A compilation of data on the elemental composition of marine phytoplankton from published studies was used to determine the range of C[ratio ]N[ratio ]P. The N[ratio ]P ratio of algae and cyanobacteria is very plastic in nutrient-limited cells, ranging from <5 mol N[ratio ]mol P when phosphate is available greatly in excess of nitrate or ammonium to >100 mol N[ratio ]mol P when inorganic N is present greatly in excess of P. Under optimal nutrient-replete growth conditions, the cellular N[ratio ]P ratio is somewhat more constrained, ranging from 5 to 19 mol N[ratio ]mol P, with most observations below the Redfield ratio of 16. Limited data indicate that the critical N[ratio ]P that marks the transition between N- and P-limitation of phytoplankton growth lies in the range 20–50 mol N[ratio ]mol P, considerably in excess of the Redfield ratio. Biochemical composition can be used to constrain the critical N[ratio ]P. Although the biochemical data do not preclude the critical N[ratio ]P from being as high as 50, the typical biochemical composition of nutrient-replete algae and cyanobacteria suggests that the critical N[ratio ]P is more likely to lie in the range between 15 and 30. Despite the observation that the overall average N[ratio ]P composition of marine particulate matter closely approximates the Redfield ratio of 16, there are significant local variations with a range from 5 to 34. Consistent with the culture studies, lowest values of N[ratio ]P are associated with nitrate- and phosphate-replete conditions. The highest values of N[ratio ]P are observed in oligotrophic waters and are within the range of critical N[ratio ]P observed in cultures, but are not so high as to necessarily invoke P-limitation. The C[ratio ]N ratio is also plastic. The average C[ratio ]N ratios of nutrient-replete phytoplankton cultures, oceanic particulate matter and inorganic N and C draw-down are slightly greater than the Redfield ratio of 6·6. Neither the analysis of laboratory C[ratio ]N[ratio ]P data nor a more theoretical approach based on the relative abundance of the major biochemical molecules in the phytoplankton can support the contention that the Redfield N[ratio ]P reflects a physiological or biochemical constraint on the elemental composition of primary production.

The production of potent toxins by bloom-, scum- and mat-forming cyanobacteria, in fresh-, brackish and marine waters, appears to be a global phenomenon. Cyanobacterial toxins can also be produced by cyanobacteria from terrestrial sources. The range and number of known cyanobacterial toxins are increasing apace as associated poisoning incidents are investigated, and increasingly powerful analytical methods are applied to complement toxicity-based studies on both natural samples and laboratory isolates of cyanobacteria. Water quality management to reduce toxic cyanobacterial mass developments, and schemes to mitigate the potential effects of cyanobacterial toxins, require an understanding of the occurrence and properties of the toxins and of the exposure routes via which the toxins present risks to health. Here, we review advances in the recognition of cyanobacterial toxins and their toxicity, and of the exposure routes with reference to human health, namely via skin contact, inhalation, haemodialysis and ingestion (the oral route).

Drying of cells leads to damage resulting from crowding of cytoplasmic components, condensation of the nucleoid, increases in the Tm of membrane phase transitions, and imposition of stress upon cell walls. Prolonged desiccation leads to oxidation of proteins, DNA and membrane components through metal-dependent Fenton reactions, while Maillard reactions generate cross-linked products between the carbonyl groups of reducing sugars and the primary amines of nucleic acids and proteins. Although such damage restricts many organisms to aqueous environments, some, including many cyanobacteria, can tolerate the air-dried state for prolonged periods. Cyanobacteria in the Tintenstrich communities of exposed rock faces, Microcoleus and Lyngbya spp. in intertidal mats, chasmoendolithic Chroococcidiopsis spp. in the rocks of hot and cold deserts, and terrestrial epilithic crusts of Tolypothrix and Nostoc are examples that show a marked capacity to withstand the removal of their cellular water. For Nostoc commune, the mechanisms of desiccation tolerance reflect both simple and complex interactions at the structural, physiological and molecular levels.

Grateloupia doryphora (Montagne) Howe, originally described from Peru, has repeatedly been reported as an invasive species in Atlantic and Mediterranean waters. Various attempts to explain this species’ route of introduction have been unsatisfactory. New evidence from comparative rbcL sequence analysis and morphology suggests that this adventive species in the NE and NW Atlantic corresponds with G. turuturu Yamada, originally described from Japan. This provenance follows a well-recognized trend of invasive marine organisms that have colonized the Atlantic Ocean and Mediterranean Sea from Pacific NE Asia.

Marine snow is a ubiquitous feature of the ocean and an important agent in the transport of energy and nutrients through marine ecosystems. Diatom aggregates, which form during blooms and, to a lesser extent, by the resuspension of benthic biofilms, are a primary source of marine snow. Genera commonly found in diatom aggregates are: Nitzschia, Chaetoceros, Rhizosolenia, Leptocylindricus, Skeletonema and Thalassionema. Most fieldwork has been restricted to a limited number of locations in the Northern Hemisphere. To quantify the global impact of diatom aggregation there is a need to conduct fieldwork in a wider range of areas, particularly in the Southern Hemisphere. Aggregates form when particles collide and stick together. Collisions in the water column are affected by turbulence, differential settlement and animal feeding, whereas diatom stickiness is affected by extracellular polymeric substances (EPS). Laboratory experiments have demonstrated that diatoms produce more EPS under nutrient limitation, although little is known about how limitation by different nutrients affects the quantity and composition of EPS and subsequent stickiness. EPS form three pools in the environment: cell coatings, soluble EPS and transparent exopolymeric particles (TEP). There is a need to investigate the dynamics of conversion between the pools of EPS by both abiotic and biological processes and how these conversions affect aggregate concentration and structure. Processes governing disaggregation have been largely overlooked, although they are as important as aggregation in determining the dynamics of aggregate concentrations in the water column. The biogeochemical significance of diatom aggregates as a means of transporting carbon and other nutrients from the euphotic zone to the seabed is well established. However, the internal biogeochemistry of aggregates is not well understood. Aggregates contain anaerobic microsites and further work is required to establish whether aggregates are significant sinks for nitrogen in the water column through anaerobic denitrification. Several hypotheses have been proposed to explain diatom aggregation in the field, but many of these are flawed because the mechanisms and adaptive explanations proposed require natural selection to operate at the level of populations rather than genes or individuals.

Changes in the ultrastructure and chemistry of the cell wall of the unicellular volvocalean green alga Haematococcus pluvialis were investigated during the transformation of flagellates into aplanospores. The motile biflagellated state exhibited a distinct gelatinous extracellular matrix. Its ultrastructure resembled the typical volvocalean multilayered architecture with a median tripartite crystalline layer. The transformation into the non-motile cell state was characterized by formation of a new layer, a primary wall, within the extracellular matrix. During this process, the initial extracellular matrix remained intact except for the outer layers of the tripartite crystalline layer, which decomposed. Further morphogenesis of the aplanospore resulted in the formation of a voluminous multilayered cell wall. A trilaminar sheath was formed inside the primary wall and the innermost and thickest part was an amorphous secondary wall, consisting mostly of a mannan. Results obtained by staining with the fluorescent dye primuline as well as by acetolysis suggest the occurrence of sporopollenin-like material (algaenan) within the trilaminar sheath of the aplanospore cell wall. The primary wall and the outer remnants of the extracellular matrix disintegrated as the aplanospores aged, and were completely absent in the resting cell state.

The production and composition of extracellular polymeric substances (EPS) in axenic batch cultures of the benthic marine epipelic diatoms Navicula salinarum and Cylindrotheca closterium were investigated. EPS was secreted into the medium and the bulk was loosely associated with the cells. Neither N. salinarum nor C. closterium formed a well-defined polysaccharide capsule. EPS of both N. salinarum and C. closterium consisted predominantly of polysaccharide but small quantities of protein were present as well. EPS also contained uronic acids and SO42− groups. Analysis of monosaccharides using gas chromatography showed that for both species glucose and xylose were the main constituents, but several other monosaccharides were present in smaller quantities. Two fractions of EPS were distinguished: a small amount was secreted into the medium and a second fraction was extracted in water at 30°C. For both species the two fractions differed somewhat in composition, indicating that they represented two different types of EPS. The EPS produced by N. salinarum and by C. closterium differed in their composition. The rate of EPS production in batch culture was highest during the transition from exponential growth to stationary growth. Negatively charged groups such as uronic acids and sulphated sugars determine the adhesion capacity of EPS and probably play an important role in the stabilization of intertidal sediments on which these diatoms grow and produce biofilms.

Different stages of the automortality in phytoplankton have been studied applying flow cytometry. These stages are, in order of expression: (1) compromised cell membranes, (2) degradation of the photosynthetic pigments and reduction of the photosynthetic activity, (3) fragmentation of the genomic DNA. The integrity test of the cell membranes is based on the inability of the DNA-specific stain SYTOX Green to pass into cells with intact plasma membranes. The reduction in photosynthetic activity was examined by sorting 14C-labelled phytoplankton cells differing in viability. Finally, DNA fragmentation was traced by measuring changes in genomic DNA. The different phytoplankton species tested showed a great variety in response when grown under the same conditions, but there was also considerable intraspecific variation. Unstained cells, fully stained cells (equivalent to full staining of genomic DNA in fixed cells) and cells with intermediate fluorescence signal occurred together within the same culture. The photosynthetic activity in cells with a reduced viability dropped by as much as 60% relative to that of the viable cells. In the subsequent stage, when photosynthetic pigments were fully degraded, this value dropped further to around 10%. Cells in this stage also showed subdiploidy as a result of genome fragmentation. Field tests using samples of phytoplankton collected in the North Atlantic Ocean (40° N, 23° W) during spring showed staining properties similar to those found in cultures grown at suboptimal growth conditions. The percentage of non-viable cells varied considerably (ranging from 5% to 60%) between the various phytoplankton groups present. The lowest value was observed for Synechococcus, but some pico-eukaryotes showed percentages as high as 60%. Moreover, the viability varied with depth (light level) and over a light–dark cycle. The present findings suggest the existence of a (genetically based) uniform process of automortality in phytoplankton. Non-viable cells are a substantial component of the oceanic phytoplankton, affecting the food-web structure and species succession.

During the last two decades, more than 100 cattle deaths have been reported from 11 alpine sites in south-eastern Switzerland. Pathological findings and the histological examination of their organs strongly indicated acute hepatotoxicosis. Clinical symptoms suggestive of neurotoxicity were also observed in some cases. To elucidate the etiology of these poisonings, different water bodies in one of the affected alpine pastures were investigated for cyanobacterial toxins. The waters were highly oligotrophic, cold and turbid, and the ice-free period was limited to 3–4 months. The algal community in these waters consisted mostly of benthic cyanobacteria forming dense mats on the surface of sediments and on submerged rocks. Oscillatoria limosa and Phormidium konstantinosum (=Oscillatoria tenuis) dominated these populations, but occasionally other species of Oscillatoria, Phormidium, Tychonema and Pseudanabaena also occurred in the mats. Samples from the cyanobacterial mats yielded positive results in a protein phosphatase inhibition assay, reacted with antibodies against microcystins in an enzyme-linked immunosorbent assay and were hepatotoxic in a mouse bioassay. The same cyanobacterial material also included neurological effects in mice. High-performance liquid chromatography was used to identify a microcystin, in these cyanobacterial samples as well as in the corresponding lake water. To our knowledge, this is the first documented example of hepatotoxicity associated with benthic cyanobacteria, and the first report of toxic cyanobacteria from the remote, oligotrophic alpine environment.

If the depletion of the stratospheric ozone shield continues, the influx of solar UV-B radiation (280–320 nm) will increase in the near future. In photosynthetic organisms there are several targets for the deleterious UV-B radiation, such as proteins, DNA and membranes. Cyanobacteria use three different types of strategies to counteract UV damage: (i) stress avoidance by gliding mechanisms, (ii) stress defence by synthesis of UV-absorbing compounds, antioxidants and extracellular polysaccharides, and (iii) repair mechanisms including DNA repair and resynthesis of UV-sensitive proteins. In the past, most studies concentrated on the physiological aspects of UV tolerance whereas the molecular basis of UV tolerance in cyanobacteria is poorly understood. We will summarize the effects of and responses to UV-B at the physiological and molecular levels and present data on the influence of UV-B irradiation on the proteome of the terrestrial cyanobacterium Nostoc commune. SDS-PAGE and high-resolution two-dimensional gel electrophoresis were used to analyse the influence of UV-B on the proteome of N. commune. The resolution of SDS-PAGE turned out to be far too low to monitor the UV acclimation process. In contrast to the latter technique, two-dimensional electrophoresis showed that the UV-B response is extremely complex, involving the induction and the repression of a large number of proteins.

In order to elucidate the phylogeny and evolutionary history of the Bacillariaceae we conducted a phylogenetic analysis of 42 species (sequences were determined from more than two strains of many of the Pseudo-nitzschia species) based on the first 872 base pairs of nuclear-encoded large subunit (LSU) rDNA, which include some of the most variable domains. Four araphid genera were used as the outgroup in maximum likelihood, parsimony and distance analyses. The phylogenetic inferences revealed the Bacillariaceae as monophyletic (bootstrap support [ges ]90%). A clade comprising Pseudo-nitzschia, Fragilariopsis and Nitzschia americana (clade A) was supported by high bootstrap values ([ges ]94%) and agreed with the morphological features revealed by electron microscopy. Data for 29 taxa indicate a subdivision of clade A, one clade comprising Pseudo-nitzschia species, a second clade consisting of Pseudo-nitzschia species and Nitzschia americana, and a third clade comprising Fragilariopsis species. Pseudo-nitzschia as presently defined is paraphyletic and emendation of the genus is probably needed. The analyses suggested that Nitzschia is not monophyletic, as expected from the great morphological diversity within the genus. A cluster characterized by possession of detailed ornamentation on the frustule is indicated. Eighteen taxa (16 within the Bacillariaceae) were tested for production of domoic acid, a neurotoxic amino acid. Only P. australis, P. multiseries and P. seriata produced domoic acid, and these clustered together in all analyses. Since Nitzschia navis-varingica also produces domoic acid, but is distantly related to the cluster comprising the Pseudo-nitzschia domoic acid producers, it is most parsimonious to suggest that the ability of species in the Bacillariaceae to produce domoic acid has evolved at least twice.

The requirements for efficient utilization of high light fluxes in cultures of Spirulina platensis have previously been elucidated. The most important of these was a short light-path coupled with a highly turbulent flow, facilitating ultrahigh cell densities (i.e. above 100 mg chl l−1). The present study shows that for each irradiance there is an optimal culture density, defined as the concentration that yields the highest output rate of cell mass under the prevailing conditions. In ultrahigh cell density cultures, a linear relationship was observed between the output rate and the irradiance, up to a photon flux density (PFD) of 2500 μmol m−2 s−1. Using a total PFD of 8000 μmol m−2 s−1, a maximal output rate of 16.8 g dry weight m−2 s−1 was obtained, which is the highest reported for a culture of photoautotrophic microorganisms exposed to direct beam radiation. Testing the effect of reduction in light-path on productivity, output rate per unit volume increased 50-fold as the light-path was reduced 27-fold, i.e. from 200 mm to 7.5 mm. Likewise, the output rate calculated on an areal basis was almost doubled, increasing from 2.5 to 4.6 g m−2 s−1, and the specific growth rate increased c. 20% as the light-path was reduced to 7.5 mm. The very significant effect of the length of the light-path on enhancing the output rate is interpreted as resulting from improvement in the light regime to which the individual cells are exposed, as reflected in the frequency and overall characteristics of the light–dark cycles.

Changes in relative abundance of estuarine epipelic microalgae along an estuarine gradient were investigated, and population densities of different species along a small-scale nutrient gradient generated by a sewage treatment outfall and in laboratory mesocosms enriched with ammonium were also studied. The relative abundance of certain species of epipelic diatoms was related to location along the estuarine salinity and nutrient gradient: Navicula gregaria and N. phyllepta were abundant at oligo- and mesohaline sites respectively, and Pleurosigma angulatum and Plagiotropis vitrea were abundant at polyhaline sites. On a smaller spatial scale, though there were no significant patterns in microalgal biomass in relation to nutrient enrichment, there were significant differences in the population densities of different epipelic species along a multivariate nutrient gradient (decreasing concentrations of ammonium, nitrite, silicate, organic content, and increasing salinity and pore-water nitrate concentrations) away from a sewage outfall along a saltmarsh creek. The diatoms Nitzschia sigma and Gyrosigma limosum and the cyanobacteria Oscillatoria limosa and O. princeps had significantly higher population densities near the outfall, and Navicula phyllepta, N. pargemina, Nitzschia frustulum, Cylindrotheca signata and Pleurosigma angulatum were significantly more abundant at the seaward end of the gradient. In laboratory tidal mesocosms, sediment cores from along the gradient had their pore-water ammonium concentrations increased to 380–450 μM NH4+. After 26 days, the population densities of Gyrosigma fasciola, G. littorale, P. angulatum, N. phyllepta, Cylindrotheca signata, C. closterium and Niztschia apiculata were significantly reduced, while those of G. limosum, N. sigma, Scolioneis tumida and O. limosa were unaffected, or were significantly higher compared with control cores. Laboratory manipulations of ammonium concentrations supported the observed field distributions, indicating that epipelic species do have different trophic preferences and ammonium concentration may be a significant factor in determining estuarine species composition of epipelic algae.

Viable diatom and dinoflagellate resting stages were recovered from sediments in Koljö Fjord on the west coast of Sweden. To determine the maximum survival time of buried resting stages, samples from sediment depths down to 50 cm were incubated at temperatures of 3, 10 and 18 °C. Sediment cores were dated by 210Pb and the age of samples containing viable resting stages was determined using the constant rate of supply model. Dilution cultures of surface sediments allowed semiquantitative estimates of the potential seed bank. Dinoflagellate cysts from species such as Diplopsalis sp., Gymnodinium nolleri, Oblea rotunda and Protoceratium reticulatum were viable down to 15 cm depth, or 37 years old. Spores and resting cells of the diatoms Chaetoceros spp., Detonula confervacea and Skeletonema costatum were viable to over 40 cm depth, and may have been buried for many decades. The seed bank of living resting stages in surficial sediments was found to be rich (c. 57000 diatom resting stages g−1 wet weight and c. 200 dinoflagellate cysts g−1 wet weight), and the percentage of viable resting stages was higher for spore- and cyst-forming species. The oxygen-deficient sediments in Koljö Fjord appear to be a natural conservator of cell viability, a condition not easily simulated in laboratory studies. These results are ecologically important since spores and cysts are a repository of genetic material able to repopulate waters if resuspended and exposed to suitable light, temperature and nutrients.

Nitrogen (N) and phosphorus (P) limitation affect the photosynthetic apparatus of Dunaliella tertiolecta in markedly different ways. When grown at 0·25 d−1 (18 % of the resource-saturated maximum rate, μmax=1·39 d−1) in chemostat cultures, N- and P-limited cells were chlorotic relative to nutrient-replete controls. The lutein-to-chlorophyll a ratio increased under both N and P limitation, whereas the neoxanthin-to-chlorophyll a ratio increased only under P limitation. The ratio of accessory photoprotective pigments (α- and β-carotene) to chlorophyll a increased under N-limited conditions. Despite differences in accessory pigment complement, chlorophyll a-specific light absorption coefficients of N- and P-limited cultures did not differ significantly, and were greater than in nutrient-replete conditions. In contrast, the initial slope of the photosynthesis–irradiance (PE) response curve (αChl) declined under nutrient-limiting conditions. There were slight reductions in the maximum quantum efficiency of photosynthesis (ϕm) in N- and P-limited cells. Reductions in ϕm were accompanied by reductions in the ratio of variable to maximum fluorescence (Fv/Fm), and the ratio of the photosystem II reaction centre protein D1 to the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Differences in light-saturated gross photosynthesis rate, as measured by light/dark oxygen exchange, could be accounted for by changes in the abundance of the carboxylating enzyme Rubisco. Oxygen exchange and 14CO2 assimilation appeared to measure different processes in P-limited and N- limited cultures. At light-saturation, 14C-bicarbonate assimilation approximated gross photosynthesis (as measured by light/dark oxygen exchange) in P-limited cultures. In contrast, 14C-bicarbonate assimilation approximated net photosynthesis in N-limited cultures. When all culture conditions were compared, there was linear covariation of the rates of reductant supply via light absorption and photochemical charge separation with the rates of reductant demand for CO2 fixation and NO3− reduction.

New holococcolith–heterococcolith life-cycle associations are documented based on observations of combination coccospheres. Daktylethra pirus is shown to be a life-cycle phase of Syracosphaera pulchra and Syracolithus quadriperforatus a life-cycle phase of Calcidiscus leptoporus. In addition, new observations from cultures confirm the life-cycle associations of Crystallolithus braarudii with Coccolithus pelagicus and of Zygosphaera hellenica with Coronosphaera mediterranea. In all four cases previous work has shown that the heterococcolithophorid species is associated with another holococcolithophorid. Two other examples of a heterococcolithophorid being associated with two holococcolithophorids have previously been identified, so this seems to be a common phenomenon. The six examples are reviewed to determine whether a single underlying mechanism is likely to be responsible for all cases. It is concluded that there is no single mechanism but rather that the six examples fall into three categories: (a) in two cases the holococcolith types are probably simply ecophenotypic morphotypes; (b) in two other cases the holococcolith types are discrete and are paralleled by morphometric differences in the heterococcolith types; (c) in the final two cases the holococcolith types are discrete but are not paralleled by any obvious morphological variation in the heterococcolith morphology. We infer that cryptic speciation may be widespread in heterococcolithophorid phases and that study of holococcolithophorid phases can provide key data to elucidate this phenomenon.

The cyanobacterial fossil record is among the oldest for any group of organisms, possibly reaching back to 3500 Ma ago. The molecular phylogeny of cyanobacteria is complementary to the fossil findings, confirming the antiquity of the group, the role of cyanobacteria in the evolution of planetary primary production, and the symbiotic origins of plastids in algae and plants from cyanobacterial ancestors. The study of fossil cyanobacteria followed the discovery of Precambrian microbial fossils by S.A Tyler and E.S. Barghoorn in 1954, and is still developing. Most fossil cyanobacteria are preserved in permineralized conditions in cherts and phosphorites or as organic compressions in shales. The interpretation of fossil cyanobacteria is aided by the study of modern counterparts, preferably within their natural habitats. These comparisons include the post mortem degradation of cellular remains. The fortuitous preservation and fossilization of ancient cyanobacterial communities in growth position, i.e. in the synsedimentary context, allows one to draw conclusions about their palaeoenvironment, including interactions between cyanobacteria and ancient sediments. These relations are based on cyanobacterial ecological requirements, and they compare well with behavioural responses of modern cyanobacteria in microbial mats and modern stromatolites. The general trend in the evolution of cyanobacteria is one of gradually increasing complexity and diversity, but the group shows a conservative maintenance of morphological adaptations to successful ecological niches. Accordingly, a large proportion of ancient morphological types is still represented among modern cyanobacteria. Fossil to Recent counterparts are identified for several coccoid and filamentous cyanobacteria. Evidence for heterocystous cyanobacteria is indirect, through identification of fossil akinetes.

Effects of UVA and UVB radiation were evaluated on two cyanobacterial strains (Phormidium murrayi and Oscillatoria priestleyi) isolated from the McMurdo Ice Shelf, Antarctica. The two isolates showed some similarities, but also major differences in their qualitative and quantitative responses to ultraviolet radiation (UVR). Growth decreased with increasing UVR, but with a 5-fold (UVA) or 10-fold (UVB) greater effect on O. priestleyi than P. murrayi. In both isolates, cellular concentrations of phycobiliproteins (measured by in vivo absorbance), and to a lesser extent chlorophyll a, diminished with increasing UVR exposure. Spectral scans of methanol extracts indicated the presence of UVR-screening compounds in O. priestleyi but not P. murrayi; however, the absorbance per unit dry weight was low, and similar in cultures with and without UVR. Carotenoid pigments increased up to a threshold UVB flux and thereafter decreased. In both isolates, moderate UVA lessened the effect of growth inhibition by UVB, consistent with a UVA-activated repair mechanism. Comparative motility tests showed that O. priestleyi is a fast gliding species that can rapidly relocate in response to changes in ambient light, while P. murrayi is non-motile. The ability of O. priestleyi to escape UVR by gliding, and the greater ability of P. murrayi to tolerate UVA and UVB exposure, illustrate the differences in UVR survival strategies even between closely related species of cyanobacteria.

Geomicrobiological analysis of calcifying biofilms of three alkaline salt lakes characterized by moderate to high carbonate alkalinity indicates that microbial carbonate rock formation is not directly linked to cyanobacterial carbon fixation. The present review summarizes results from two published case studies that have been carried out at Pyramid Lake, USA, and Lake Nuoertu, PR China. New observations and data are presented for a current project on Satonda Crater Lake, Indonesia, that revise previous conclusions concerning the relationship between cyanobacteria and biofilm calcification. Extracellular polymeric substances (EPS) in the investigated lakes are mostly produced by cyanobacteria; their properties are discussed as key factors in biofilm calcification. In particular, EPS are capable of binding divalent cations (e.g. Ca2+) from the liquid phase by their carboxylate and sulphate groups. Therefore, despite a high supersaturation of the lake water with respect to calcium carbonate minerals, precipitation does not take place immediately. A delayed onset of precipitation can be achieved by a continuous Ca2+ supply that exceeds the Ca2+-binding capacity of the EPS, and/or an exoenzymatic degradation (decarboxylation, cleavage) of mucous substances that reduces the binding capacity and causes secondary Ca2+ release. The resulting microcrystalline precipitates are randomly distributed within the EPS, usually away from any of the living cyanobacteria. This suggests that the effect of photosynthetic CO2 fixation in increasing supersaturation is of secondary importance at high alkalinities. In contrast to biofilm-covered surfaces, calcium carbonate minerals nucleate and grow rapidly at surfaces poor in EPS when the critical supersaturation level for non-enzymatically controlled carbonate precipitation is reached. Examples of such surfaces poor in EPS are dead, lysed green algal cells and thin, discontinuous biofilms in voids of microbial reef rocks. Calcium carbonate crystals directly linked to cyanobacterial cells or filaments have been observed only exceptionally, e.g. on Calothrix.

Microphytobenthos are the major intertidal primary producers in many estuarine systems and assessment of their biomass is essential in ecological studies. Algal biomass can be assessed by the use of proxy measurements, e.g. chlorophyll a (Chl a), using a lengthy and destructive extraction procedure. Fluorescence is a non-destructive and in situ alternative, recently applied to microphytobenthos, using the minimum fluorescence signal (Fo) as a measurement of Chl a. In this study, Fo after 15 min of dark adaptation (Fo15) showed a significant positive correlation with Chl a (r = 0.84, p<0.001, n = 24). The relationship between Fo15 and Chl a extracted from sediment samples of increasing sample depth (0.2–2 mm) became less significant as sediment depth increased. The strongest relationship between Fo15 and Chl a, expressed as concentration (mg m−2) and content (mg kg−1), was found when all data were combined, but nearly comparable levels of association were found for in situ measurements using sediment cores up to 2.0 mm deep. Results suggested that full dark adaptation was not always achieved because a significant negative correlation was observed between Fv/Fm and ambient irradiance for seasonal data. In addition, Fo15 showed a more significant correlation with Chl a than with Fm15, suggesting some residual level of non-photochemical quenching after 15 min in the dark. Despite the incomplete dark adaptation, the highly significant relationships between Fo15 and both Chl a concentration and content suggest that the use of Fo15 is a suitable in situ proxy method of estimating algal biomass.