Agriculture Reference
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such as the application of probiotics, prebiotics, phytobiotics and immunostimulants, would
lead to a more environmentally friendly aquaculture sector, in accordance with the Ecosystem
Approach for the sustainable growth and expansion of aquaculture, promoted by FAO in
The
State of World Fisheries and Aquaculture
(FAO 2007).
The present chapter discusses bacterial aspects of live feed and reviews the use of probiotics
and prebiotics as a strategy to improve live feed cultures, and also considers how live feed can
be used as a vector for prebiotics and probiotics to fish larvae cultures.
16.2 BACTERIAL ASPECTS OF LIVE FEED
16.2.1 Microalgae
Extracellular compounds resulting from the photosynthetic activity of algae constitute the
carbon source of heterotrophic bacteria, forming an algae-bacteria interaction region known
as the phycosphere. These interactions range from mutualism to parasitism (Cole 1982).
Algae can produce compounds that promote or inhibit bacterial growth and bacteria can have
a negative or positive effect in the algae culture (Austin
et al.
1992; 1990; Fukami
et al.
1997; Naviner
et al.
1999). In mass cultivation systems, it is almost impossible to cultivate
microalgae under axenic conditions. Taking advantage of the high organic load accumulated
in the system, high numbers of bacteria attach to microalgae cells or proliferate in the water.
The abundance and diversity of bacterial microbiota in microalgae are determined by the
method of culture, the species cultured, the growth media used, the quality of seawater and
the growth phase of the culture (Salvesen
et al.
2000). Culturable bacteria in microalgae
cultures range from 10
3
to 10
8
colony forming units (CFU) ml
-1
(Nicolas
et al.
1989; 2004;
Salvesen
et al.
2000). Analysis by culture-independent methods showed that different algae
species (
Pavlova lutheri
,
Isochrysis galbana
,
Chaetoceros calcitrans
,
Skeletonema costatum
,
Chaetoceros gracilis
and
Chaetoceros muelleri
) are associated with a large spectrum of
bacteria with bacteria belonging to the α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria,
Cytophaga-Flavobacterium-Bacteroides (CFB) group,
Actinobacteria
and
Bacillus
(Nicolas
et al.
2004). Members of the
Roseobacter
clade and the CFB group are dominant in microal-
gae cultures (Nicolas
et al.
2004; Porsby
et al.
2008) and
Vibrionaceae
are generally absent
or in very low numbers (Salvesen
et al.
2000).
16.2.2 Rotifers
Rotifers are filter feeders with a high bacterial load and a variable bacterial microbiota on their
external surface and in digestive tract. The microbiota of rotifers is similar to the microbiota
of the water in the rearing system, indicating a non-selective uptake of bacteria (Muroga and
Yasunobu 1987; Skjermo and Vadstein 1993; Vadstein
et al.
1993; Makridis
et al.
2000a). The
most usual approach for cultivation of rotifers involves the method of batch culture, where a
tank is inoculated and harvested in its entirety after some days. Another less common approach
is the continuous culture, where after an initial stage with increasing population density the
culture is partially harvested and replaced with water at regular time intervals (Dhert
et al.
2001). Continuous cultures may result in lower microbial load and more predictable micro-
biota, which is associated with the rotifers rather than with batch cultures (Rombaut
et al.
2001). In addition to the method of cultivation, the diet of rotifers has a strong influence on their
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