An ecological perspective of microbial secondary metabolism

Bacteria and fungi produce a remarkable array of bioactive small molecules. Many of these have found use in medicine as chemotherapies to treat diseases ranging from infection and cancer to hyperlipidemia and autoimmune disorders. The applications may or may not reflect the actual targets for these compounds. Through careful studies of microbes, their associated molecules and their targets, a growing understanding of the ecology of microbial secondary metabolism is emerging that exposes the central role of secondary metabolites in many complex biological systems.     

Viruses and the microbial loop

The abundance of viral-like particles in marine ecosystems ranges from <104 ml^–1 to >10⁸ ml^–1. Their distribution in time and space parallels that of other biological parameters such as bacterial abundance and chlorophyll a. There is a lack of consensus between methods used to assess viral activity, i.e., rate of change in viral abundance (increase or decrease). The highest rates, 10–100 days^–1, are observed in experiments with short sampling intervals (0.2–2 h), while lower rates, on the order of 1 day^–1, are observed in experiments with longer sampling intervals (days). Few studies have been carried out, but viruses appear, at least in some cases, to have a significant impact on carbon and nutrient flow in microbial food webs. Viruses have also been demonstrated to exert a species specific control of both bacteria and phytoplankton populations in natural waters.     

Carbon fluxes in the microbial loop: Comments

The heterotrophic bacterial community of oceanic aggregates which mediates particle solubilization, displays features (low carbon demand and low turnover) that are difficult to reconcile with the observed high enzyme activities and cell numbers. Hypotheses are proposed to explain this discrepancy. Furthermore, production of both free and attached bacteria may have been underestimated by neglecting the ultramicrobacteria (UMB). Production of UMB may represent up to 28% of the total bacterial production.     

Responsible estuarine finfish stock enhancement: an Australian perspective

The responsible approach to marine stock enhancement is a set of principles aimed at maximising the success and benefits of artificially re‐stocking depleted fisheries. The benefits of such an approach are evident in the 400% increase in survival of stocked striped mullet in Hawaii through refinement of release techniques, however financially or temporally constrained stocking programs in Australia have not adhered to all principles. A pragmatic approach to address these principles is proposed, using international examples and Australian marine finfish pilot stockings of barramundi, mulloway, sand whiting, dusky flathead and black bream. Biological ranking of candidate species by estuarine residency, a low natural‐mortality to growth ratio, a large L _∞ and comparison by recreational value and available rearing technologies, show that mulloway, barramundi and sea mullet are ideal species for stocking in Australia. Australian intermittently closed opening landlocked lagoons and recreational fishing havens, especially near cities, provide experimental opportunities to apply this approach and stock suitable species through small‐scale pilot experiments. This would allow evaluation of production and carrying capacity, and density dependent processes with respect to optimal stocking strategies unconfounded by emigration and commercial fishing practices. Twenty per cent of Australians fish each year, and harvest approximately 27 000 t of finfish. Stocking recreationally important species in Australia should give a greater financial benefit, which is spread across a larger cross‐section of the community, compared to stocking to enhance commercial fisheries. The pragmatic application of the responsible approach, and stocking of fast growing estuarine residents into recreational fishing havens would enhance the benefit from marine stocking.     

Effect of Kepone on estuarine microbial activity

In situ heterotrophic uptake of mixed¹⁴C-amino acids and direct viable cell (DVC) count of Chesapeake Bay water samples were not significantly affected by the insecticide Kepone at concentrations 0.01 mg/1. Maximum inhibition of heterotrophic uptake,ca. 85–90%, and DVC count, 45–97%, was evident at concentrations of Kepone exceeding 0.2 mg/1. A specific activity index (Metabolic Activity/DVC or Kepone-resistant DVC), heterotrophic uptake, and DVC count were found to be statistically correlated (a=0.05) to one another, but negatively correlated with concentration of Kepone. The direct viable cell count proved to be a rapid, simple method for estimating the effect of Kepone on in situ estuarine microbial activity.     

Modeling the microbial food web

Models of the microbial food web have their origin in the debate over the importance of bacteria as an energetic subsidy for higher trophic levels leading to harvestable fisheries. Conceptualization of the microbial food web preceded numerical models by 10–15 years. Pomeroy's work was central to both efforts. Elements necessary for informative and comprehensive models of microbial loops in plankton communities include coupled carbon and nitrogen flows utilizing a size-based approach to structuring and parameterizing the food web. Realistic formulation of nitrogen flows requires recognition that both nitrogenous and nonnitrogenous organic matter are important substrates for bacteria. Nitrogen regeneration driven by simple mass-specific excretion constants seems to overestimate the role of bacteria in the regeneration process. Quantitative assessment of the link-sink question, in which the original loop models are grounded, requires sophisticated analysis of size-based trophic structures. The effects of recycling complicate calculation of the link between bacteria or dissolved organic matter and mesozooplankton, and indirect effects show that the link might be much stronger than simple analyses have suggested. Examples drawn from a series of oceanic mixed layer plankton models are used to illustrate some of these points. Single-size class models related to traditional P-Z-N approaches are incapable of simulating bacterial biomass cycles in some locations (e.g., Bermuda) but appear to be adequate for more strongly seasonal regimes at higher latitudes.     

The microbial loop in flowing waters

The microbial loop in flowing waters is dependent on allochthonous sources of carbon, which vary in quality. The proportion of dissolved organic carbon (DOC) that can be degraded ranges from <1 to over 50%, and the bioavailability of DOC (micrograms bacterial biomass produced per milligram DOC present) ranges over two orders of magnitude. Bioavailability of DOC is predictable from the ratio of H/C and O/C of the DOC, but further work is needed to develop simple predictors of bioavailability of DOC in a range of environments. Consumers of bacteria in streams range in size from protists to insect larvae, with highest rates of bacterial consumption found among the meiofauna and certain filter feeders and grazers. Because there appear to be fewer trophic transfers in the lotic microbial loop, it functions more as a link in flowing waters than it appears to do in the marine plankton.     

Inorganic nutrients,bacteria, and the microbial loop

The realization that natural assemblages of planktonic bacteria may acquire a significant fraction of their nitrogen and phosphorus via the uptake of dissolved inorganic nutrients has modified our traditional view of these microorganisms as nutrient remineralizers in plankton communities. Bacterial uptake of inorganic nitrogen and phosphorus may place bacteria and phytoplankton in competition for growth-limiting nutrients, rather than in their traditional roles as the respective source and sink for these nutrients in the plankton. Bacterial nutrient uptake also implies that bacterivorous protozoa may play a pivotal role in the remineralization of these elements in the microbial loop. The overall contribution of bacterial utilization of inorganic nutrients to total nutrient uptake in the ocean is still poorly understood, but some generalizations are emerging with respect to the geographical areas and community physiological conditions that might elicit this behavior.     

Incorporating the microbial loop in a simple plankton model

The microbial loop, which recycles nutrients in the upper layers of the ocean, is an integral part of plankton dynamics. The usual method for modelling the complex patterns involved has been to consider the ''Z'' in N/P/Z (nutrient/phytoplankton/zooplankton) models as containing all possible grazers on P and, implicitly, relegate the carnivorous metazoans to the loss term on Z. I propose the opposite approach (to define Z explicitly as the metazoans responsible for export fluxes) and to simulate the effects of the microbial loop implicitly in terms of grazing and excretion rates. The reasons for taking this alternative route are (i) the importance of copepods in the carbon/nitrogen flux from the euphotic zone to deeper water compared with (ii) the predominantly internal role of the microzooplankton in recycling nutrients; and (iii) the problems of sampling the microbial component, compared with sampling larger metazoans. Finally, there is the need to keep plankton models as simple as possible for later use in coupled physical/biological systems.     

Toxicity of methyltins to microbial populations in estuarine sediments

Summary The toxicities of three organotin compounds were examined on natural populations of microorganisms in sediments from Boston Harbor. Mono-, di-and trimethyltins were toxic to organisms from these sediments, and the di-and trimethyl compounds were more toxic than the monomethyl compound as measured by either viable counts or by [³H]thymidine uptake. Approximately three to eight times as much organotin was required to achieve the same effect measured by thymidine uptake as measured by viable counts. The results of replica plating experiments suggest that most estuarine organisms which are resistant to one methyltin will be resistant to other methyltins. LC-values suggest that at concentrations reported for methyltins in aquatic environments, methyltins alone are not likely to cause major alterations in the microbial flora. However, these compounds may combine with other stressors to alter the composition of natural populations.     

Fish species diversity in southern African estuarine systems: an evolutionary perspective

Synopsis The ichthyofauna of southern African estuaries consists primarily of juvenile marine species that use these habitats as nursery areas. The abundance and biomass of fishes in estuarine systems are typically high but species diversity is generally low, with only a few taxa dominating the community. This relatively low species diversity is attributed to the fact that estuaries in the region are unpredictable environments which lack any degree of permanence and are dominated by mobile marine eurytopes. Although stenotopes, represented mainly by small resident species from the families Gobiidae, Clinidae and Syngnathidae, are present in southern African estuaries, little speciation appears to have occurred. A possible reason for this lack of speciation, apart from the seasonal and annual variability of the abiotic environment, is that the lifetime of individual systems is usually limited to a few thousand years. In addition, fishes utilising southern African estuaries need to remain flexible (eurytopic) in their responses to an external environment which is unlikely to become more stable in the future. Thus the lack of permanence and fluctuating nature of southern African estuaries on both a spatial and temporal scale, together with the dominance of eurytopes in these systems, does not favour the evolution of new species. A preliminary examination of the available literature indicates that a detailed review of estuarine ichthyofaunal communities on a global basis will probably mirror the trends outlined above, and reveal a domination of these dynamic ecosystems by eurytopic taxa with low speciation potential.     

Anaerobic microbial methylation of inorganic tin in estuarine sediment slurries

Estuarine sediment slurries and microorganisms were examined for the ability to methylate inorganic tin. Under controlled redox conditions, tin was methylated only in oxygen-free sediment slurries. Monomethyltin usually comprised greater than 90% of the alkyltin products formed, although dimethyltin was also produced. Autoclaved anoxic sediments did not produce organotins. Several bacterial cultures, most notably sulfate-reducing bacteria isolated from anoxic estuarine sediments, formed monoand dimethyltin from inorganic tin in the absence of sediment. The results suggest that inorganic tin methylation in estuarine environments is an anaerobic process catalyzed primarily by sulfate-reducing microorganisms.     

Interactions between algae and the microbial loop in experimental microcosms

We conducted a short-term microcosm experiment to study the direct and indirect effects of a bacterivore on bacteria and the dynamics of two species of green algae. We introduced Scenedesmus , Chlorella and Colpidium , a bacterivorous ciliate, successively in a carbon-rich medium. Bacteria were introduced with Scenedesmus , Chlorella and Colpidium . The experiment lasted 40 days, preventing us from detecting whether the populations had reached equilibrium. The bacterivore had a positive effect on both species of algae by limiting the abundance of bacteria. In absence of the bacterivore, bacteria did not exclude the two algal species, despite the high carbon:nutrient ratio of the medium. Unexpectedly, by the end of the experiment the bacterivore declined in all microcosms. Also, Chlorella growth was impeded by the presence of Scenedesmus . These two observations might be explained by allelopathic interactions. Our experiment suggests that the functioning of such a simple community is far more complex than assumed in previous theoretical and experimental models. Studying the dynamics of the system, however, allowed us to disentangle species interactions.