Association of turbidity and organic carbon with bacterial abundance and cell size in a large, turbid, tropical lake

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Suspended clay particles inhibit autotrophic processes by reducing illumination, but they simultaneously provide surfaces for the adsorption of dissolved organic material. The aggregated organic matter can be used by heterotrophic bacteria which consequently modifies the microbial loop. Large (up to 65-µm diam) clay-organic matter aggregates are abundant in the water of tropical Lake Chapala, Mexico, primarily as a result of wind mixing and sediment resuspension. Because bacterial abundance and cell size each vary with available substrate, we hypothesized that the bacterial community associated with these suspended clay-organic aggregates would be numerous and large celled as compared with those living free in the water. A persistent in-lake turbidity gradient (from 20 to 90 NTU) was used to evaluate the relationships of particle-aggregate abundance to bacterial cell size and planktobacterial community size. Particle aggregates are colonized by bacteria to the extent that <20% of the planktobacteria are free in the water. Bacterial biomass increased with increasing turbidity, and bacteria associated with aggregates were larger than those free in the water. Both the number of bacteria and their community biomass were significantly greater in more turbid regions of the lake than in the less turbid regions. Variation in number of particle aggregates explained variation in the number of bacteria, whereas variation in the organic matter content of the aggregate explained variation in bacterial cell size. The abundance of large clay-organic matter-bacterial aggregates suggests their trophic significance and provides a possible explanation for the observed discrepancy between phytoplankton production and fishery yield in the lake.