type A,
b
being the number of species in season/sample type B (this
index is equal to 1 in cases of complete similarity and 0 if
they have no species in common).
Kruskall-Wallis test was employed to explore differences
in frequency and abundance. Differences observed in the
diversity between seasons were tested for significance with
the H-t test.
A principal component analysis (Legendre and Legendre
1998) was performed to explore trends in the physicochem-
ical environment and in the zoosporic organisms composi-
tion and abundance.
RESULTS
Physicochemical characteristics.—
Water physical and
chemical characteristics are shown (TABLE I). Spring
was characterized by a slight increase in pH and high
nitrite, nitrate, ammonia, sulfate and soluble reactive
phosphorus (SRP) values. In fall SRP, BOD and COD
were high, whereas nitrite, sulfate and ammonium
were low. In summer ammonium was low, whereas
nitrate was high; COD and BOD reached the highest
levels in some of the sampling sites. In winter DO
levels were the highest recorded and BOD and COD
were the lowest observed. Sulfate and nitrite were low,
the ammonium presented fluctuations among sites,
whereas nitrate and SRP were the lowest encountered.
PCA ordination of the sampling sites according to
the stream physicochemical variables showed that axis
1 and axis 2 accounted for 74%of the total variance
(FIG. 1). The sites were grouped in spring, winter and
692 MYCOLOGIA
fall-summer. Axis 1 was negatively correlated with
nitrite (20.96), sulfate (20.91) and pH (20.91). Axis
2 was negatively correlated with DO (20.61) and
ammonium (20.66) and positively correlated with
COD (0.78) and BOD (0.90).
Species composition and diversity analysis.—
A total of
21 taxa were identified, 14 Chytridiomycota and 7
Peronosporomycota (TABLE II). Thirteen out of the
14 Chytridiomycota were Chytridiales (93%), with the
Cladochytriaceae family accounting for 69%and the
Chytridiaceae family accounting for 31%of them.
Only
Rhizophlyctis rosea
was found within the Spizel-
lomycetales.
The total number of isolations recorded was 758,
therefore the abundance of zoosporic organisms was
63%. Four hundred thirty-eight of these isolations
were Chytridiomycota (58%) and 319 Peronospor-
omycota (42%). The number of positive sample units
was 179, thus showing that zoosporic organisms were
common (FC 75%). The diversity of the community
was 3.49 and their evenness (E) was 0.79. The most
abundant and frequent taxa was
Dictyuchus
sp. (A
17.4%, FC 31.2%), followed in abundance by
N.
elegans
(9.7) and
Nowakowskiella
sp. 1 (9.3). As
regards frequency
Nowakowskiella
sp. 1 (20.8%)was
more frequent than
N. elegans
(17.1%), which, like
the rest of the species found, was rare, according to
the Braun-Blanquet scale. Few species showed values
of abundance (A) .10; most of them were found
unusually and were recovered in fewer than 30
isolations (A ,5).
Although higher abundance and frequency were
recorded respectively in winter and spring, the
differences were not significant between seasons (
P
.0.05, FIG. 2).
Species richness (S) was greater in spring (17
species) than in summer (16), winter (15) and fall
(11). The greatest diversity was observed in winter,
followed by spring and summer samples. The diversity
was significantly lower only in fall (
P
,0.05, FIG. 3).
While similar evenness was obtained in fall, summer
and spring, fall showed the lowest species richness (S
11), determining the highest value of Simpson’s
dominance index (D 0.22).
Based on species composition Sorensen’s index
showed higher values in summer-spring (SI 0.84),
followed by winter-spring (0.80), winter-summer
(0.76) and winter-fall (0.72) samples. The most
different were summer-fall (0.61) and spring-fall
samples (0.52).
PCA ordination of the seasons according to species
composition and abundance showed that axis 1 and
axis 2 accounted for 78%of the total variance
observed (FIG. 4). On axis 1 spring and fall were the
seasons placed on the extremes.
Ch. hyalinus
,
C.
TABLE I. Average values of the environmental variables measured at each season
Fall Winter Spring Summer
Width (m) 6.82 61.71 6.63 61.31 7.27 62.09 9.57 61.77
Depth (m) 0.34 60.13 0.34 60.15 0.34 60.16 0.88 60.27
Flow (m. s
21
) N/A 0.02 60.01 0.03 60.02 0.05 60.01
Temperature (C) 20.33 61.51 9.17 60.41 18.50 61.22 21.67 61.63
pH (range) 6.74–7.45 7.04–7.13 7.59–8.31 6.70–7.35
DO (mg. L
21
) 2.90 61.67 5.20 60.12 2.87 61.39 3.98 62.26
BOD
5
(mg. L
21
) 8.33 60.82 2.17 60.75 6.17 61.72 9.67 61.75
COD (mg. L
21
) 49.83 61.33 22.5 61.05 23.17 612.07 40 617.23
Sulfate (mg. L
21
) 2.31 61.34 0.41 60.28 19.08 60.24 10.69 61.98
Nitrate (mg. L
21
) 0.30 60.02 0.05 60.01 0.57 60.21 0.62 60.17
Nitrite (mg. L
21
) 0.01 60.002 0.01 60.003 0.10 60.01 0.02 60.03
Ammonium (mg. L
21
) 0.05 60.01 0.59 60.50 0.66 60.12 0.004 60.007
SRP (mg. L
21
) 0.51 60.01 0.17 60.04 0.57 60.01 0.28 60.06
FIG. 1. PCA ordination of the samples according to the
stream physicochemical variables.
MARANO ET AL:DIVERSITY OF ZOOSPORIC ORGANISMS 693
TABLE II. Species composition and their presence (*) in each sample type. (In gray, taxa that are restricted to one
sample type.)
Sample type Reference Water Organic Matter In situ
Phylum CHYTRIDIOMYCOTA
Order Chytridiales
Family Chytridiaceae
Chytriomyces
sp. CHYSP *
Chytriomyces hyalinus
Karling CHYHYA *
Cylindrochytridium johnstonii
Karling CYLJOH ***
Karlingiomyces lobatum
(Karling) Sparrow KARLOB *
Karlingiomyces marilandicus
Karling (Sparrow) KARMAR ***
Family Cladochytriaceae
Cladochytrium replicatum
Karling CLAREP ***
Nowakowskiella
sp. No. 1 NOWSP1 ***
Nowakowskiella
sp. No. 2 NOWSP2 **
Nowakowskiella elegans
(Nowak.) Schroeter NOWELE ***
Nowakowskiella hemisphaerospora
Shanor NOWHEM ***
Nowakowskiella multispora
Karling NOWMUL **
Nowakowskiella ramosa
Butler NOWRAM **
Septochytrium variabile
Berdan SEPVAR ***
Order Spizellomycetales
Family Spizellomycetaceae
Rhizophlyctis rosea
(de Bary and Woronin) Fischer RHIROS ***
Phylum PERONOSPOROMYCOTA
Order Saprolegniales
Family Saprolegniaceae
Achlya
sp. ACHSP **
Aphanomyces
sp. APHSP **
Dictyuchus
sp. DICSP ***
Dictyuchus monosporus
Leitgeb SAPSP ***
Saprolegnia
sp. * * *
Order Leptomitales
Family Leptolegniellaceae
Aphanomycopsis saprophytica
Karling APHSAP ***
Order Peronosporales
Family Peronosporaceae
Pythium
sp. PYTSP ***
FIG.2. Abundance(A%) and frequency (FC%)of
zoosporic organisms in the sampled dates. Data were
transformed with arcosin of (x). Error bars 5SE. The
same letters above bars indicates that the values do not
differ significantly as determined by Kruskall-Wallis test (
P
,0.05).
FIG. 3. Diversity index calculated for each season. Error
bars 5SE. The same letters above bars indicates that the
values do not differ significantly as determined by H-t test (
P
,0.05).
694 MYCOLOGIA
replicatum
,
Cy. johnstonii
and
N. elegans
correlated
positively with spring samples, whereas
Dictyuchus
sp.,
K. lobatum
and
Saprolegnia
sp. were negatively
associated with axis 1. Axis 2 separated winter and
summer samples;
N. ramosa
,
R. rosea
,
Aphanomyces
sp.
were associated with winter samples whereas
Ap.
saprophytica
,
Nowakowskiella
sp. 1 and
Se. variabile
were associated with summer. Other taxa had the
greatest abundance in winter-fall (
Pythium
sp. and
K.
marylandicus
), fall-summer (
Achlya
sp. and
D. mono-
sporus
) and summer-spring samples (
Nowakowskiella
sp. 2 and
N. multispora
).
Dictyuchus
sp. showed high abundance in all
seasons, being greater in winter, decreasing in fall,
summer and spring, when frequency was also lower
(17%) than in the remainder of seasons (35–37%).
N.
hemisphaerospora
was more abundant in fall,
K.
marylandicus
and
Pythium
sp. in winter,
N. elegans
and
Cy. johnstonii
in spring and
Nowakowskiella
sp. 1
and
Se. variabile
in summer (TABLE III). In fall the
taxa exhibited the lowest abundance, being greater
for
Pythium
sp. (5.7) and
N. hemisphaerospora
(5.0). In
winter the remainder of the taxa showed a low
abundance (,10) with a maximum for
Pythium
sp.
(8.7),
N. elegans
(7.7) and
R. rosea
(7.3). These taxa
were also the most frequent ones in this season,
colonizing 17–25%of the samples.
N. elegans
(26.0),
R. rosea
(8.3),
Nowakowskiella
sp. 1 (8.0) and
Cy.
johnstonii
(6.7) were the most abundant in spring,
whereas
Nowakowskiella
sp. 1 (A 19.7%and FC 40%),
Ap. saprophytica
(6.0) and
Se. variabile
(5.3) were the
most abundant and frequent taxa in summer.
Some prevalent taxa were present at the four
sampled dates (TABLE III) and others that were
recovered only in fall (
K. lobatum
) or in spring
(
Chytriomyces
sp. and
Ch. hyalinus
). Some taxa with
a constancy of 3 were not recorded in fall (
C.
replicatum
,
N. ramosa
,
Se. variabile
and
Ap. sapro-
phytica
), in spring (
D. monosporus
)orwinter
(
Aphanomyces
sp.).
N. hemisphaerospora
and
Saproleg-
nia
sp. were found in fall and winter,
Cy. johnstonii
,
Nowakowskiella
sp. 2 and
N. multispora
in spring and
summer, whereas only
Achlya
sp. was found in
summer and fall.
With regard to the sample types recovered, the total
number of isolations was greater for in situ (336) than
for organic matter (261) and water samples (205),
whereas species richness, evenness and diversity were
greater for in situ (S 18, E 0.82, H93.44) than for
water (S 15, E 0.76, H92.98) and organic matter
samples (S 14, E 0.71, H92.71). In fall diversity and
evenness were higher for water than for in situ and
organic matter samples, whereas the species richness
was ordered as listed: in situ (9), organic matter (8)
and water samples (7). In other seasons in situ
samples were more diverse and exhibited more
species richness (12 in winter, 11 in summer and 10
in spring) than organic matter and water (FIG. 5).
Chytriomyces
sp. and
Ch. hyalinus
were restricted to in
situ samples and
K. lobatum
to water (TABLE II). The
greatest number of isolations from organic matter was
obtained in winter and the smallest in spring, whereas
in water samples it was the opposite.
Sorensen’s index showed that species composition
among sample types was similar, being the highest for
FIG. 4. PCA ordination of the seasons according to their
species composition and abundance. (See references of the
species in TABLE II.)
TABLE III. Abundance of the species recovered. Species
are listed in decreasing order of constancy and abundance
Fall Winter Spring Summer
Dictyuchus
sp. 20.7 26.3 4.3 18.3
K. marylandicus
3.0 4.0 0.3 0.7
N. elegans
1.3 7.7 26.0 3.7
Nowakowskiella
sp. No. 1 4.0 5.3 8.0 19.7
Pythium
sp. 5.7 8.7 1.0 2.3
R. rosea
2.7 7.3 8.3 0.3
Ap. saprophytica
0 3.7 3.0 6.0
Aphanomyces
sp. 0.3 2.0 1.3 0
C. replicatum
0 4.3 4.0 2.3
D. monosporus
3.3 0.7 0 3.3
N. ramosa
0 2.7 0.7 0.7
Se. variabile
0 2.0 1.3 5.3
Achlya
sp. 2.0 0 0 2.3
Cy. johnstonii
0 0 6.7 1.0
Nowakowskiella
sp. No. 2 0 0 1.7 1.3
N. hemisphaerospora
5.0 0.7 0 0
N. multispora
0 0 1.7 1.7
Saprolegnia
sp. 1.0 0.3 0 0
Ch. hyalinus
0 0 1.0 0
Chytriomyces
sp. 0 0 0.3 0
K. lobatum
0.3 0 0 0
MARANO ET AL:DIVERSITY OF ZOOSPORIC ORGANISMS 695
in situ-organic matter (0.79), followed by in situ-water
(0.76) and the lowest for water-organic matter
samples (0.69).
DISCUSSION
Zoosporic organisms were abundant and common
and most of their diversity at this community lies on
the Chytridiomycota, which predominates on the
Peronosporomycota in frequency and abundance.
Taxa that exhibited high frequencies also showed
high values of abundance and therefore could be
considered as indicators of this community. Domi-
nant taxa, in this case
Dictyuchus
sp.,
N. elegans
and
Nowakowskiella
sp. 1, characterized the community
structure whereas the ones with rare occurrence
determined the diversity of the stream analyzed.
More prevalent taxa, such as
Dictyuchus
sp.,
Now-
akowskiella
sp. 1,
N. elegans
,
K. marylandicus
,
Pythium
sp. and
R. rosea
, also characterized community
structure because of their presence in all dates
analyzed. In agreement with Letcher and Powell
(2001) the results show that microfungal communi-
ties have characteristic species structure that consist of
few abundant taxa and a larger number that are
uncommon. In most habitats a few chytrid species are
relatively frequent and abundant (i.e.
Ch. hyalinus
in
freshwater and
R. rosea
in agricultural soils) whereas
most species are infrequent and scarce to rare
(Letcher and Powell 2001, Letcher et al 2004).
Particulary in streams running through riverine
forests fallen leaves are the major source of cellulosic
materials that enter the aquatic system (Kaushik and
Hynes 1968, Park 1972). Although little is known
about the role of zoosporic organisms in litter
breakdown (Willoughby 1974), their ubiquity and
enzymatic potential testify their importance in cellu-
lose decomposition (Mitchell and Deacon 1986,
Steciow 1993). Some of the taxa recorded in the
present study, such as
N. elegans
and
R. rosea
, are
regarded as cellulose decomposers of plant materials
(Willoughby 1964, Schoenlein-Crusius and Milanez
1998).
Changesinthesupplyofallochthonouslitter
influence zoosporic organisms frequencies. In
spring we observed an increase in the frequencies
of colonization, probably due to major inputs of
organic matter of allochtonous origin (Dascanio et al
1994), which offer a great amount of available
substrates.
Fluctuations were observed in species composition,
richness and diversity; maximum diversity was ob-
tained for winter and the lowest for fall.
Similarly to that observed by Czeczuga et al (2003) a
different mycobiota was identified for spring and fall
samples, as reflected in the lowest similarity index
obtained. In this sense
Chytriomyces
sp. and
Ch.
hyalinus
were restricted to spring and
K. lobatum
to
fall samples.
Rattan et al (1980), Misra (1982) and Smith et al
(1984) have reported that water temperature plays a
key role in zoosporic organisms populations, deter-
mining the number of isolations recovered. Accord-
ing to our results, more abundance and a higher
diversity were found at low temperatures (9–10 C)
than at moderate ones (18–24 C); in contast frequen-
cy and species richness was greater in spring and
summer. Czeczuga et al (2002) recorded a higher
diversity in winter when water temperature was low.
El-Hissy and Khallil (1991) found that these organ-
isms exhibited an increase in the number of records
up to a maximum in winter and then a gradual
decrease until summer, when the species apparently
dissapeared. Petersen (1910) and Alabi (1971) found
that spring was the most favorable season for
collecting zoosporic organisms in temperate areas.
Steciow (1997) reported from Rio Santiago area
(Argentina) the highest frequencies of Peronospor-
omycota in spring and fall, with middle values in
summer and a minimum in winter. In this way our
results are in agreement with previous works, suggest-
ing greater abundance and diversity under low water
temperature and higher frequency and species
richness in moderate temperature conditions.
Ismail et al (1979) reported that
Saprolegnia
was
FIG. 5. Diversity (A) and eveness (B) for each sample
type in the sampled dates. Error bars 5SE.
696 MYCOLOGIA
more abundant at low to moderate temperatures and
disappeared in summer months. In addition Ziegler
(1958) and Hughes (1962) listed all
Saprolegnia
species as ‘‘cold weather’’ organisms that predomi-
nate in winter; in contrast Lund (1934) found that the
abundance distribution of this genus was not temper-
ature dependent.
El-Hissy and Khallil (1991) found that
Achlya
sp.,
Dictyuchus
sp.,
Pythium
sp. and
Saprolegnia
sp. were
the taxa with higher frequencies and constancy. In
agreement with these authors we found that
Dictyu-
chus
sp. and
Pythium
sp. exhibited the maximum
constancy within the Peronosporomycota.
K. mary-
landicus
,
N. elegans
,
Nowakowskiella
sp. 1 and
R. rosea
were the most frequent and constant within the
Chytridiomycota; the remainder of species were of
low-to-rare occurrence among seasons.
In situ and organic matter samples yielded more
isolations than water, probably due to the presence of
larger amounts and diversity of propagule types
(mycelium, resistant structures and encysted zoo-
spores). Water samples are subjected to the rigors of
sampling and transport that may destroy a great
number of zoospores if a suitable substrate is not
available for colonization within a few hours (Wil-
loughby 1962, Mu¨eller et al 2004). By contrast baiting
in situ favors the zoospore chances of encountering
the substrate by chemotaxis and keeping this loss to a
minimum.
Species richness and diversity was greater for in situ
baits, being related to the course of the substrate
succesion, which becomes progressively suitable for
the development of later colonizers mainly because of
their different nutrient requirements, the alteration
of the physical conditions and/or possible organisms
interrelationships (e.g. antagonism). Also in this case
organisms are subjected to more realistic conditions
of germination and growing.
Although similarity indexes between samples were
high some taxa appeared only in a particular type of
sample (e.g.
Chytriomyces
sp. and
Ch. hyalinus
were
restricted to in situ baits). Because different methods
do not always reveal the same taxa the recovery of
multiple types of samples will increase the probability
to find most species.
Biological and physicochemical relationship.—
Water
physicochemical variables exhibited some trends
reflected on the structure of the zoosporic organisms
community. The mycobiota developed on in situ
samples was more directly exposed to those changes.
The abundance of organic matter in the stream
water (particularly in spring) possibly favored the
conditions for the development of decomposers, thus
increasing BOD and decreasing DO levels.
Winter and fall presented opposite DO, COD, BOD
and nutrient levels (mainly SRP). Those factors
probably affect the diversity, changing the species
composition and making a replacement of species
among seasons.
In fall
N. hemisphaerospora
and
Saprolegnia
sp.
presented a higher abundance and
K. lobatum
was
recovered only in this season when ammonium,
nitrate, nitrite and sulfate were low and BOD, COD
and SRP were high.
N. ramosa
and
Aphanomyces
sp.
were more abundant in winter when nutrient
concentrations were low and SRP, BOD and COD
were the lowest recorded.
N. elegans
and
Cy. johnstonii
presented the greatest abundance in spring when
ammonium, nitrate, nitrite and SRP were high.
Ap.
saprophytica
,
Nowakowskiella
sp. 1 and
Se. variabile
characterized summer samples, where the nitrite,
ammonium and SRP were low, sulphate, nitrate and
DO were high and COD and BOD were the highest
recorded. In winter we recorded the greatest diversity,
which might be influenced at least by high DO and
low SRP (in contrast in fall we found the lowest
diversity related to low DO and high SRP levels).
On the other hand the species richness was greater
inspring,whensulphateandnitratewerehigh.
Czeczuga and Muszyn´ska (2001) found the largest
number of species in samples where nitrate and SRP
were low. El-Hissy et al (2001) obtained the greatest
species richness in habitats with low nitrate, and Park
et al (1978) found a higher number of isolations in
sites with high ammonia content. The influence of
water nitrogen content might be related to the use of
inorganic nitrogen compounds (i.e. nitrate and
ammonium) by these organisms (Cantino and
Turian 1959, Schoenlein-Crusius et al 1999).
N.
elegans
and
N. ramosa
areabletometabolize
inorganic nitrogen as nutrient source (Goldstein
1961). Sulphate and nitrate also affected the occur-
rence and species richness, being greater in spring
when those concentrations were high. In contrast
Khulbe and Bhargava (1983) found a negative
relationship between nitrate and sulfate concentra-
tions and the frequency of zoosporic organisms.
Steciow (1998) observed a decrease in the frequency
of zoosporic organisms when SRP concentrations
increased. The results by Czeczuga and Muszyn´ska
(2004), Steciow (1998) as well as our own indicate
that sulphate influences the number of taxa recov-
ered. Some chytrids have the ability to reduce sulfates
because sulfur sources for nutrition and some
Saprolegniales usually depend on the presence of
organic sulfate compounds for growth (Cantino and
Turian 1959).
Lund (1934) mentioned that the Saprolegniaceae
family and some
Pythium
species require high DO
MARANO ET AL:DIVERSITY OF ZOOSPORIC ORGANISMS 697
levels for growing. In addition Steciow (1997) found
that an increase in the oxygen content generates an
increase in the frequency of zoosporic organisms
(particularly Saprolegniales). We found the greatest
diversity in well oxygenated conditions, such as those
found in winter, in which the lowest COD and BOD
were recorded; in fall by contrast those concentra-
tions were the highest obtained and zoosporic
organisms were less represented. However because
facultative anaerobiosis is widely distributed among
Chytridiomycetes and Peronosporomycetes in pollut-
ed environments Alabi (1971), Rattan et al (1980)
and Emerson and Natvig (1981) concluded that DO
levels do not influence the occurrence and abun-
dance of zoosporic organisms. El-Hissy et al (1994)
and El-Hissy et al (2001) pointed out that organic
matter content influenced their occurrence, revealing
a close relationship in organic matter-species rich-
ness. Spring water conditions were similar to those
obtained in high trophic state habitats (with high
nutrient levels and pH and low DO concentrations)
but zoosporic organisms exhibited more species
richness and frequency. Many studies evaluating the
influence of abiotic factors on the occurrence of these
organisms have shown that organic and inorganic
pollution affected their diversity (Pires-Zottarelli
1999, Silva 2002, Rocha 2004).
Our results suggested that the greatest abundance
and diversity were obtained under low water temper-
atures, nutrient levels and high DO concentrations
whereas the highest species richness and frequency
were related to moderate temperatures and abundant
nutrients.
Nowadays the knowledge about the ecology of
these organisms is scanty and thus further studies are
needed to elucidate the factors governing the
establishment of zoosporic organisms communities
in streams. This work provides baseline data for future
comparative studies of the distribution of zoosporic
organisms in aquatic habitats. However, if we attempt
to represent the diversity of a given site, the use of a
single bait has restrictions because it will influence
which zoosporic organisms are recovered, excluding
those that require other nutrients. Species adapted to
restricted and different nutrient sources can coexist;
then cellulophilic, keratinophilic and chitinophilic
species can be part of the same community. Difficul-
ties in assessing biodiversity and abundance might be
mitigated by developing protocols that help reduce
sampling errors. Such protocols, involving the recov-
ery of multiple types of samples and the use of
different baits (such as pollen grains, keratin and
chitinic baits) over a longer period of time, are
neccesary to identify patterns of distribution and to
characterize this community as a whole.
ACKNOWLEDGMENTS
This research was supported by grants of the Argentine
National Research Council (CONICET, PIP 5931) and the
National University of La Plata (N 11/440 Proyect).
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