more:
http://www.int-res.com/articles/meps2004/275/m275p129.pdf
Seasonal variability of prooxidant pressure and antioxidant adaptation to symbiosis in the Mediterranean demosponge Petrosia ficiformis
F. Regoli1,*, C. Cerrano2, E. Chierici1, M. C. Chiantore2, G. Bavestrello3
1Istituto di Biologia e Genetica, UniversitÃ_ Politecnica delle Marche, Via Ranieri, Monte Dââ"šÂ¬Ã¢"žÂ¢Ago, 60100 Ancona, Italy
2Dipartimento per lo Studio del Territorio e delle sue Risorse, UniversitÃ_ di Genova, Via Balbi 5, 16126 Genova, Italy
3Dipartimento di Scienze del Mare, UniversitÃ_ Politecnica delle Marche, Via Brecce Bianche, 60100 Ancona, Italy
ABSTRACT: In symbioses between invertebrates and microalgae, host tissues are exposed to
increased levels of photosynthetically produced oxygen. The biochemical consequences of symbioses
have been poorly investigated in Mediterranean species, but a general increase in antioxidant
defences has been recently reported in the demosponge Petrosia ficiformis as an adaptive response
to the cyanobacterium Aphanocapsa feldmanni. Since Mediterranean symbioses naturally experience
marked seasonal variations in symbiont content, light intensity and seawater temperature, the
aim of this work was to investigate if these fluctuations modulate the prooxidant challenge to sponge
tissues. Antioxidant efficiency was characterised on a monthly basis by combining an analysis of the
main antioxidants (superoxide dismutase, catalase, glutathione S-transferases, glutathione reductase,
glutathione peroxidases) with measurements of the total oxyradical scavenging capacity
(TOSC), thus achieving a more holistic assessment of the capacity of sponge tissues to absorb different
forms of reactive oxygen species. Symbiotic sponges showed significant seasonal changes in
antioxidant efficiency, with more marked variations in tissues directly exposed to photosynthetically
produced reactive oxygen species. The greatest variations were observed during the summer
months, with the highest seasonal values for some defences (i.e. catalase) and the lowest for others
(i.e. glutathione peroxidases). The marked increase in catalase and TOSC in summer suggests
greater production of H2O2 in the symbioses during this period, supporting the hypothesis that seawater
temperature can significantly modulate the prooxidant challenge in Mediterranean symbioses.
The results suggest that species with lower antioxidant efficiency may be less tolerant of conditions
effecting oxidative damage; e.g. increases in temperature during the summer months.
KEY WORDS: Mediterranean symbioses ââ"šÂ¬Ã‚¢ Oxyradicals ââ"šÂ¬Ã‚¢ Antioxidants ââ"šÂ¬Ã‚¢ Adaptation ââ"šÂ¬Ã‚¢ Sensitivity ââ"šÂ¬Ã‚¢
Temperature ââ"šÂ¬Ã‚¢ Demosponge
INTRODUCTION
Symbioses have been largely described between
photosynthesising organisms (e.g. cyanobacteria and
dinoflagellates, diatoms and algae) and several marine
invertebrates including poriferans, cnidarians, ascidians
and molluscs (Douglas 2003). These associations,
particularly frequent in tropical coral reefs, have also
been reported from temperate and even polar environments
(Shick & Dykens 1985, Dunlap & Shick 1998,
Cerrano et al. 2000a, 2003).
Photosynthetic products secreted by the symbionts
represent an additional food source for the host tissues,
while benefits for microalgae include the use of
animal waste for nutrients, exposure to light and a
protected habitat during their life cycle (Trench
1993). An association with algae is thought to be necessary
for some species such as adult giant clams,
which are never devoid of zooxanthellae, while in
other organisms the presence of symbionts is influenced
more by physical and environmental factors
like depth-dependent light irradiance and tempera- ture, and seasonal fluctuations in these parameters
(Shick et al. 1996).
Considerable interest has developed in the biochemical
consequences of symbioses to invertebrate tissues,
which are exposed to increased levels of oxygen produced
during photosynthetic processes. These reactions
involve the generation of reactive oxygen species
(ROS) such as O2
ââ"šÂ¬Ã¢â‚¬Å“, H2O2 and ââ"šÂ¬Ã‚¢OH, the formation of
which is considered proportional to the partial pressure
of molecular oxygen, pO2 (Jamieson et al. 1986).
Several studies on tropical invertebrates have revealed
increased efficiency of antioxidant defences in response
to symbionts, and variations in such defences
have been measured in organisms collected along
depth transects or exposed under field and laboratory
conditions to different regimes of light and temperature,
both of which factors are well known to influence
the generation of oxyradicals (Lesser et al. 1990, Shick
et al. 1995, Douglas 2003).
In such symbioses, ROS are mainly produced within
the chloroplasts by several mechanisms associated
with the electron transport chains of Photosystems I
and II; among these, hydrogen peroxide (H2O2) is generated
in the Mehler reaction by the oxygen-evolving
complex (Mehler 1951, Badger 1985, Richter et al.
1990), and from the algae cell this molecule can easily
diffuse into the host cytoplasm (Downs et al. 2002). If
not adequately neutralised by antioxidant defences,
hydrogen peroxide can induce direct oxidative damages
or react with superoxide anion and/or transition
metals to originate the hydroxyl radical (ââ"šÂ¬Ã‚¢OH), by far
the most toxic and reactive oxyradical.
In symbiotic corals, it has been proposed that antioxidants
can compensate the algae-induced prooxidant
pressure within a certain threshold of ROS concentration.
Above this threshold, the increased
antioxidant efficiency can be overstretched by ROS
production, and oxidative damage will occur. The
ââ"šÂ¬Ã‹Å“oxidative theory of coral bleachingââ"šÂ¬Ã¢"žÂ¢ proposes that
bleaching is the final defence of corals against oxidant
injury (Downs et al. 2002): when algal production of
oxyradicals is exacerbated (i.e. by elevated temperature),
corals will remove the main source of oxidative
damage by expelling their endosymbiotic algae.
In coral ecosystems, symbioses are also greatly
affected by temperature variations, and increased sea
surface temparature has been associated with mass
coral bleaching events (Lesser 1996, 1997, Stone et al.
1999, Wilkinson 1999, Douglas 2003). In different
colonies of the star coral Montastera annularis, accumulation
of oxidative damage products, antioxidants
and cellular stress capacity were correlated with
increases in temperature and coral bleaching intensity
(Downs et al. 2002) confirming that high temperatures
may contribute to triggering oxidative stress and
bleaching in coral reef systems (Lesser et al. 1990,
Dykens et al. 1992, Goreau & Hayes 1994, Downs et al.
2000, 2002). Anomalous seawater temperatures lead to
bleaching events also in the Mediterranean corals
Cladocora caespitosa, Balanophyllia europea and
Oculina patagonica (Metalpa et al. 2000, Kushmaro et
al. 2001). Several species of sponges and gorgonians
underwent a strong mass mortality during summer
1999 (Cerrano et al. 2000b, 2001, Perez et al. 2000).
The biochemical consequences of symbioses have
been less well investigated for Mediterranean species. A
general enhancement of antioxidant defences was described
in the demosponge Petrosia ficiformis (Poiret,
1789) as a counteracting response to the more elevated
levels of oxygen photosynthetically produced by the
cyanobacterium Aphanocapsa feldmanni (Regoli et al.
2000a). The intensity of light irradiance did not appear to
be an additional prooxidant stressor, and the levels of antioxidant
defences in symbiotic sponges were similar in
specimens exposed to high and to low solar irradiance.
Compared to tropical symbioses, exposure to UV radiation
is more limited at temperate latitudes, and the presence
of photosynthesising symbionts appears to be the
primary factor inducing an antioxidant response in the
Mediterranean sponge P. ficiformis.
Mediterranean symbioses also experience marked
seasonal variations in seawater temperature, but the
influence of these fluctuations has never been assessed
in regard to biochemical adaptations to prooxidant
challenge. In this study, specimens of Petrosia ficiformis
were sampled on a monthly basis from symbiotic
colonies and the main antioxidants analysed:
superoxide dismutase (SOD, which catalyses the
dismutation of O2
ââ"šÂ¬Ã¢â‚¬Å“ to H2O2 and O2), catalase (which
reduces H2O2 to H2O and O2), glutathione S-transferases
(which catalyses conjugation reactions of glutathione
to electrophilic centres of organic substrates),
glutathione peroxidases (as the sum of Se-dependent
and Se-independent forms, which reduce inorganic
and organic hydroperoxides with oxidation of reduced
glutathione), glutathione reductase (which converts
oxidised glutathione GSSG to the reduced and functionally
active form GSH). The results for the individual
antioxidants were combined with the total oxyradical
scavenging capacity (TOSC) which quantifies the
capacity of the whole antioxidant system to absorb different
forms of oxyradicals such as peroxyl radicals
(ROOââ"šÂ¬Ã‚¢) and hydroxyl radicals (HOââ"šÂ¬Ã‚¢).
The aim of this work was to provide a better characterisation
of the antioxidant defences in symbiotic
colonies of Petrosia ficiformis and to investigate the
possible presence of variations reflecting the seasonality
of environmental prooxidant factors. The overall
results were expected to indicate whether seasonal
variation in both prooxidant pressure and sensitivity of antioxidant defences might be useful for predicting
differential susceptibility to oxidative damages in
Mediterranean symbioses.
I'd have to have a SPS guy come over & identify a few of them. I always blamed the leathers for this.
