Microorganisms in marine environments inhabit surfaces and
form biofilms. The formation of biofilms involves adsorption of organic and
inorganic molecules to submerged surfaces. The role of first colonisers is important
as they enable the biofilm to stay intact or detach. Biofilms can be altered by
biotic and abiotic factors.
Global warming is becoming an increasing concern and an
Increase in temperatures could influence the structure and functions of
biofilms in coral reef systems. Increases in sea temperature can alter the
distribution, abundance, function and community dynamics in marine microbial
systems both positively and negatively by affecting the biofilm recruitment and
inductive capabilities of invertebrate larvae. As a biofilm grows it develops a
complex architecture that contains different micro habitats which facilitates
the growth of diverse microbial communities and the settlement and growth of
invertebrate larvae. Previous studies have shown that living in biofilms can be
advantageous. Biofilms can provide nutrients and protection against external
factors such as toxins, heavy metals, dehydration and UV-radiation. Gamma-proteobacteria
have been found to produce chemical cues that facilitate the induction of coral
larvae. Crustose algae harbour or produce biofilms that can produce morphogens
and crustose coralline algae precipitate calcium carbonate which is Important
in reef building.
The Arabian Gulf, one of the hottest bodies of water on the
earth, is characterised by the highest variability in annual temperatures.
Corals suffer thermal stress when water temperatures exceed 32 °C or drop below
18 °C but corals in the Gulf can survive extreme temperature fluctuations of 35
°C in summer and below 11 °C in winter. The coral spawning season in the
Arabian Gulf occurs between may and august when temperatures are highest. Because microbial biofilms provide cues for
larvae to settle and metamorphose it is important to know how biofilms develop
at these temperature extremes.
The experiment was conducted in the inshore reef system of
Qit'at Benayah, north of the Arabian Gulf. Glass slides were used as a substratum for
biofilm growth. Replicates were collected every 2 weeks from both sides of the
glass slides. The physical and chemical properties of the seawater were checked
onsite and showed a continuous increase in water temperature during the
sampling period.
Biofilms were removed from the glass slides and placed into
petri dishes. Biofilms were divided into two and either used to measure the
abundances of phototroughs and heterotroughs or used in molecular analysis.
The abundances of heterotroughs and phototroughs were
investigated using total and viable count techniques using epifluorescence
microscopy. Developed biofilms were plated onto marine agar to determine the
culturable bacterial abundance and developed bacterial colonies were
subcultured to purify them. PCR was then used with 16s r/RNA to identify the
isolates.
Four Vibrio isolates, Vibrio
harveyi, Vibrio owensii, Vibrio ponticus and Vibrio sinaloensis, were inoculated on a marine agar to assess the
ability of vVbrio to precipitate calcium carbonate. Calcium carbonate crystals
were removed and examined under an emission scanning microscope to assess size
and number of crystals.
The results showed a linear increase in the number of
heterotrophs on both sides of the glass slides as the age and temperature of
the biofilm increased. Biofilms kept at 33 °C for longer times saw heterotroph
numbers start to decrease. Phototroph numbers fluctuated overtime along with
levels of phototroughs detected in seawater samples.
The study found that biofilms were similar in the diversity
of microbes but were found grouped together by age. Biofilms of a younger age
were seen to cluster together and remained separate from mature biofilms no
matter which side of the substrata they were growing on suggesting the age
groups had different bacterial communities.
Light microscopy was used to examine the diversity of
phototrophs in the biofilms. Phototrophs which have the potential to produce substances
involved in coral larvae settlement and metamorphosis was found colonizing the
substrata. Bacillariophyceae (diatom)
and Cyanobacteria dominated the
biofilms. Bacillariophyceae dominated
young biofilms but decreased in number with age whilst cyanobacterial abundance
increased with age and dominated later.
For all biofilm age groups Gamma-Proteobacteria dominated with vibrio being the most dominant
genus of that group. Invertebrate Larvae were found in 6-week-old biofilms
suggesting that this may be the age the biofilm becomes mature.
There were 20 Vibrio species isolated from the biofilm
samples 4 of which could precipitate calcium. This calcification is important
in providing stability for the biofilm, the Vibrio produced larger sized
crystals at 30°C than at 23°C and there were more of them.
In conclusion, the study found that bio-films grown on glass
substrata over 14 weeks contained bacterial and algal species capable of producing
morphogens and stabilising biofilms through calcification. Natural elevations
of prolonged temperature at 33°C did not affect structural diversity of the
bacterial communities within the biofilm. The study found that the microbial
community shifted as the bio-film aged with mature biofilms containing key species
such as Vibrio important in calcification and the production of morphogens. During
the study invertebrate larvae were found in 6-week-old bio-films but no coral
larvae were found despite the experiment being carried out during coral spawning.
The paper stated that it may have no found coral larvae due
to the method of removing the biofilms. They said they could have looked under
a microscope to see if they were there but it would have meant they couldn’t have
used other methods to assess the biofilm diversity. I feel that because this
paper was based on corals they could have assigned extra slides just for
looking at coral larvae settlement so they could see if they do settle or not
and at what point which could have improved our knowledge on how these corals
persist at higher temperatures.
Mahmoud (2015) ‘Variations
in the abundance and structural diversity of microbes forming biofilms in a
thermally stressed coral reef system’, Marine
Pollution Bulletin 100, 710–718. doi.org/10.1016/j.marpolbul.2015.10.030
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