通过可培养方法进行藻类计数

Algae are a heterogeneous group of photosynthetic organisms unified by possession of photosynthetic pigments, commonly chlorophyll. Most algae are microscopic, though the group includes macroscopic seaweeds. Unlike heterotrophic bacteria, algae are phototrophic, obtaining energy from photosynthesis and carbon from carbon dioxide, allowing growth in inorganic media without organic substrates.

Algal populations in soils average around ten thousand cells per gram, lower than bacteria, fungi, or actinomycetes because algae require sunlight for photosynthesis. Sunlight cannot penetrate far below the soil surface, limiting algal growth to shallow depths where light is available for photosynthetic activity.

Soil samples are serially diluted tenfold to 10-6 g soil per mL and cultured in growth media with multiple replicates at each dilution. Tubes are incubated in well-lit areas for up to four weeks. The serial dilution dilutes algae to extinction, meaning at some dilution, no algal growth occurs, enabling accurate enumeration using MPN analysis.

Algal growth appears as a green slime in the culture medium, resulting from photosynthetic activity. This visible green coloration indicates the presence of algae in that particular dilution and serves as the primary indicator for determining which tubes show positive growth during MPN analysis.

The highest dilution with the most positive replicate tubes is labeled p1, followed by p2 and p3 from higher dilutions. Values are located in an MPN table using p1 and p2 columns, then intersected with p3 across the top to obtain organisms per mL. This value is divided by soil concentration in the p2 dilution to calculate organisms per gram of original soil.

Algal enumeration assesses algal levels in rivers and lakes to evaluate risks of harmful algal blooms. It also monitors cleanliness and safety of human-use waters including swimming pools and drinking water sources, where ideally no algae should be present. The MPN method can similarly assess water quality using indicator organisms like coliforms or E. coli.

Blue-green algae, or cyanobacteria, fix nitrogen gas from the atmosphere, increasing soil nitrogen in semi-arid environments. Some algae serve as biofertilizers, acting as symbionts with plants to enhance nitrogen uptake, particularly useful in areas with poor soil. Algae are also cultivated for biofuels and as nutrient-rich livestock feed.