Coating matters in the ‘invisible sea’: Biofilms, marine snow and bioengineering

(Conclusion)

‘Marine snow’

Anyone familiar with biological sewage treatment plants can imagine that marine sediments are not the only sites of organic matter recycling in the sea. The more natural counterpart of activated sludge flocs in seawater is called "marine snow." The high salt content of seawater facilitates adhesion and attachment of microbial cells to interfaces. Abundant attachment sites on freely suspended particles lead to the formation of aggregates of partially decaying plankton forming marine snow. These marine snow aggregates (of living cells and decaying plankton debris) support the formation of planktonic biofilms containing a battery of immobilized enzymes with recycling functions. Representing a conspicuous biochemical factory in the water column, marine snow is responsible for short-circuited organic matter recycling. This kind of "on-site-recycling" stands in contrast to "outsourcing strategies" that are characterized by sedimentation of dying plankton cells to the sea floor. Predominance of the latter in traditional ecosystem models has now been challenged. The role of microbial biofilms in sustaining ecosystem functions is gradually emerging, as previously "invisible" or neglected pieces of Nature’s recycling machinery receive more attention.  

A walk over slippery intertidal rocks can always remind us of the real existence of biofilms, integral parts of Nature’s "bioengineering" capacity that account for the self-purification of coastal marine waters. The abundance of biofilms in marine environments also complicates monitoring of environmental health. Anyone who tries to detect certain microbes in the sea must be aware that microbial biofilms serve as protected refuge and survival sites in microbial life cycles. Analyses of water samples would even become useless, if the target organisms to be monitored reside in biofilms at interfaces and suspended materials.

Can biofilms be controlled?

On a multitude of man-made constructs other than waste water treatment devices, however, biofilm formation is viewed as undesirable. Biofilms in general cause adverse effects on a variety of liquid-exposed objects, from ocean oil drilling gear to medical catheters. Hence, many new insights into structures and functions of biofilms stem from researchers who are exploring ways and means to prevent them, including the medical profession.

A few years ago, it was discovered that certain seaweeds produce chemical compounds of the furanone family. As signal analogues these molecules block the signal compounds required in gene expression to initiate the bacterial colonization of surfaces. This signaling system is based on measuring critical cell densities. This finding aroused interest not only among applied, but also academic researchers: "Cell to cell communication" in microbial biofilms via chemical signals has stirred speculations about the scope of "supraorganismic" capacities among the smallest forms of life with the by far longest history of evolution. On the applied side, a non-antibiotic compound that merely prevents microbial cells from colonizing surfaces, might be turned into an environment-friendly weapon against biofouling in marine environments.

Most marine organisms wear ‘microbial coats’

Despite increasing organic pollution along our coasts, short supply of organic nutrients and energy in bulk offshore ocean water continues to limit the growth of freely suspended microorganisms. Hence, microbes that require organic matter for growth, take advantage from attaching themselves to interfaces with adsorbed nutrients. Enhanced nutrient levels at metabolically active surfaces select for colonizers with particularly high nutrient demands. This feasting type of bacteria is typically associated with higher marine organisms and relatively easy to isolate from seawater. So it is not surprising that most of the known (cultivated) marine bacteria are found among the feasting biofilm associates of marine animals and plants.

"Hot spots" of microbial growth and activity in the sea are associated with metabolism (as well as decay) of marine animals and plants. The specific role of most microbial biofilms in the life of higher organisms remains yet to be investigated. A few conspicuous cases of symbiosis highlight the role of rather specific biofilms: For example, the "light organs" of squids are run by symbiotic light-emitting bacteria.

The capacity to decompose chitin, the polysaccharide construction element of crab and copepod shells, is common among bacterial colonizers of these animals. While healthy chitin-armored animals are hardly affected, stressed and weakened specimen may invite chitin-degrading colonizers to intrude through lesions in their outer "skin." At this point the primarily harmless colonizer has turned into an opportunist pathogen. Extraordinary stress in intensive shrimp mariculture can render cultivated shrimps vulnerable to attacks by such opportunistic pathogens. Probably all pathogenic bacteria of shellfish and fish are opportunistic scavengers and surface colonizers.

In principle, however, bacterial lawns covering marine animals need not pose a particular health risk for their host. On the contrary, several detritus-feeding animals inhabiting marine sediments seem to depend on their "bacterial gardens" as a source of protein-rich diet. Other sea floor dwellers owe their survival in toxic hydrogen sulfide-rich sediments to "fur coats" composed of a sulfide-oxidizing bacterium.

Thus, aquatic ecosystems and especially the "invisible sea" rely on microbial biofilms. These are instrumental for sustaining indispensable ecosystem functions, such as organic matter recycling and detoxification. Some of these natural designs have already been copied and exploited for man-made technologies, like in biological waste water treatment. On the other hand, numerous man-made constructs suffer from economically undesirable, material-deteriorating effects of biofilms. Support and development of biotechnologically desirable biofilms is counterbalanced by prevention of undesirable biofilm formation on man-made constructs, including those in medicine. Therefore, microbial biofilms constitute a unique case in which support of research can be expected from two opposing ends: From the conservationists and copiers of Nature’s design to preventionist protectors of man-made constructions. While "biofilm" has become a popular buzz word in today’s world of microbiology, it is already an "old acquaintance" for marine scientists. Nevertheless, to those concerned about environmental health and marine resources management, microbial strategies of biofilm formation will continue to teach a lesson.

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Wolfgang T. Reichardt, Ph.D. in Botany, is a research professor (DAAD-Lectureship) at the UP Marine Science Institute, Diliman, Quezon City. His research interests are in marine microbial ecology, biogeochemical recycling and mariculture. Send comments and queries to [email protected].