MAP Technical Reports Series No. 106 UNEP

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7.4 General facts on eutrophication, bacteria and human health
Relationships between eutrophication, bacteria and public health are very complex and
little studied. In order to understand them it is necessary to take into consideration the nutrient
effects of bacterial growth, the effects of bacteria on algae, the effects of phytoplankton,
phytobenthos and macrophytes produced by eutrophication on sea bacteria.
7.4.1 Bacterial cycle eutrophication
Among the schemes and patterns of cyclical processes that characterize the cultural
eutrophication of coastal areas, in the study of bacterial role Aubert (1988, 1990, 1992)
distinguished between a planktonic cycle (eutrophication) and a bacterial cycle eutrophication.
The bacterial cycle eutrophication, in which the bacteria play a major role, begins with
sea green because of phytoplankton growth, followed by its disappearance and transparent
water; then a development of sulpho-reducer, sulphate-reducer (Desulphovibrio) and sulphitereducer
(Clostridium) bacteria begins with production of H 2S and a decrease in 0 2, in results of
which we shall have transition waters, then whitish waters, precipitation of sulphur on bottom
with whitish deposits, and, soon afterwards, appearance of red colonies of flavobacteria on
bottom and red water through the action of sulphur-oxidative bacteria: reappearance of sulphites,
sulphates, increase in O 2 and clarified water. Unfortunately, we have no precise knowledge
about this scheme. It can't be excluded that some phenomena of red water observed without
dinoflagellates are due to a process of this kind.
7.4.2 Marine bacteria and the red tide link
In addition to the promoting blooms of dinoflagellata from the vitamins B 12, thiamin and
biotin and chelating agents produced from marine bacteria (Provasoli, 1979), recent research
has suggested that a strong link may exist between the activity of marine sediment bacteria and
the accelerated phytoplankton growth resulting in red tides. Various researchers have revealed
that plant growth hormones can be produced by a wide range of marine bacteria. 45-55% of
sediment bacteria tested were found to produce cytokinin, a principle plant growth hormone. The
algae responsible for the "red tide" phenomena are known to be dependent on substances
exuded from the sediment surface, and the phytoflagellates causing red tides have been shown
to respond to cytokinin. Heterotrophic bacterial activity is influenced by nutrient supply, and an
influx in nutrients or rise in temperature will accelerate this activity. It is possible that observed
correlations between nutrient/temperature increase and red tide outbreaks may be at least
partially due to increased hormone production by marine bacteria. Translocation of hormones
from the sediment bacteria to the phytoflagellates may be accounted for by upwelling currents
in deep water or by simple diffusion or stratification in shallow.
7.4.3 Effects on bacteria of active principles produced by phytoplankton
It is possible to separate substances produced by phytoplankton and macroalgae
bioactive on bacteria into three large groups: (1) antibiotics (Duff et al., 1966; Allen and Dawson,
1969); (2) growth promoting substances (Lelong et al., 1980); (3) compounds that are inhibitors
of mineralization processes of organic substances (biomass) (Chriost, 1975a-b).
Particular researches on this reactive principle have been carried out not only in
laboratory and in various marine ecological conditions, but also with hygienic-sanitary aims in
coastal areas. Active principles produced by algae during their bloom or released by cells