Chapter 19 Epiphytes of Seagrasses
443
(Goering and Parke, 1972; Iizumi and Yamamuro,
2000; Pereg-Gerk et al., 2002). In most cases,
the main N 2 -fixing organisms on the leaves are
cyanobacteria and it has been estimated that they
can supply 4–38% of the nitrogen needed for primary production in T. testudinum beds (Capone and
Taylor, 1977). This nitrogen may be made available by grazers feeding on the cyanobacteria thus
releasing the nitrogen for uptake by the seagrass
(Yamamuro, 1999). Pereg et al. (1994) have also suggested that anaerobic photosynthetic bacteria of the
Rhodospirillaceae also contribute to N 2 -fixation observed on seagrass leaves.
Seagrass epiphytes are also likely to be nutrient
‘sinks’. It has been estimated that the epiphytes of
T. testudinum are responsible for about 17% of the
total NH
+
4 removed from the water column by the
whole seagrass meadow (Cornelisen and Thomas,
2002). However, little is known of how much epiphytic algae compete with the seagrass for available nutrients and more studies are needed to assess the importance of seagrass epiphytes to nutrient
cycling (see Mateo et al., Chapter 7).
III. Distribution and Abundance
of Epiphytic Organisms
The epiphytic organisms found on seagrass leaves,
stems, and rhizomes generally show distinct patterns
in their distribution.
A. Bacteria, Fungi, and Protozoa
Bacteria, fungi, and protozoa are ubiquitous members of the epiphytic community but remain little
studied. The bacterial flora of Zostera marina has
been the object of several studies (Sieburth and
Thomas, 1973; Kirchman et al., 1984; Barnabas,
1992). A bacterial film is discernible on new seagrass leaves within 1 h (Novak, 1984) and young
leaves of Z. marina and Posidonia oceanica have
a significantly lower bacterial density than older
leaves (Kurilenko et al., 2001), with the bacteria
most abundant on the leaf tip. On mature leaves this
gradient is reversed and bacterial density is highest
at the leaf base (Novak, 1984). Wahbeh and Mahasneh (1984) also observed differences in viable
counts of heterotrophic bacteria attached to various
parts of Halophila stipulacea. Maltas and Borowitzka (unpublished results) have also found distinct
seasonality in the abundance and diversity of culturable bacteria on the leaves of P. australis and
A. antarctica.
Bacteria found on seagrass leaves include the genera Vibrio, Alteromonas, Moraxella, Pseudomonas,
Marinobacter, and Brochothrix (Maltas and Borowitzka, unpublished, Kurilenko et al., 2001) as well as
several nitrogen fixing bacteria (Pereg et al., 1994).
New methodologies such as restriction fragment
length polymorphism analysis of 16S rRNA genes
(Weidner et al., 1996) will provide further important information on the diversity of the prokaryotic
epiphytes of seagrasses and their possible roles.
B. Algae
The most abundant and diverse epiphytic organisms on seagrasses are algae. These range from unicellular diatoms and dinoflagellates found on almost all seagrasses (Kita and Harada, 1962; Buia
et al., 1996; Jernakoff and Nielsen, 1997; Mazzella,
1999) to large macrophytes such as Laurencia spp.,
Metagoniolithon stelliferum, and Hypnea spp. found
attached to the stems of Amphibolis spp. (Lethbridge et al., 1988). Seagrasses are also host to
a wide range of other epiphytes including fungi
(Belofsky et al., 1999), protozoa (Aladro-Lubel and
Mart´ ınez-Murillo, 1999; Saraswati, 2002), sponges,
bryozoans, hydroids, and ascidians (see later). Furthermore the primary epiphytes may have dease populations of microalgae, such as bacteria and diatoms,
and bacteria. Other invertebrates such as crustaceans
and molluscs live amongst the sessile epiphytic flora
and fauna.
With few exceptions, the diversity and biomass
of epiphytic algae is highest on the leaves, especially on the oldest leaf and near the leaf apex (Buia
et al., 1985; Cullinane et al., 1985; Borum, 1987;
Mazzella et al., 1994; Reyes et al., 1998; Trautman and Borowitzka, 1999; Uku and Bjoerk, 2001).
This pattern has been reported for most species
with strap-like leaves, including Z. marina, Posidonia sinuosa, P. australis, P. oceanica, Cymodocea
rotundata, Thalassodendron ciliatum, and Thalassia hemprichii. A similar apico-basal pattern has
also been observed in Syringodium isoetifolium (Yamamuro, 1999). Some notable exceptions to these
generalizations are species of Amphibolis and Thalassodendron, and P. oceanica. In Amphibolis, for
example, the majority of epiphytic algae occur on
the long-lived stem, with relatively few species and
443
(Goering and Parke, 1972; Iizumi and Yamamuro,
2000; Pereg-Gerk et al., 2002). In most cases,
the main N 2 -fixing organisms on the leaves are
cyanobacteria and it has been estimated that they
can supply 4–38% of the nitrogen needed for primary production in T. testudinum beds (Capone and
Taylor, 1977). This nitrogen may be made available by grazers feeding on the cyanobacteria thus
releasing the nitrogen for uptake by the seagrass
(Yamamuro, 1999). Pereg et al. (1994) have also suggested that anaerobic photosynthetic bacteria of the
Rhodospirillaceae also contribute to N 2 -fixation observed on seagrass leaves.
Seagrass epiphytes are also likely to be nutrient
‘sinks’. It has been estimated that the epiphytes of
T. testudinum are responsible for about 17% of the
total NH
+
4 removed from the water column by the
whole seagrass meadow (Cornelisen and Thomas,
2002). However, little is known of how much epiphytic algae compete with the seagrass for available nutrients and more studies are needed to assess the importance of seagrass epiphytes to nutrient
cycling (see Mateo et al., Chapter 7).
III. Distribution and Abundance
of Epiphytic Organisms
The epiphytic organisms found on seagrass leaves,
stems, and rhizomes generally show distinct patterns
in their distribution.
A. Bacteria, Fungi, and Protozoa
Bacteria, fungi, and protozoa are ubiquitous members of the epiphytic community but remain little
studied. The bacterial flora of Zostera marina has
been the object of several studies (Sieburth and
Thomas, 1973; Kirchman et al., 1984; Barnabas,
1992). A bacterial film is discernible on new seagrass leaves within 1 h (Novak, 1984) and young
leaves of Z. marina and Posidonia oceanica have
a significantly lower bacterial density than older
leaves (Kurilenko et al., 2001), with the bacteria
most abundant on the leaf tip. On mature leaves this
gradient is reversed and bacterial density is highest
at the leaf base (Novak, 1984). Wahbeh and Mahasneh (1984) also observed differences in viable
counts of heterotrophic bacteria attached to various
parts of Halophila stipulacea. Maltas and Borowitzka (unpublished results) have also found distinct
seasonality in the abundance and diversity of culturable bacteria on the leaves of P. australis and
A. antarctica.
Bacteria found on seagrass leaves include the genera Vibrio, Alteromonas, Moraxella, Pseudomonas,
Marinobacter, and Brochothrix (Maltas and Borowitzka, unpublished, Kurilenko et al., 2001) as well as
several nitrogen fixing bacteria (Pereg et al., 1994).
New methodologies such as restriction fragment
length polymorphism analysis of 16S rRNA genes
(Weidner et al., 1996) will provide further important information on the diversity of the prokaryotic
epiphytes of seagrasses and their possible roles.
B. Algae
The most abundant and diverse epiphytic organisms on seagrasses are algae. These range from unicellular diatoms and dinoflagellates found on almost all seagrasses (Kita and Harada, 1962; Buia
et al., 1996; Jernakoff and Nielsen, 1997; Mazzella,
1999) to large macrophytes such as Laurencia spp.,
Metagoniolithon stelliferum, and Hypnea spp. found
attached to the stems of Amphibolis spp. (Lethbridge et al., 1988). Seagrasses are also host to
a wide range of other epiphytes including fungi
(Belofsky et al., 1999), protozoa (Aladro-Lubel and
Mart´ ınez-Murillo, 1999; Saraswati, 2002), sponges,
bryozoans, hydroids, and ascidians (see later). Furthermore the primary epiphytes may have dease populations of microalgae, such as bacteria and diatoms,
and bacteria. Other invertebrates such as crustaceans
and molluscs live amongst the sessile epiphytic flora
and fauna.
With few exceptions, the diversity and biomass
of epiphytic algae is highest on the leaves, especially on the oldest leaf and near the leaf apex (Buia
et al., 1985; Cullinane et al., 1985; Borum, 1987;
Mazzella et al., 1994; Reyes et al., 1998; Trautman and Borowitzka, 1999; Uku and Bjoerk, 2001).
This pattern has been reported for most species
with strap-like leaves, including Z. marina, Posidonia sinuosa, P. australis, P. oceanica, Cymodocea
rotundata, Thalassodendron ciliatum, and Thalassia hemprichii. A similar apico-basal pattern has
also been observed in Syringodium isoetifolium (Yamamuro, 1999). Some notable exceptions to these
generalizations are species of Amphibolis and Thalassodendron, and P. oceanica. In Amphibolis, for
example, the majority of epiphytic algae occur on
the long-lived stem, with relatively few species and